INTRODUCTION.

1. Physiology is that part of physical science which treats of the nature, properties, and func- tions of living bodies; comprehending under this term, animals and vegetables. The word is derived from φύσις, "nature," and λόγος, "a discourse;" and signi- fied originally what we may call natural knowledge.

The object of this science is to examine and compare the phenomena of life; to discover the properties, powers, and operations of the bodies that are actuated by this principle, and to pursue the development, pro- gress, and decay of vital energy, from brute matter, which possesses no portion of vitality, to the most perfect animal, which seems to have it in the greatest perfec- tion.

2. Physiology may be considered under three heads; historical, philosophical or rational, and practical phy- siology.

3. Historical physiology is occupied in giving a simple relation of the facts and phenomena that take place in living bodies; in bringing them together, and com- paring those which succeed each other without inter- ruption during the existence of vitality.

4. It is the business of philosophical physiology to consider the nature of these phenomena, and endeavour to deduce from them some general conclusions, by which they may be explained or elucidated; to draw from them natural consequences, and unfold successively their analogies and relations; to arrange, distribute, and clas- sify them, and thus acquire sufficient data by which to discover the causes which produce them.

The practical part of physiology is intended to point out the application of the principles of the science to the useful purposes of life, especially to medicine and agricul- ture. Of these divisions the first is the most impor- tant, as, until we have acquired a pretty complete knowl- edge of the facts relating to living beings, and arranged these in a natural manner, it cannot be expected that we should make any great progress in explaining them, or investigating their causes. From the multitude, variety, and complex nature of these phenomena, a complete view of them is extremely difficult, and re- quires the united efforts of genius, dexterity, patience, and discernment.

Physiology is intimately related to several other de- partments of natural knowledge. Its relation to ana- tomy is the most strict and natural; and indeed the knowledge of the structure of living bodies is a necessary introduction to that of their properties and functions. So close is the union between these two sciences, that it is generally thought that the study of them should go hand in hand. Certain it is, that, without physiology, ana- tomy is a dry and uninteresting study; while, on the other hand, physiology, unaided by anatomy, is obscure and uncertain. It is by means of anatomy that we learn the structure and organization of the animal machine; the disposition and form of its several members; the parts that concur in the composition of them; the ar- rangement and the connection of these: it is by means of anatomy that we know how to estimate the advanta-

(c) The germ of every organ must exist in embryo, if the expansion of that organ is to be afterwards called forth. ges of any particular conformation; that we estimate the proportion between the solid and fluid parts of the body, and the adaptation of the organs to the various uses for which they are defined. But we must remember, that the aid of anatomy in physiological researches, extends no farther than to the mechanical arrangement and relation of the several parts of the body; it teaches us nothing of its intimate composition, and much less does it inform us respecting the vital powers and mental energies displayed by the living body. The most accurate inspection of the brain will afford us no light respecting the obscure function of sensation, and the hidden operation of thought and judgment; nor will the nicest direction of the eye, or of the tongue, shew us how the former is enabled to convey to us the ideas of external objects, or how the latter imparts to us the thousand varied flavours of sapid bodies. In short, we may conclude, that the more the exercise of any function depends on the structure and organization of the organs that perform it, the more capable is it of illustration from anatomy.

The science of chemistry has proved of eminent use in explaining several functions of the animal economy, which, without its affluence, would have been very imperfectly understood. The action of the air on the blood during respiration, and the digestion of food in the stomach, are found to depend chiefly on chemical operations; while this science has explained to us, in a most satisfactory manner, the nature, composition, and reciprocal action of the component principles of organized beings, and the nature of several changes which they undergo, both in the living and the dead state. But if we extend the application of chemistry too far, and imagine, as some of our modern chemical physiologists have done, that it is capable of explaining life, motion, and even sensation, we shall only betray our own presumption and our ignorance of vital phenomena. When the performance of any function depends on the intimate composition of its organs, or the combinations and decompositions that take place among their component principles; then, and then only, is it susceptible of chemical illustration.

Mechanical philosophy is employed with advantage in explaining some of the phenomena that take place in living beings. The strength and solidity of the bony compasses; the force and direction of the muscular motions; the propagation, direction, and mechanical effect of light, sound, odours; the effects of the gravity and elasticity of atmospheric air, and a few other circumstances, may be submitted to calculation, and illustrated on physical principles: but the laws of mechanical and of vital action are, in general, so different, and even the most mechanical phenomena of living beings are often so modified and counteracted by the agency of the vital powers, that, on the whole, we must consider general physics as one of the least useful auxiliaries of physiological researches.

Of all the branches of physical science, physiology certainly makes the nearest approach to the region of metaphysics; but yet there is a difference between these, though it may not be very easy to point out the precise line of termination. Physiology, as already defined, being that science which has for its object the organic economy of living bodies, the word organic, we think, here should mark the distinction.

Whenever the economy of living bodies indicates design, and cannot result from any combination or structure of organs, it must be supposed the effect of something different from matter, and whose explanation belongs to that science which is called metaphysics, or which we might term the philosophy of the mind. By ascribing, indeed, to the glandular contents within the cranium, and to that fiction animal spirits, the motives of action, the superficial and ill-informed may have been led to an opinion that perception, memory, and imagination, are the functions of the cerebrum, the medulla oblongata, and cerebellum; that the soul is a consequence of organization; and the science which treats of it only a particular branch of physiology. But mind and its faculties are now so well understood and investigated, that this opinion can seldom prevail but where penetration is not remarkable for its acuteness, or where reflection, reading, and research, have long been confined within the limits of a narrow circle.

Some metaphysical physiologists contend, that every living system of organs supposes mind, and indeed in the study of such systems the physiologist must often meet with many phenomena that are less singular than simple perception, and yet for which he cannot account by any knowledge he possesses of organic powers. This truth we partly acknowledge, when, like ancient Athens erecting her altars to unknown gods, we retreat to those altars of ignorance, the vis inflata, the vis nervosa, the vis vitalis, the vis medicatrix, and a number of others of the same kind.

the general sense in which we have and naturally defined it, is a science that investigates the nature and history, the functions of all living beings. It is, therefore, reasonable to suppose that it must have an intimate connection with natural history, and in fact, it is to this branch of physics that it has been perhaps more indebted than to any other. A comparative view of the various gradations among organized beings has taught us to appreciate the value of the several functions that characterize vitality, and has shewn us, that in proportion as the structure is more complex, the functions are more numerous, and more complete. Repeated observations, and multiplied experiments on various tribes of animated nature have cleared up many doubtful and obscure phenomena in the economy of man, and a continuation of this truly philosophical method of research promises to place physiology on the solid basis of experience, and enable us to reason, where only we can reason with safety, by a deduction from facts. The more numerous these facts, and the more complete their arrangement, the more extensive, and the more secure will be the foundation which they afford for physiological conclusions. In short (to use the language of Dumas, who has illustrated this relation at great length), "the physiologist who is conversant with natural history, is so much the better fortified against uncertain opinions, inasmuch as he has more fully observed the operations of nature in connection and in detail. An hypothesis which to others appears perfectly adequate to the object in view, is not convincing to him. He rises above the particular object to which it is accommodated, in order to appreciate its value; and it is often among circumstances which are foreign to the original subject, that he seeks for exceptions or contradictions that overturn the hypothesis. Every thing that may serve to complete the knowledge..." knowledge of the animal economy enters into his plan; and as the nature of man is so much the less comprehensible as we employ a greater number of comparative ideas in its exposition, it is doubtless in the power of natural history to elucidate that subject, by revealing a multitude of unknown relations between man and those beings which resemble or which differ from him.*

The importance and utility of physiology will scarcely be questioned, and need therefore but little illustration. To all who desire to become acquainted with the operations that take place in the animal economy, or to trace the progress of vegetation, and examine the various changes produced on the seed or bud from the action of air, heat, and moisture, (and what studies can be more deserving of a rational and an enlightened mind?) physiology must afford the most interesting subjects of contemplation. To the anatomist and the botanist, it relieves the tedium of dry description and severe classification; to the physician it holds out the surest lights to direct his researches into the circumstances that are favourable to life and health, into the nature and phenomena of death, and of course, the means of avoiding or delaying its attack; to the agriculturist it furnishes some of the most certain principles to direct him (with the aid of chemistry) in the choice of soils, and the application of manures; while to the genuine naturalist, no subject presents such a field of amusement and instruction. When it shall have been rendered as complete as the state of contemporary science will allow, it will exhibit the general result of all those experiments and observations that have purposely been made to illustrate the phenomena of animated matter, or have accidentally contributed to that illustration; and when it shall reach that summit of perfection to which the efforts of genius may carry it, it may diffuse a light, of which at the present day we can form no just or adequate conception. On many occasions it may introduce order for confusion, certainty for doubt; and may establish science, in various departments that are now occupied by fancy and conjecture.

After having pointed out the nature, divisions, relations, and utility of physiology, it may not be improper to make a few remarks on the best methods of pursuing the study of it, and the works that are most worthy of a perusal.

From what has been said of the relations between physiology and other sciences, it will be inferred that the student of our present subject should come prepared with a moderate share of knowledge in anatomy, both human and comparative, of chemistry, of mechanical philosophy, especially dynamics, optics, pneumatics, and acoustics; and natural history, especially zoology and botany. At least the rudiments of these branches should be well understood, and the student will then have laid a foundation on which to raise a firm and durable superstructure.

He has now to make himself acquainted with what is already known; and in this inquiry it is of much consequence that he should select those works that embrace the whole subject, without being too diffuse on the one hand or too brief and general on the other. The Elementa Physiologiae of Haller contains a mass of information that will ever render it valuable as a book of reference, though it will scarcely at the present day be studied as a system of physiology. His Prime Lineae

Physiologiae, though first written, is chiefly a compendium of the larger work, and is better adapted to the general student, though, from its not containing the later discoveries in the science, it is far from complete. The Institutiones Physiologicae of Blumenbach is a useful work, though it has now given place to the later and more accurate publications of Cuvier and Dumas. The Anatomie Comparee of the former writer contains an excellent digest of comparative physiology, and the preliminary observations prefixed to the anatomical details contained in this work, may be read with considerable advantage. Probably, however, the Principes de Physiologie of Dumas is the most perfect and scientific modern production that has appeared on the subject. We cannot say too much of the Elements of Physiology lately published by Richerand, and translated into English by Mr Kerrison; for although it contains considerable information, and a great display of reading, and even of original observation and experiment, it is neither scientific nor always very accurate. The works of Bichat, especially his Anatomie Generale, Anatomie Descriptive, and his Recherches Physiologiques sur la Vie et la Mort, abound with excellent physiological remarks, and, allowing for the great extent to which he has carried some peculiar doctrines, are among the best that have appeared on the animal economy.

In our own country, we have many valuable treatises and papers on different parts of physiology, and the names of Hunter, Monro, Home, Cooper, Abernethy, Carlile, Sanders, Barclay, Jones, and many others, will ever reflect honour on the country and on the age in which they lived. We can scarcely, however, point out a complete work in our language on the general subject of physiology, though we doubt not that many will be disposed to consider the Zoonomia of Dr Darwin as entitled to that appellation. We allow that this is a stupendous monument of the genius and industry of its author, and it contains an ample store of valuable facts, which, if they could be divested of the hypothesis with which they are so much blended, would be extremely useful to the cause of physiological science. At present many of them tend to mislead, by a few metaphysical acutenesses, and by the new sense in which several terms are employed. Another of Dr Darwin's works, not less valuable, in a physiological point of view, is his Phytologia, in which he treats of the economy of vegetation with ability and success.

He who is desirous of advancing and improving the science of physiology, must, in the first place, have recourse to a patient, and, as far as may be, an accurate observation of the phenomena that take place in organized beings; but the multitude, the variety, and complicated nature of these phenomena, place in his way obstacles that it is difficult to surmount. It is only through time, and patience, and assiduity, that he can attain his object; and it requires considerable dexterity and acuteness to detect the appearances under which these phenomena sometimes present themselves, to pierce through the obscurity in which they are often involved, and to avoid, in a route so uncertain, both the illusions of sense and the errors of genius. The living body has properties peculiar to itself, while it also possesses others that are common to it with brute matter. The phenomena by which it manifests these two orders of properties, are therefore of two kinds, as they relate particularly larly to the state of vitality, and as they are found in every object that exists. These latter are subject to the general laws of matter, are confounded with the phenomena of universal nature, and may be denominated physical phenomena. Among the former, some are confined to the arrangement or disposition of the parts in organs, and depend on the structure or form of these organs. These may be called organic phenomena. Others depend on the particular laws that govern vital beings, and are not the result of any peculiar organization; these are vital phenomena.

Observation alone is sufficient to indicate the presence or the existence of these phenomena; but to unveil them fully there is required an unceasing attention, that is resolved to pursue them through the changes produced by age, sex, climate, situation, and all those circumstances that can affect the living system.

To observation, he must add, wherever this can be done with a chance of accuracy, a patient investigation of nature by experiment. From the experiments of Spallanzani and Stevens on digestion; of Goodwin, Menzies, Spallanzani, and Davy on respiration; of Monro, Galvani, Volta, and a hundred others on animal irritability, with many other experiments made both at home and abroad, more light has been thrown on the economy of living bodies, than by all the hypotheses and theories that the most ingenious speculations have contrived since the first dawn of infant science.

In following out Bacon's great plan of observation and experiment, we must, however, take care in physiological, as in all other physical inquiries, not to be too hasty in our conclusions, and not to suppose that we have reached the bottom of the well of truth, when we have barely got within its verge. Further observations on this subject are unnecessary here, as we have already treated it at some length in the articles PHILOSOPHY and PHYSICS.

We shall conclude these introductory remarks with a brief sketch of the principal arrangements of modern physiologists, and a tabular outline of the subject as we propose to treat it in the following pages.

There are two modes of arrangement that have usually been adopted in treating physiology; one according to the order of the functions, and another according to that of the organs by which these are performed. The latter of these was adopted by Haller; the former is that of Dumas, Cuvier, and most of our later physiologists.

Dumas, after a long introductory discourse, in which he treats of the best method of pursuing the study of anatomy and physiology, divides his subject into six parts. In the first of these he offers some general views respecting anatomy, physiology, and all the branches of physics which are employed in illustrating the nature and properties of organized and living beings. In this part he gives a compendious history of the progressive improvements in anatomy and physiology, points out the relations that take place between these sciences, and the auxiliary branches of mathematics, mechanical philosophy, chemistry, and natural history; he considers the principal differences that distinguish organized from inorganic matter; the nature, effects, and duration of life, and of the general and particular powers or faculties of nature, both in living and brute matter.

In the second part he lays down the fundamental principles on which the physical constitution and par-

cular economy of man depend; treats of man considered individually, of his formation, structure, and varieties; of the modifications produced in the nature of man by age, sex, habit, and temperament; of the relations between man and external objects; of the action and reaction of the organic systems on each other; of the organic structure of man, and of its several varieties in the different parts and organs; of the natural composition of the different fluids and solids of the human body; and gives a methodical division of the functions, with a critical examination of the modes of classification commonly received.

In the third part he treats of the phenomena of the animal economy, in the relation which they bear to the perpetual commerce established between man and the organs that surround him, or of sensation and motion. Here he considers the action of external objects upon man, whence result the phenomena of sensation, and the action of man on external objects, from which arise the phenomena of motion.

In the fourth part he treats of the phenomena of the animal economy, in the relation which they bear to the confluence of the fluids, the cohesion of its solids, and the temperature of the whole system. Here he considers the mutual action between the vessels and the blood, from which result, both in the solids and fluids, that degree of cohesion and pliability that favours the necessary expansibility of the living body, or the function of circulation; the action of the air, and of caloric, on the solids and fluids, from which results the degree of expansion necessary to life, or the function of respiration.

In the fifth part he treats of the phenomena of the animal economy in the relation which they bear to the healthy and entire state of the material substance and composition of the body. Here he considers the action of alimentary substances on the human body, in repairing its loss, and preserving its substance, from which result the phenomena of digestion, absorption, and nutrition; and of the action of certain organs on the fluids of the body, in separating those which do not serve the purposes of nutrition, from which result the phenomena of secretion and excretion.

In the sixth and last part, he treats of the phenomena of the animal economy in the relation which they bear to the commerce established between the individual and the species. Here he considers the mutual physical action of the two sexes, from which arise the phenomena of generation; and the mutual moral action of several individuals, from which result the phenomena of speech, and mutual intelligence.

From this sketch of the arrangement of Dumas it will be seen, that although he takes a very extensive view of the subject, his observations are chiefly confined to the human body.

Though the lectures of Cuvier do not contain a complete system of physiology, the anatomical matter of them is, however, so much blended with observations on the animal economy, that it will be of importance for the physiological student to be acquainted with his arrangement.

The whole work is divided into 30 lectures: the first of which is occupied with preliminary observations on the animal economy, comprehending a general view of the functions of animal bodies; a general idea of the organs of which the animal body is composed; a view of of the principal differences exhibited by these organs, and of the relations which exist among those variations, together with a division of animals founded on their organization. The second lecture treats of the organs of motion in general; the third, fourth, fifth, and sixth lectures are merely anatomical, exhibiting a comparative view of these organs in the several classes of animals. The seventh lecture is strictly physiological, and treats of the organs of motion considered in the several actions of standing, walking, seizing and climbing, leaping, swimming, and flying.

The eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, and fifteenth lectures, are occupied in considering the anatomy and physiology of the function of sensation. Of these, part of the ninth treats of the nervous system in general, and of its action; part of the twelfth gives the physiology of vision; part of the thirteenth, that of hearing; part of the fourteenth, that of touch; and part of the fifteenth, that of smell and taste.

The sixteenth, seventeenth, eighteenth, nineteenth, twentieth, and twenty-first lectures treat of the organs and phenomena of digestion, mastication, inflation, and deglutition. The twenty-second lecture treats of what have been called the affluous chylopoietic viscera, namely, the liver, the pancreas, the spleen, and their offices. The twenty-fourth treats of the organs and phenomena of circulation in general; the twenty-fifth of those of respiration in general; the twenty-eighth of the organs of voice.

The twenty-ninth treats of the organs and phenomena of generation, and the thirtieth, of those of excretion, comprehending a general view, both of secretion and excretion.

Subjoined to Bichat's introduction to his *Anatomie Générale*, there is a tabular view of physiology, in which, after some preliminary outline of the general structure of the organs and of the phenomena of vitality, he divides the functions into classes, orders, and genera.

The first class consists of the functions that relate to the individual; the first order of which, comprising the functions of animal life, comprehends five genera, viz., sensations, cerebral functions, locomotion, voice, and nervous transmission, besides sleep. The second order of this class contains the functions of organic life, and comprehends eight genera, viz., digestion, respiration, circulation, exhalation, absorption, secretion, nutrition, and calorification.

The second class contains the functions that relate to the species in general, and is divided into three orders. The first of these, comprising the functions peculiar to the male, comprehends only one genus, viz., the production of the seminal fluid. The second comprises the functions peculiar to the female, and contains three genera, viz., menstruation, the production of milk, and of the female generative fluids. The third order comprises the functions that relate to the union of the two sexes, and the product of that union; and it comprehends also three genera, viz., generation, gestation, and delivery.

Respecting the peculiar doctrines of this writer we shall speak hereafter.

There is still another mode of arranging the phenomena of living bodies, that deserves to be noticed, namely, that in which they are arranged according to the artificial systems of natural history. This mode of arrangement, though of infinite advantage to the zoologist, by shewing him at once the extent of his subject, and giving to his memory a power of recollection which it could not otherwise possess, is yet not such as the physiologist would wish to be observed. Zoological arrangements are useful chiefly as they facilitate the study of the manners, dispositions, and habits of different animals; and all that part of the outward economy which indicates something of the wisdom and design displayed by the Creator, in their structure and adaptation to the modes of life which they are intended to pursue; but they do not sufficiently illustrate the internal structure on which this outward economy depends, nor do they sufficiently explain the more secret functions, which being independent of the will of the creature, only display the power and omniscience of him who made it. This will be readily conceived from considering the difference between zoology and physiology, as we have defined it. Zoology is chiefly led to examine the animal kingdom as it usually presents itself to the eye, including a great variety of objects; physiology examines only that part of the animal economy which is principally made known by anatomy and chemistry. Zoology has been accustomed to divide its kingdom into so many classes or orders of animals; physiology would naturally divide its economy into so many functions. Zoology has usually subdivided its classes by certain obvious external marks, as the teeth and claws; physiology would naturally subdivide its functions by the varieties of those organs which are destined to perform them, as the several kinds of lungs and stomachs. Zoology mentions the functions only in a cursory manner, as forming a part of the history of animals; physiology takes notice of animals, only when they are of use to illustrate the functions. To these differences we may add another; that physiology, in the extended sense which we have given it, goes beyond zoology in comprehending the economy of the vegetable creation. From this comparison it will be admitted, that things which are primary in a zoological method, will often be secondary in a physiological arrangement, and vice versa. This is very conspicuously the case in one of the grand divisions of Linnæus, viz., mammalia, where the important secretory organs of the milky fluid are noticed only like the colour of the hair, or the length of the tail, as a good outward mark of distinction, and likewise in the excellent table by D'Aubenton in his introductory view of natural history, in the *Encyclopédie Méthodique*, in which the function of digestion is not even mentioned.

It is, however, extremely useful, both to the naturalist Utility and physiologist, that the arrangements of both sciences of the application should be, as far as possible, adapted to each other, by cation of marking the relative importance of the several functions of zoology in the various classes of living beings. This has been a great advantage performed by Vieq d'Azyr, a modification of arrangement whose table has been given in the comparative part of our *Anatomy*. See Vol. II. p. 280.

With respect to all physiological arrangements we may observe, that as the phenomena of living beings are so intimately dependent on each other, as to form the links of one continued chain, it is of little consequence which of the functions or phenomena we make the point from which we set out in our examination. But as the organs of sensation in most animals, and those of diges- Introduction in almost all living creatures, arc among the first that are evolved, it appears most convenient to begin with one of these. The following is the arrangement that we shall adopt in the present article.

After giving a sketch of the progress of physiological discoveries and opinions, we shall divide the remaining part of the article into 16 chapters. In the first of these we shall treat of the characteristic marks, general phenomena, duration, and principle of life. In the second we shall consider the phenomena of sensation, the action of the nervous system, and the external senses of feeling, tasting, smelling, hearing, and sight. In the third and fourth chapters we shall treat of irritability, and the phenomena of motion. In the fifth we shall treat of digestion; in the sixth of absorption; in the seventh of circulation; in the eighth of respiration and voice; and in the ninth of nutrition, as completed by the successive performance of the four preceding functions. In the tenth chapter we shall treat of the phenomena of secretion; and in the eleventh, of those of excretion. In the twelfth we shall consider the various means by which living beings defend themselves from external injury, or the phenomena that attend the evolution and change of the integuments, to which function we shall give the name of integumentation. In the thirteenth chapter we shall consider the transformations that take place in some tribes of living beings, especially insects and reptiles. In the fourteenth we shall briefly examine the phenomena of reproduction, and the hypotheses to which they have given birth. In the fifteenth we shall consider the nature of sleep, and the phenomena of dreams; and in the sixteenth we shall terminate our inquiries by a few observations on the nature and phenomena of death.

The following table is intended to exhibit an outline of the principal circumstances attending the phenomena of life, in the order in which we have enumerated them.

| 1. Life—is either | |-------------------| | Universally diffused through the body. Polypi, &c. | | Most concentrated in certain organs. | | Continued for only a few hours. Ephemeræ, and some other insects. | | ———— about a year. Annual plants. | | ———— two years. Biennial plants. | | ———— several years. Perennial plants, and most animals. | | ———— about a century. Elephants, pikes, &c. | | ———— several centuries. Oaks, chestnuts, &c. |

| 2. Sensibility—Appears to | |---------------------------| | Exist in a very low degree in plants. Sensitivity. | | Exists in a greater or less degree in all animals. | | Confined to the senses of feeling and taste. Most zoophytes, | | Extended besides these to the senses of smelling, sight, and hearing. | | Appears farther extended by an additional sense. Bats. |

| 3. Irritability—Affected by | |----------------------------| | Stimulants invisible. | | ———— unknown. | | ———— unthought of. | | The nervous influence. | | Light. | | Heat. | | Moisture. | | Electricity. | | Salts. | | Gales. | | Bodies that act mechanically. |

| 4. Motion—Performed by | |------------------------| | Legs. | | Wings. | | Fins. | | The tail. | | Organs which fall not properly under these descriptions, bats, flying opossums, &c. | | The springines of the body or of some part of it, maggots, fleas, &c. | | Contrivances which fit living bodies for being moved by foreign agents. |

5. Digestion—

Without teeth. With teeth in the mouth. — in the stomach. — stones or artificial teeth in the stomach. — glands in the mouth for secreting a liquor to be mixed with the food. — pouches in the mouth where the food is kept and moistened. — a sac or bag where the food is kept and moistened. — a membranous stomach. — a muscular stomach. — an intermediate stomach.

5. DIGESTION— Performed by an alimentary canal.

Without a cæcum or blind gut. With a cæcum. — two cæca. — three cæca. — four cæca. — one entrance or mouth. — many entrances by absorbers. Plants have many alimentary canals. Some polypes have alimentary canals that branch through the body. The alimentary canals of plants and worms distribute the fluids without the aid of a circulating system.

6. ABSORPTION— Performed by

Vessels beginning from the alimentary canal. — the cavities. — the surface. Veins in the penis and placenta. Re-absorbents originating from all the parts of the system.

7. CIRCULATION— Performed by a system with

One heart. A heart for distributing the blood through the respiratory organs, and an artery for distributing it through the system. One heart for the respiratory organs, and one for the system, both in one capsule. Two hearts for the respiratory organs, and one for the system. A pulmonary heart, or a heart for the respiratory organs in the course of circulation. A pulmonary heart within or without the course of circulation at pleasure. A heart situated in the breast. — near to the head. — in the opposite extremity. Some animals and all plants have no circulating system.

Diffused through the system. Confined to one place. Situated externally. — internally. In the course of the circulation. Not in the course of the circulation. Within or without the course of circulation at pleasure. Without tracheæ. With tracheæ ramified through the system where the respiratory organs are generally diffused.

8. RESPIRATION— Performed by organs.

not ramified through the system where the respiratory organs are confined. formed by rings. — by segments of rings on one side, and a membrane on the other. — by continuous rings running spirally like a screw. admitting air by one entrance. — by several entrances. wholly concealed in the body. partly projecting from the body. opening at the head. — at the opposite extremity. — upon one side. — both sides.

NUTRITION— 9. Nutrition—Food prepared by

The alimentary canal. The lacteals. The respiratory organs. The circulating system. The cellular membrane. The glands. And by the several parts in which it becomes finally assimilated.

10. Secretion—Performed by

Vessels. Exhaling vessels. Excretory organs. Organic pores. Glands. And by all the parts of which the system is composed.

11. Excretion—Excrementitious matters thrown out by

The integuments chiefly. The common opening of the alimentary canal. Two openings of this tube. By the lungs and other emunctories.

12. Integumentation—Some living bodies have integuments which are

Scaly. Shelly. Membranous. Corneous. Cretaceous. Ligneous. Covered with down, hair. prickles, feathers. a viscid matter. Change their colour. their covering. Changed themselves.

13. Transformation—Takes place

By a change of proportion among the parts. of their form. throwing off old parts. an addition of new ones of a different use, structure, and form. a change of the whole form together. of qualities, propensities, manners.

The temporary union of two sexes. The spontaneous separation of parts. Organs situated in the breast. in the side. near to the head. in the opposite extremity. An intrant organ of the male, and a recipient organ of the female. An intrant organ of the female, and a recipient organ of the male. The stamina and pistils of flowers. The seminal secretion of the male thrown into the organs of the female. sprinkled at the entrance of the female organs. thrown upon them from a distance. transported to them by the winds. sprinkled on the embryo after emission. diffused in a fluid secreted by the female before it can rightly perform its office. diffused perhaps sometimes in air, as in the case of dioecious plants, where it probably acts like an aroma.

15. Sleep—Natural sleep is occasioned by

Quietness. The absence of stimuli. The fatigues of stimuli when long continued. Deficient assimilation. Deficient irritability, which is owing sometimes to the weaknesses, inattention, or confined powers of the mental principle.

16. Death. See Life. PHYSIOLOGY.

HISTORY.

The early history of physiology can be little more than an account of the opinions of the ancient philosophers respecting the nature and functions of the human body; but as their opinions reflect considerable light on the progressive improvement of the philosophy of man, the history of physiology, even in its early stages, is curious and interesting.

Of the origin of physiology as a science we know nothing. On examining the writings of the earliest philosophers, we meet with little more than a collection of abstract principles and hypothetical reasonings, especially previous to Pythagoras. He considered man as a microcosm, or an epitome of the universe, in which were produced the same phenomena as in the larger world, only to a less extent. He admitted more than one intelligent principle, conducting all the operations of the human body. He supposed that the human soul, nourished by the blood, fixed by the veins, the arteries, and the nerves, as so many visible situations, became obedient to the general laws of universal harmony. He did not pretend that the eternal power of numbers had prescribed all the phenomena of nature, and that the force of numerical harmonies regulated the motions of the bodies which filled the universe, though he has been made so to express himself by his disciples. He was contented with asserting, that everything in nature was brought about according to the qualities and proportions of numbers, without attributing to them an intrinsic virtue and a positive existence. He perceived that the phenomena of the animal economy succeeded each other with a strict regularity, by which they concurred in maintaining order; and in this order he found the principle of the existence and preservation of all beings; a principle without which they could not exist. He considered the souls of men as emanations from the general soul of the universe, or anima mundi.

Alcmeon considered the brain as the seat of the soul. He supposed it to be produced by the reverberation of the air within the cavity of the ear; and he thought that taste was owing to the moisture of the tongue. He compared the body of a fetus to a sponge, which obtained its nourishment by the suction established over every part of its substance. According to him, the motion of the blood was the essential principle of life; and he supposed that the stagnation of this fluid in the veins produced sleep, and its active expansion brought back the waking state of the body. Health consisted in the equilibrium and well proportioned mixture of certain primary qualities; and that whenever any of these became too predominant, disease was the consequence.

Empedocles involved himself in a multitude of absurd hypotheses, in order to explain the formation of man, and the combination of the elements from which he was produced. He too, like the disciples of Pythagoras, sought among the properties of numbers, for the general principles both of physical and moral science. In uniformity with this system it was, that he reckoned the four elements, and admitted among the particles of these material principles a kind of affection and aversion, of desire and antipathy, capable of separating and reuniting them, as occasion might require. He believed that respiration commenced within the uterus, where the infant was provided from each parent with certain organic particles, which tended to unite into one uniform whole. Anaxagoras, convinced that we must attribute the arrangement of matter to the intelligence of a superintending being, imagined that the body of every animal was formed of homogeneous particles, which were brought together by a sort of affinity. It appeared to him, that bodies which were endowed with thought, were composed of sensible elements; that these elements remained unalterable, and that no power in nature could exert any action on them.

Democritus dedicated his life to repeated experiments on plants and animals. He explained the principal phenomena of organized bodies by the action and reaction of atoms, which he supposed to be endowed with powers essentially active, and susceptible of repelling and attracting each other. According to him, generation consisted in the cohesion of homogeneous atoms. He conceived the heat inherent in the elements of the body to be the sole active principle with which man was animated; and that by increase of this he became capable of life and motion. He compared the organs of the senses to mirrors, on which were painted the images of things, and he reduced all sensations to the sense of feeling, which he supposed to be more or less delicate according to circumstances.

These philosophers, who lived before the time of Hippocrates, had, as we see, but very rude and indistinct notions of the animal economy; nor were those of the great father of physic much superior. Excellent as was his practice, and acute his knowledge of the symptoms and progress of diseases, the physiology of Hippocrates was very lame and defective. He seems, indeed, to have understood the function of nutrition better than most others; he traces the aliment into the stomach, seems aware of the processes it has to undergo there, and hazards a conjecture that part of the chyle is taken up immediately by the pores of the cellular texture; and that the juices admitted into this membrane, served for the production of milk, the matter of which is afterwards transported, and laid up within the breasts. He attributes to each vital part an attractive force, which it exerts on the nutritious particles, in order to incorporate and appropriate to itself those which bear to it a certain analogy. He thought that the heat generated in a living body was kept up entirely by the powers of vitality; and that the external air introduced by respiration, served rather to check it, by exerting a cooling effect on the pulmonary organs. He represents the human body as agitated in all its parts by an alternate flux and reflux, which carried the matters from within outwards, or brought them from without inwards. From this some have supposed that he understood the circulation of the blood, a supposition made two hundred years ago, and lately again brought forward by the French physiologists. We shall not at present stay to canvass this opinion, which, however, we conceive to be founded on very unsatisfactory arguments.

After Hippocrates, the science of man was again left Plato to the schools of philosophy, from which he had first separated it. Plato is the first philosopher whose opinions merit particular notice. He wrote on the physiology of man with his accustomed elegance and splendour of diction; and he assumed the tone of an inspired prophet in describing, with the force of enthusiasm, the grand images that suggested themselves to his mind. Accord- According to him, the human body does not contain within itself the cause of the phenomena which are the consequence or the attendants of life. It is only a passive subject, on which the soul expresses the series of its functions, like the canvas on which the painter traces the conceptions of his inventions. He distinguishes two principles of action in man, a rational soul, on which depend reflection and intelligence, and an irrational soul, on which depend life and motion. The latter is diffused through every part of the body; and it is by means of these parts that it feels, suffers pain, or enjoys pleasure. Thus it is by means of the heart it is susceptible of courage and of passion; by the liver of desire. The head is the seat of reason; the chest, and especially the heart, the seat of strength and anger; the lungs, the general coolers of the body. One division of the irrational soul, which possesses an appetite for food, and all the necessary refreshments of the body, resides in the epigastric region; which, in the language of Plato, is a sort of stable, in which resides a voracious animal. During nutrition, the vital parts assimilate to their substance the aliment which are presented to them; and this assimilation is the consequence of an affinity that takes place between these parts and the nutritious juices. He thus seems to regard nutrition as the effect of a combat between the aliments and the parts of the animal. A young animal will receive more nourishment than one which is old, because the force of its body has more effect in overcoming the force of the nutritious substances.

As the reciprocal action of the soul and the body on each other did not appear to him capable of being explained on the supposition of immateriality, he proposed the idea of a plastic nature, which he supposed to be an intermediate principle connecting the soul and the body.

The human body, which is entirely spongy, is exposed through every part to opposite currents of air and fire, which traverse and penetrate it, being introduced alternately by the lungs and by the skin. Hot, cold, dense, rare, and the other sensible properties of bodies, are only the causes of the phenomena which we perceive, and are, as it were, the occasions or accidents that are required to keep in play the intelligent force disseminated through nature.

Aristotle, the disciple of Plato, for a long time disputed with him the palm of genius and celebrity; but, as his physiological doctrines differed very little from those of his master, it is unnecessary to detail them, except to remark, that he attributed to the four three faculties, a nutritive, a sensitive, and a rational faculty; in the first of which life is the only principle; in the second, feeling is produced; and the third is peculiar to man, and is that part of him which knows or judges. This part is either an active or a passive intellect, of which the first may be separated from the body, and is immortal; whereas the second perishes together with the body. Life, according to this philosopher, is a permanence of the soul, retained by the natural heat, the principle of which resides in the heart.

About the period which we are now considering, philosophy was divided into two sects; the materialists, who attributed the formation of all beings to the fortuitous concourse of atoms; and the spiritualists, who held that the soul enjoyed an existence anterior to that of the body, which was no other than a passive organ, in which the phenomena that previously existed in the soul, in an abstract, latent manner, became evident and sensible. To the former sect belonged Democritus and Epicurus; to the latter, Zeno and Plato.

The professors of the Alexandrian school, though they did much for the improvement of anatomy, added little to physiology. Of these Herophilus brought to some degree of perfection the doctrine of the pulse, and seems to have understood the action of the pulmonary organs more correctly than his predecessors, attributing to them a sort of natural appetite, by which they attracted and rejected the matter of respiration. He considered the nerves, the muscles, and the arteries, as the moving powers of the body.

In Galen, also a disciple of this school, we find the most scientific physiologist that has yet come under our notice. He seems first to have ascertained by experiment, that the arteries contain blood, and not air, as had been the opinion of Herophilus and his predecessors; and that they possessed a moving force, independent of that which the heart exercises on the mass of blood, and he found that the contraction of the heart always alternated with a proportional dilatation. He even tried some delicate experiments, in order to ascertain the influence of the nervous system upon the sensitive and motive powers of the body, by which he found, that when a nerve was intercepted with a ligature, the part to which it led became deprived of sense and motion. He believed that the stomach, in a state of contraction, applied itself to the aliment that had been taken in; that the mesenteric veins absorbed a portion of the chyle prepared in the intestines; that the ductus choledochus carried the bile from the gall bladder into the duodenum; that the kidneys separated a part of the urine; and he supposed, that another part of this fluid passed immediately from the stomach to the bladder, through some unknown passage. He believed that the lungs transmitted to the blood contained within them, an aerial principle, destined to free them from fuliginous vapours, and to temper the excesses of heat generated within the body. The obscure function of generation did not entirely escape his researches; and he made some curious attempts to find out how the sexual organs prepared the seminal fluid, and how this acted in reproduction.

For more respecting the doctrines of Galen, see the History of Medicine.

The commencement of the 13th century is the epoch of a material revolution in physiology. Chemistry having penetrated into Europe, soon exerted its influence on most of the sciences, and especially on those connected with medicine, the doctrines of which were totally changed from their ancient simplicity, and became a farce of the most wild and fanciful opinions. The Peripatetics and the Galenists sunk into oblivion; and the primitive qualities and occult faculties of the ancient school gave way to the fermentations and effervescences of the chemists. Albertus Magnus and Roger Bacon, when they introduced the science of chemistry, scarcely dreamed of applying it to medicine; but Arnoldus de Villa Nova undertook this application, and sought for the foundation of medical theory amid the processes of his laboratory. Paracelsus followed, and surpassed him in this chemical delirium. An enlightened chemist and a credulous

credulous astrologer, his head burning with the fire of his furnaces, and his imagination filled with magical reveries, he believed himself capable of constructing a new system of philosophy, from examining the course of the stars, and the products of his alchemies. With the daring assurance of inspiration, he declared man to be composed of sulphur, mercury, and salt; and, having traced the origin of all diseases to certain chemical operations, he flattered himself, that by means of his arcana he could preserve health, and prolong, to an indefinite extent, the natural duration of human existence.

Van Helmont, the disciple of Paracelsus, not less fanciful, but more scientific than his master, saw the necessity of something more than chemical principles to explain the functions of the animal machine. He therefore introduced his archon, an intelligent being who established his throne in the epigastric region, having several subaltern ministers under him, who presided over the several functions of the body, and whose chief seats were, the head, the chest, and the belly.

In the philosophy of Des Cartes, the separate existence of the vital principle is entirely rejected. He availed himself of the progress that had been made by Willis, and some other anatomists, in the investigation of the nervous system, to form an hypothesis of the vital functions, founded on the supposition of the nervous fluid, or what was then called the animal spirits; and this nervous fluid was assumed independently of the sensitive soul, to explain the appearances of sensation and voluntary motion.

The discovery of the valves in the veins by Fabricius; of the lymphatics by Rudbeck and Bartholin; of the lacteals by Aellius, and of the circulation of the blood by Harvey, all of which took place during the 17th century, gave to physiology an interest and a clearness which it never possessed before that period. Some account of the discoveries in the circulating and absorbing systems, hath been already given under Anatomy; but as these discoveries have been productive of great advantages, both in general physiology, and in medicine, it will be worth while briefly to trace their origin and progress.

To begin with the circulation of the blood. Hippocrates speaks of the usual and constant motion of the blood, of the veins and arteries as the fountains of human nature, as the rivers that water the whole body, and which if they be dried up man dies. He says, that the blood-vessels are, for this reason, everywhere dispersed through the whole body; that they give spirits, moisture, and motion; that they all spring from one; and that this one has no beginning and no end, for where there is a circle there is no beginning. In such language was the prince of physicians accustomed to express his vague ideas of a circulation; for so far was he from having acquired accurate conceptions on this subject, that when he saw the motions of the heart, he believed that the auricles were two bellows to draw in air, and to ventilate the blood.

When after his time anatomy came to be more studied, the notions of the ancients reflecting the blood were better defined; and, however chimerical they may seem to us, they were partly derived from dissection and experiment. On opening dead bodies, they found that the arteries were almost empty, and that very nearly the whole of the blood was collected in the veins, and in the right auricle and ventricle of the heart. They therefore concluded that the right ventricle was a sort of laboratory; that it attracted the blood from the cavae; by some operation rendered it fit for the purpose of nutrition, and then returned it by the way that it came. From the almost empty state of the arteries, they were led to suppose that the right ventricle prepared air, and that this air was conveyed by the arteries to temper the heat of the several parts to which the branches of the veins were distributed.

To this last notion, entertained by Erasistratus, Galen added an important discovery. By certain experiments, he proved, that the arteries contained blood as well as the veins. But this discovery was the occasion of some embarrassment. How was the blood to get from the right to the left ventricle? To solve the difficulty in which his new discovery had involved him, he supposed that the branches of the veins and arteries anastomosed; that when the blood was carried to the lungs by the pulmonary vein, it was partly prevented by the valves from returning; that therefore during the contraction of the thorax it passed through the small anastomosing branches to the pulmonary vein, and was thence conveyed along with the air to the left ventricle to flow in the aorta. This opinion, so agreeable to fact, unfortunately afterwards gave place to another that was the result of mere speculation. This notion was, that the left ventricle received air by the pulmonary vein; and that all its blood was derived through pores in the septum of the heart.

The passage through the septum being once suggested, and happening to be more easily conceived than one through the lungs, it was generally supposed the only one for a number of centuries; and supported likewise, as it was thought, by Galen's authority, it was deemed blasphemy in the schools of medicine to talk of another. In 1543, however, Vesalius having published his immortal work upon the structure of the human body, and given his reasons in the fifth book why he ventured to differ from Galen, he particularly showed how it was impossible that the blood could pass through the septum of the heart. His reasoning roused the attention of anatomists; and every one grew eager to discover the real passage which the blood must take in going from the right to the left ventricle. The discovery of this was first made by Michael Servede, a Spanish physician, who published his opinion in 1553. He expressly says, that the blood does not pass through the septum of the heart, as is commonly believed, but is conveyed by an admirable contrivance from the right ventricle of the heart, by a long passage through the lungs. This opinion was deemed heretical, and Servede's book was suppressed by public authority. Soon after, however, the same discovery was made by Realdo Columbus, an Italian professor, who published his account in 1559. It farther appears, that Andreas Cawalpinus, who published in 1571, and again in 1593, was acquainted, not only with the lesser circulation, but observed, that the blood sometimes flowed from the branches of the veins towards their trunks; and that when a vein was tied with a ligature, it swelled between the ligature and the distal extremity of the vein, and not between the ligature and the heart. He thence inferred, that the veins and arteries opened into each other, and ventured to affirm We learn from Galen, that certain vessels had been seen in kids by Erasistratus, which appear to have been lacteals, though he called them arteries. These lacteals were, however, first accurately distinguished in 1622, by Afellius, who printed his account in 1627. In 1651, Poquet published his account of the thoracic duct, which appears, however, to have been seen before by Eustachius. In 1653, Bartholin published on the lymphatics, which had been some time before discovered by Rudbeck. In 1654, Glisson ascribed to these vessels the office of carrying back the lubricating lymph from the arteries into the blood, or considered them as absorbents. In 1664, the valves of these vessels were discovered by Swammerdam, and a year after, an account of them was given by Ruych. The farther discoveries of Nuck, Nouges, Warton, Steno, Hunter, Monro, Hewson, Cruikshank, Sheldon, Macagni, &c., have nearly completed our knowledge of the absorbent system, and its uses.

In the latter end of the 17th century, some important discoveries were made on the subject of respiration, by our countryman Mayow; and these were supported by the observations of Lower, Verheyen, and Borelli. These discoveries, however, lay dormant till they were brought into recollection a hundred years after in consequence of the experiments of Priestley and Lavoisier.

During the 17th century, considerable progress was made in completing the knowledge of the internal organs of generation. Much was done in this way by De Graaff and Malpighi, and Leuwenhoek, the two latter of whom made several discoveries with the assistance of their microscopes, though Leuwenhoek founded on his observations a theory of generation which at this day appears not a little ridiculous.

The beginning of the 18th century is remarkable for the promulgation of a new physiological doctrine, founded on a mistaken application of the circulation of the blood. We allude to the system of Boerhaave. This great physician supposed that all the functions of the living body, excepting the will, are carried on by mechanical movements, susceptible of rigid calculation, which necessarily succeed each other in the organs, from the time that life commences. These movements are brought into action as soon as the animal begins to respire, and are the consequence of an impulsive power in the heart, renewed by means of the influence of the nervous fluid brought from the brain. He conceived the living body to be merely a hydraulic machine, in which the heart performs the office of a piston, and that the alternate contractions and dilatations that take place without intermission in that organ, are owing to the alternately increased and diminished compression of the nerves that are distributed to the heart. When a contraction takes place, the blood fills the large arteries, and thus distends and compresses them; when the principal nerves of the heart, which pass between these arteries, must of course become compressed, and thus their influence being diminished, a relaxation takes place. But in proportion as the heart is relaxed, the large arteries become empty, and consequently cease to compress the nerves, which thus recovering their influence, reanimate the heart to a new contraction. Thus succeed each other without interruption the movements which form the mechanical principle of all the sensible motions that we observe in the animal machine.

Proceeding Proceeding on these principles, Boerhaave conceived some very strange notions respecting the constituent properties of the living fluids, in which he saw no other mark of vitality than the globular form of their particles. He confined all the functions of the several organs to the operation of rounding into spheres the particles of the fluids which were presented to them, or of preserving that form in those which they already contained. He thought that the lungs were chiefly of advantage, because they contained within them a complete series of vessels, in which the particles of the blood can receive all those dimensions which may fit them to circulate through the rest of the body. The greater or less velocity with which the fluids circulate through the secretory organs, constitutes the principal difference in the nature of the secretions. Various orders of vessels receive the blood and other fluids which pass through these divisions, subject to the laws of hydraulics; and when a fluid got by chance into an order of vessels that was not fitted to receive it, some disease was the consequence. Every thing in the animal machine was reduced to an assemblage of conduits, canals, cords, levers, pulleys, and other mechanical contrivances, put into action by mechanical means.

Thus was completed the system of mechanical physiology, which was begun some time before by Bellini and Borelli; and this system maintained its ground in defiance of observation and common sense, till about the middle of the 18th century. In the mean time, however, there arose two men, whose enlarged ideas and acute genius induced them to dissent from the received opinions of the day, and to think for themselves. These were Hoffman and Stahl, who, though they did not, any more than Boerhaave, form complete or unobjectionable theories, contributed much to improve our ideas of the animal economy.

Hoffman saw, that in the living body we ought not to separate the principle of vitality from the general properties of matter. He believed that that principle, susceptible in itself of activity and motion, was sufficient for all the occasions and all the functions of the body which it animated. The animal body was not, in his eyes, an hydraulic elastic machine, formed of solids and canals, differing only in size, form, elasticity and force. He saw, that if the solids act upon the fluids, these must, in their turn, react upon the solids; and that life could subsist only by these mutual actions and reactions. The essential cause of life, according to Hoffman, is the progressive motion of the blood, occasioned by the impulse of the heart, and kept up by the alternate contractions and dilatations of the vessels. These contractions and dilatations are the consequence of the force of an elasticity inherent in the vascular fibres, and this force is still farther promoted by the different structure of these elastic fibres, which is such that they can be penetrated by the blood and the nervous fluid. This last fluid he imagined to be composed of aerial and ethereal particles enveloped in a certain portion of a very pure subtile lymph, that served them as a vehicle. By this fluid the cavities of the nerves are filled, and it constitutes the sensitive soul, in which resides the seat of the passions. Now, all the functions, even those which we attribute to a sentient principle, are the effect of physical powers, whose mechanism has, however, something more sublime and more exalted with respect to the animal operations than to others. If all the nervous, vascular, and membranous parts, preserve a moderate degree of action, and a moderate state of tension and relaxation, the solids are subjected to oscillatory motions which balance each other, and produce a proper equilibrium in the system. In this state, all the operations of the body and the mind take place with proper regularity; and this happy harmony, by affording to the animal the entire plenitude of its existence, becomes the foundation of health. This degree of moderate tension is always more or less altered in a state of disease.

Little satisfied with all the theories founded on a gross mechanism, and convinced of their insufficiency to explain the phenomena of vitality, Stahl admitted forces that were something more than mechanical, and that were directed by an intelligent principle which applies them to their destined uses, and which, by distributing them with a wise economy, proportions or accommodates them to the different occasions of the individual. His disciples consider Stahl as the first modern writer who has treated the science of man on a general plan, and according to a philosophical arrangement; and as his doctrine has still numerous advocates in the medical schools of France, we shall be somewhat more particular on it than on that of Hoffman.

In determining the limits between medicine and the other physical sciences, Stahl commences with separating from the former all those principles which, though true in themselves, have no relation to the nature of that science, which he considers as originating in observation alone. The knowledge of the physical state of the animal body cannot, he thinks, throw any light, either on the injuries to which it is exposed, or on the means of preventing or removing them. Consequently it is of little use in medicine, and has no right to govern an art, the object of which is, to remedy those injuries that threaten the human body. He proves that living bodies are freed from the necessary laws of mechanics, because all their actions tend to one common end—an end which embraces the whole chain of the movements essential to life, and the means established for its preservation. The human body, by means of this mixture of mechanical and vital powers, tends naturally to self-destruction; but, on the other hand, the organic structure to which is attached the exercise of the actions peculiar to the human species, is founded on this mixture. It is therefore necessary that the body should be in a state of resisting this tendency, in order that it may be sustained; and as the corruptibility inherent in its nature, pursues it through every period of its existence, the opposing action necessary to prevent the corruption from taking place, must also be exercised without intermission. It is this preserving action that constitutes the essence of life.

The preservation of the body is indeed effected by a sort of mechanical action; it requires the corporeal organs as its instruments, and it depends on different coexistent and successive actions. Health is the result of that just conformation of the organs which enables them to perform their functions with ease.

The exact conformity which subsists between the structure of each organ, and the functions it is destined to perform, demonstrates to the philosophical eye an intelligent and wise principle, that in the formation of organized bodies directs and prescribes every thing in the manner most favourable to the end which it proposes. A speculative metaphysician, accustomed to wander over the field of abstraction, to enlarge the sphere of his intellectual notions, to transform sensible objects into ideas, this author could never persuade himself that a being could not proportion and adapt its organs to the operations they are to perform, without possessing a knowledge of these operations, and having already exercised a judgment with respect to them. It is from this that he confounds the principle of life with the thinking soul, which being incessantly present in every part of the body, directs and disposes them according to its own views, and to the end that it proposes in the continual development of the actions it is to conduct.

The formation, the structure, duration and movements of the body, do not belong peculiarly to it, as it is only a passive subject on which the soul impresses and realizes the idea of the phenomena that she has conceived. Every thing is derived from the union of the body with the active foreseeing principle, which governs, according to special laws, those phenomena which are more particularly vital, and which are most independent of the will. The immediate action of this latter faculty does not require the assistance of any other substance. The intervention of an intermediate principle would be there superfluous; and Stahl rejects that of the animal spirits, which had been introduced to explain the mechanism of vitality, and which, by overcharging the science, embarrasses it with a useless hypothesis.

Two faculties are sufficient for the soul to act upon the body, and to preserve it in a living state, viz. those of sense and motion. By the former the animal learns to know the properties of the objects by which he is surrounded, or in which he is interested, and to estimate the relations that subsist between these objects and himself; the latter produces the motion of the whole machine, and determines all the changes of situation which it has to undergo in its whole, and in its parts.

The faculty of sensation has two modifications, relative to the two kinds of knowledge which the soul may receive by means of that function. The first of these resides in the organs of sense, and is adapted to external objects; the second establishes its seat in the interior organs, and refers to objects that are within, or ideas. Sometimes the moving power enveloped in the muscular system is displayed by the sensible actions that regulate the position of the body with respect to the objects of the universe, of which it makes a part; sometimes concentrated within these organs, it excites intestinal motions, which maintain among their constituent parts, those relations, and that equilibrium, which are necessary to preserve the healthy state, confidence, and tone of each organ. The muscular apparatus is subservient to the exercise of the senses; and the different motions which it impresses on the body, for the purpose of transporting it towards, or to a distance from, certain objects, are always determined by the convenience or inconvenience which the body, by means of the senses, experiences from those objects. The tonic motion, determined by the confused ideas of the principle of life, is displayed in the most hidden organic parts, in the most perfect repose and profound silence of the voluntary movements.

The soul gives to its organs the disposition that is favourable to the sensations it wishes to receive, by virtue of the judgment that it exerts respecting these sensations, before it has experienced them. This judgment is exerted on the relations between the objects that excite these impressions, and the actual state of the body; and it is the intuitive knowledge of these relations that determines, in all their infinitely diversified shades, the pleasure or the pain which the animal experiences from the objects that surround it.

Stahl regards the excretions as the means employed by nature to counteract the natural tendency of the body towards putrefaction. He believes that the animal humours are exceedingly disposed to thicken, and that the circulation of the blood is the means made use of by nature to keep up their original fluidity. One of the causes that most favour the tendency of the humours to putrefaction, is plethora, to which nature opposes, sometimes the motion of the solids that dilute the blood; sometimes the hemorrhagic fluxes which unload the vascular system. These latter opinions are the principal foundations of what has been called the humoral pathology, which prevailed so long in most of our medical schools, and which, with certain modifications, is still maintained in many parts of the continent.

The favourable impulse given to physical science in general, by the philosophical writings of Bacon and Newton, extended itself at length to physiology; and physiological writers became convinced that it was better to collect and arrange the facts that related to the economy of living beings, than to frame hypothetical systems concerning them. The honour of forming a rational digest of the phenomena of the animal economy was reserved for Haller, who perceived the importance of assembling under one view, the experiments, facts, and observations of preceding writers, and of substituting them in the place of hypothetical reasonings. He traced the plan of the immense edifice that he designed to construct in his First Lines of Physiology, and executed it on a grand and extensive scale, in his Elements, in which he has brought together into a body of doctrine, as complete as could be expected in his time, all the materials of the science. He perceived the inconvenience of a too strict application of the laws of mechanical philosophy to the living system. He admitted an active force, which he considered as peculiar to the animal body, viz. irritability, which contains the reason or the experimental cause of muscular motion. He maintained that irritability should never be confounded with sensibility, and that the irritable fibre differs as much from the sensible fibre, as the function of motion from that of sensation. Lastly, in his Opera Minor, he lays down many new and important points of doctrine respecting the structure of our organs, and the mechanism of our functions; and he relates a number of experiments made on living animals, for the purpose of drawing from nature the secret of those phenomena which she appears most desirous to conceal. We owe to Haller some curious researches respecting the formation of bone, and the production of callus, as well as some important elucidations of the manner in which the embryo contained in the egg is developed, and passes through the successive stages of its organization. He has left us many experiments and details reflecting the structure of the heart, the circulation of the blood, and the pulsation of the arteries; on the mechanism of the ribs, and the action of the intercostal muscles during respiration; on the differences between the sensible and irritable organs; on the action of the brain and nerves, &c. The latter half of the 18th century is remarkable for many able physiologists, who will be admired by posterity, either for the acuteness of their genius, or the important improvements that they have made in the science. We may mention the names of Bordeu, La Caze, Bonnet, Vicq d’Azyr, Bichat, Dumas, and Cuvier in France; of Fontana and Spallanzani in Italy, and of Whytt, Cullen, Brown, and Darwin in Britain. We cannot pretend to enumerate all the opinions and discoveries of these celebrated men, but must content ourselves with giving a sketch of the three rival systems of Cullen, Brown, and Darwin, and a brief outline of the opinions of Bichat.

The physiological system of Cullen was founded chiefly on that of Hoffman. He placed the principle of the whole animal economy in the movement of the vital fluids, regulated by the fundamental laws of the nervous system. This notion of the vital fluids, according to him, originates in the nerves, and being almost always united in the fenforium, is easily transmitted from one nervous part to another, as long as the medullary substance of the nerves continues in its natural state of life and continuity. The contraction of the moving fibres connected with the sensible organs through the medium of the brain, is the direct effect of a movement that commences with those objects. It is on the contractility inherent in the moving fibres, excited by their own extension, by the application of various stimuli, and often by the immediate influence of the animal or nervous powers, that all the physical actions of a living being depend. He regards this contractile force as distinct from all those which are possessed by the simple solid, and the inorganic elastic parts of the body.

Of the theory of Brown, we have given a sufficient detail under his life, and need not repeat it here.

It is not easy to give a compendious view of the system of Dr Darwin, that shall be intelligible to those who have not examined his celebrated work, the Zoönomia; but we shall endeavour to give as brief and perspicuous an account of it as possible. It is necessary first to notice the definitions given of the terms to be employed, which are as follows.

The immediate organs of sense are, by Dr Darwin, affixed to consist, like the muscles, of moving fibres. The contractions of the muscles and of the organs of sense, are comprehended under what are called fibrous motions, in contradistinction to the sensorial motions, or the changes which occasionally take place in the fenforium. By this latter term is understood, not only the medulla of the brain and nerves, but also at the same time that living principle or spirit of animation, which resides throughout the body, and which we perceive only in its effects. An idea is defined to be a motion of the fibres of some immediate organ of sense; and hence is frequently termed also a sensuous motion. Perception comprehends both the fibrous motion or idea, and the attention to it. When the pain or pleasure arising from this motion and this attention produces other fibrous motion, it is termed sensation; thus limiting this term to an active sense. Ideas, not immediately excited by external objects, but which recur without them, are termed either, 1. Ideas of recollection, as when we will to repeat the alphabet backwards; or 2. Ideas of suggestion, as when we repeat it forwards, A suggesting B, B suggesting C, &c. from habit.

After mentioning a number of experiments to prove the fibrous motions of the organs of sense, Dr Darwin proceeds to lay down the following laws of animal causation.

1. The fibres which constitute the muscles, and organs of sense, possess a power of contraction. The circumstances attending the exertion of this power of contraction constitute the laws of animal motion, as the circumstances attending the exertion of the power of attraction constitute the laws of inanimate matter.

2. The spirit of animation is the immediate cause of the contraction of animal fibres. It resides in the brain and nerves, and is liable to general or partial diminution or accumulation.

3. The stimulus of bodies external to the moving organs is the remote cause of the original contractions of animal fibres.

4. A certain quantity of stimulus produces irritation, which is an exertion of the spirit of animation exciting the fibres to contraction.

5. A certain quantity of contraction of animal fibres, if it be perceived at all, produces pleasure; a greater or less quantity of contraction, if it be perceived at all, produces pain. These constitute sensation.

6. A certain quantity of sensation produces desire or aversion. These constitute volition.

7. All animal motions which have occurred at the same time or in immediate succession, become connected, that when one of them is reproduced, others have a tendency to accompany or succeed it. When fibrous contractions succeed or accompany other fibrous contractions, the connection is termed association; when fibrous contractions succeed sensorial motions, the connection is termed causation; when fibrous and sensorial motions reciprocally introduce each other, it is termed catenation of animal motions. All these connections are said to be produced by habit; that is, by frequent repetition. These laws of animal causation are, according to our author, evinced by numerous facts, which occur in our daily exertions, and are employed by him to explain the diseases and decay of the animal system.

The four sensorial powers, upon which all the actions or motions depend, are thus characterized:

Irritation is an exertion or change of some extreme part of the fenforium, presiding in the muscles or organs of sense, in consequence of the appulses of external bodies.

Sensation is an exertion or change of the central parts of the fenforium, or the whole of it, beginning in some of those extreme parts of it which reside in the muscles or organs of sense.

Volition is an exertion or change of the central parts of the fenforium, or of the whole of it, terminating in some of those extreme parts of it which reside in the muscles or organs of sense.

Association is an exertion or change of some extreme part of the fenforium, residing in the muscles or organs of sense, in consequence of some antecedent or attendant fibrous contractions.

To these four faculties correspond so many classes of fibrous contractions, named irritative, sensitivite, voluntary, and associate. But all muscular motions, and all ideas, are originally irritative, and become causable by sensation and volition from habit, i.e., because pleasure or pain, or desire or aversion, have accompanied them; those ideas or muscular motions which have been frequently excited together, ever afterwards have a tendency to accompany each other.

Of these motions the associate seem most to have excited Dr Darwin's attention. He divides them into three kinds; irritative associations, as when any part of the extracted heart of a frog is irritated by puncture, the whole heart contracts regularly; sensitive associations, or the trains or tribes of motions established by pain or pleasure; and the voluntary associations, or those produced by volition.

The activity of this power of volition is supposed to form the great difference between man and the brute creation; the means of producing pleasure and avoiding pain given to man by this power being denied to brutes.

Corresponding to these four classes of motions, there are four classes of ideas; irritative, preceded by irritation; sensitive, preceded by the sensation of pleasure or pain; voluntary, preceded by voluntary exertion; and associate, preceded by other ideas or muscular motions.

It has been observed in Hudibras that

"A rhetorician's rules Serve nothing but to name his tools."

So we find that a considerable part of Darwin's works is taken up in establishing the new meaning which he attaches to terms well understood and long adopted.

We cannot enter more fully at present into the opinions of the Zoonomia, but we shall have occasion to notice some of them in the succeeding part of this article.

Bichat's system, which has made so much noise on the continent, is chiefly founded on the division of life into two kinds, organic and animal; the former of which is common to all organized beings, while the latter, as its name imports, is peculiar to animals. Each of these two kinds of life may be considered as composed of two orders of functions, which succeed each other in an inverse order. The first of these series in animal life commences with external objects, and proceeds towards the brain; the second begins in the brain, and is thence propagated to the organs of motion and voice. In the first order of functions, the animal is passive; in the second he is active. External objects act on the body through the medium of the first; by the second, the body reacts on the external objects.

Two kinds of motion take place in organic life. In the first the formation of the body is constantly going on; in the second there is a constant decomposition: hence the elements of the body are continually changing, while the organization continues the same. Organic life is accommodated to the continual circulation of matter. The one order of functions assimilates to the nature of the animal, the nutritious particles received into the system; the other rejects what is useless, or is so much altered as to become noxious. The assimilating order of functions consists of digestion, circulation, respiration, and nutrition; all of which processes the matter received into the body must undergo, before it can become a part of the animal substance. When it has for some time constituted a part of the body, it is taken up by absorption, conveyed into the circulation, and thrown out thence, by cutaneous or pulmonary exhalation, or by some other emunctories. Hence, the second order of organic functions, or dissimilating functions, consist of absorption, circulation, exhalation, secretion, and excretion. The brain is the centre of animal life; the heart of organic life.

Bichat considers the proper balance of life to be preserved by the proportion which exists between the action of surrounding bodies, and the reaction of the system. This reaction is greatest in youth, hence the principle of life is at that time in excess. It is least in old age, and then the vital principle is defective. The measure of life is therefore the difference which exists between the efforts of external powers to overturn life, and the internal resistance to support it. The excess of the former shows the weakness of life; that of the latter indicates its strength.

The following table exhibits Bichat's distribution of the organs, or, as he calls them, appareils, belonging to animal and organic life, and to generation, which is common to both.

### I. Organs of Animal Life.

| Organ | Including | |------------------------|------------------------------------------------| | 1. Locomotive | 1. The bones and their dependances. | | | 2. The muscles and their dependances. | | 2. Vocal | | | | 3. The eye. | | | 4. The ear. | | | 5. The nostrils. | | | 6. The tongue. | | | 7. The skin and its dependances. | | 3. External sensitive | | | 4. Internal sensitive | | | 5. Conducting sensation and motion | |

### II. Organs II. Organs of Organic Life.

1. Digestive, including 1. The mouth. 2. The pharynx and oesophagus. 3. The stomach. 4. The small intestines. 5. The large intestines. 6. The peritoneum and epiploon.

2. Respiratory, including 1. The trachea. 2. The lungs and their membranes.

3. Circulatory, including 1. The heart and its membranes. 2. The arteries. 3. The veins of the general system. 4. The veins of the abdominal system.

4. Absorbent, including 1. The absorbent glands. 2. The absorbent vessels.

5. Secretory, including 1. The lacrimal ducts. 2. The salivary and pancreatic ducts. 3. The biliary and splenic ducts. 4. The urinary passages.

III. Organs of Generation.

1. Male. 2. Female. 3. Produced by this union, including 1. The membranes and placenta. 2. The foetus.

We have now taken such a view of the progressive state of physiological science, as we deemed consistent with the nature and extent of this article. It has taught us that the prevailing spirit of every age has had a marked influence on the productions both of art and science that have appeared during that period; and that physiology has always been impressed with the character of the science that was most prevalent at any particular period. While the doctrines of Aristotle prevailed in the schools, physiology never extended beyond the bounds that had been set to it by Galen; and the belief in occult qualities universally prevailed. When a taste for metaphysical speculations began to gain ground, this science was given over to the most abstract subtleties and absurd fictions. When Descartes had reformed the principles of the ancient philosophy, the study of the animal economy, like all the other branches of physics, was fettered by the trammels of the Cartesian doctrines. After the genius of philosophers was directed to chemistry, physiology also took a chemical turn, which it quitted only to take a new direction pointed out to it by the taste for mathematics and mechanical philosophy, which prevailed among all the literary at the end of the 17th and beginning of the 18th century; and now that the study of chemistry is become so general, we see that physiologists are for reducing the functions of the animal economy to the analytical and synthetical operations of the laboratory, and converting the living body into a furnace where a constant combustion is going on while life remains.

We are now to enter on the phenomena of life, and the functions of organized beings; and here we must premise, that in our illustration of these phenomena and functions we shall occasionally refer to every class of living creatures; it being our object rather to give a comparative view of physiology in general, than to confine our remarks to the human economy in particular. Indeed much of the physiology of man has already been given under Anatomy and Medicine; and of that of the inferior animals, we have treated of the physiology of the order Cete under Cetology; of that of Reptiles under Reptology; of that of Fishes under Ichthyology; of that of Serpents under Ophiology.

Chap. I. Of the General Phenomena of Life.

When we take a general view of the objects of nature, we see that they differ from each other in many important particulars, and we soon find that they may be conveniently divided into two great classes; one capable of growth, nourishment, and reproduction; the other not susceptible of these changes. We perceive that all those substances which are found in the bowels of the earth, and many of those which appear upon its surface, continue for an indefinite time in the same circumstances, until they are acted on by each other, when they undergo certain changes which entirely alter their nature and former properties.

Sulphur, in its natural state, is a solid substance insoluble in water, and possessing little activity when applied to the human skin; but if it be subjected to the action of heat, in contact with atmospheric air, or any other gas containing oxygen, it becomes a fluid, very miscible with water, and of a most corrosive quality, namely sulphuric acid. The hydrogenous gas found in the upper part of mines, would remain for ever uncombined with the oxygenous gas which forms part of the atmosphere in which it floats, were it not subjected to the action of caloric, or electricity in a very concentrat- General Phenomena of Life.

We find that all the bodies to which we give the name of minerals, possess no power in themselves which can enable them to resist the operation of external agents; each individual of them is composed of a small number of principles, and their texture appears to be made up of independent particles. Every other body in nature, comprehending the almost infinite variety of plants and animals, though under certain circumstances subject to the same changes which take place among minerals, have, when these circumstances do not exist, an innate property by which they are enabled to resist the production of these changes. They do indeed undergo certain alterations, but by these their original habit and essential properties are not changed. From the time that a plant springs from the seed, till it ceases to vegetate, it is perpetually receiving an accession of new matter, and giving out a part of its former composition: but the new matter is affiliated to it, and becomes a part of the plant; the identity of the plant is preserved, though its component parts are perpetually changing. The same in a still higher degree takes place in animals. The individuals of this latter class, comprehending plants and animals, possess peculiar structure, very different from that of the former. Their texture is fibrous, and the fibres arranged and interwoven, so as to form parts called organs, by means of which they carry on certain operations or functions necessary for their preservation, or for the reproduction of the species. Hence these have been called organized bodies, while the others have been denominated brute or inorganic matter. See NATURAL HISTORY, No. 7.

The component principles of organized beings are much more numerous in each individual than those of inorganic matter, though their absolute number in the former class is smaller than in the latter. In order to present, under a compendious point of view, the distinguishing characteristics of these two classes of beings, we shall give the following table:

| Organized Matter | Inorganic Matter | |-----------------|-----------------| | Mobility | Impression | | Repose | Sensation | | Aggregation | Perception | | Cohesion | Affection | | Gravitation | Sympathy | | Condensation | Action | | Distillation | Locomotion | | Combination | Digestion | | Diffusion | Circulation | | | Respiration | | | Affiliation | | | Accretion | | | Reproduction |

The differences that are found to prevail between organized beings and inorganic matter, have always been attributed to something of a superior nature, called vitality or life. This term life forms one of those simple ideas which it is difficult to define, and as all understand the meaning of the expression, a definition is the least necessary; but if it be required, it cannot be expressed more accurately than in the language of Bichat, who calls life the sum of those functions which resist death. In short, life is best described by the effects produced on a body while it resides in it, contrasted with the appearances which take place in the same body when life is no longer present.

One of the most general effects of the presence of life is, as we have said, the resistance which living beings are by it enabled to oppose to the operation of external agents; and this is most remarkably seen with respect to temperature. Every living being possesses, in a greater or lesser degree, the power of preserving nearly a uniform temperature, which is always a few degrees greater than that of the medium in which it lives. In plants, this power seems to exist but in a low degree. Some of the lower animals which inhabit the air, particularly insects, possess it much more completely. The great heat generated in a hive of bees is a familiar illustration of this. In birds this property is very remarkable, the heat of their bodies being greater than that of any other species of animals. The heat of fishes, worms, and of most reptiles, very little exceeds the temperature of the medium in which they reside; but when the water in which fishes live is frozen, they are capable of resisting, for a long time, the consequences of the diminished temperature. The power which many animals possess of resisting high degrees of heat without any considerable increase of their own temperature, seems still more remarkable, and probably led to the fable of the salamander, which was supposed able to endure the heat of fire, and even extinguish it, when thrown into it for that purpose.

Life seems to pervade almost every part of a living being. In animals, every part, except the cuticle, hair, and nails, exhibits marks of vitality; but it seems to be distributed...

The function which appears to be most universally diffused in living beings, is digestion, (including nutrition) or that by which the substances intended for their nourishment are assimilated to the nature of the body which they enter. This function varies considerably in the different classes. Plants merely attract water from the earth in which they grow, by means of the fibrous parts of their roots, whence it is conveyed by innumerable capillary vessels throughout the whole plant, in which it appears partly to be decomposed, and partly to remain in the state of water, diluting some of the vegetable principles, and thus forming the juices of the plant. In some of the inferior animals, digestion seems to be almost the only function which they are capable of performing. Thus, many of the zoophytes, as the polyps, appear to be almost entirely composed of a stomach, resembling the finger of a glove, into which the aliment is received, the nutritive part extracted, and the excrementitious part thrown out by the same opening. In most other animals, the alimentary canal has two distinct openings, one for the reception of the food, and the other for the ejection of the excrement.

By some animals the food is swallowed entire, and digestion is performed by a simple solution or trituration in the stomach; while in others the mouth is furnished with teeth, or other hard parts, capable of reducing the aliment to a pulpy state, in order to render its further digestion more easy and expeditious. In most animals, the food having undergone some change in the digestive organs, is taken up from them by certain very minute vessels, and carried to every part of the body; but in some it appears rather to exude through pores in the sides of the alimentary canal.

The function of circulation, by which the fluids are constantly moved through every part of the body, is not so general as either of the former functions. In plants there is no proper circulation; for although there are numerous vessels by which water enters into the substance of the plant, and in which the peculiar juices of the vegetable move, the motions of these fluids are not uniform, and do not tend towards one centre. The same defective circulation appears in many of the inferior animals, as in zoophytes and insects. As we rise, however, to the higher classes, we find a perfect circulation. In these there is always a peculiar organ from which the fluids are conveyed to the rest of the body, and to which they again return in a perpetual round. In some animals this central organ is single, while in others it consists of two similar organs joined together, from one of which the whole of the fluids proceed through one particular organ in a lesser circulation, and thence return to the other part, before they are distributed to the general system.

All organized beings require more or less the presence of atmospheric air for their subsistence, or at least for the due performance of the vital functions. In some, the agency of this fluid is conveyed merely by pores upon the surface; as in plants, in which the leaves absorb the air; and in several of the inferior animals, as insects and worms, over the surface of whose bodies are distributed numerous openings, by which the air enters. In animals of the higher orders there are peculiar organs called... called lungs or gills, through which air, or water containing air, enters, and from which its beneficial influence is imparted to the fluids which are circulating through them. In general, these beings exist for a very short time, when deprived of atmospheric air, or when the element in which they live is deprived of oxygen; but in some of the lower classes of animals the absence of oxygen is much less injurious; and there are instances of reptiles in particular having been preserved in a state susceptible of life and motion, while buried for many years in the heart of a tree, or in the middle of a block of stone. Respiration, then, though in general necessary to the continuance of vitality, may, in many tribes of organized beings, be suspended for a considerable time, without the principle of life being entirely destroyed.

A function equally general, and equally indispensible with that of digestion, and one which forms another characteristic of living beings, is the function of regeneration, a function more peculiarly necessary, as all organized beings, though capable of resisting for a considerable time the operation of external agents, tend ultimately to corruption and decay; and as they cannot, like inorganic matter, be regenerated by a reunion of their component principles, it was necessary that they should possess the capacity of producing, while in existence, a creature similar to themselves, to supply their place in the scale of being.

It has been a very general opinion among naturalists, that all living beings, both plants and animals, originate from seeds or eggs produced by the parent. This, although very generally true, is not a universal fact. Most plants, indeed, with which we are acquainted, appear capable, in their natural state, of producing seeds, which form the embryo of a future plant. But in a great many of them new plants are obtained from buds, slips, or suckers, furnished by the parent. In some animals too, as the polypi, reproduction may be effected by dividing the parent into several pieces; and even the natural generation of these animals is performed by protuberances which grow from the body of the parent, and seem to drop off spontaneously, when capable of an independent existence.

There are two striking differences in the manner by which living beings are generated. In some, two distinct sets of organs are necessary, and by the mutual action of these generation is effected; while in others, as in the instances we have mentioned of the polypus, this act of copulation appears to be unnecessary. Almost all plants possess distinct sexual organs, and in most both male and female organs are situated in the same individual. In these plants the female ovum is impregnated by a very fine powder, which is contained in part of the male organs, and is applied to those of the female. We are fully convinced of the necessity of the vegetable copulation, by observing that the females of those plants which have the sexual organs situated in distinct individuals are not capable of producing fruit, or at least do not produce this in perfection, if they are excluded from the influence of the male; and that an artificial impregnation may be brought about by bringing the male and female organs in contact. Many animals are hermaphrodite; and among these the individuals of some species generate without the assistance of another individual of the same species. This appears to be the case with the bivalve shell-fish. Others again, as snails, and most of the mollusca, which crawl upon the earth, copulate reciprocally, or each individual performs the double office of male and female. In most animals, however, the sexes are distinct, and probably a real hermaphrodite in the superior classes never existed. Another striking difference with respect to generation in animals is the more or less perfect state in which they bring forth their young. A large proportion of animals, among which are the insect tribe, fishes and birds, produce eggs, which are afterwards hatched by the heat of the parent, or by that of the sun. Other classes again, as some of the amphibia, and the whole of the mammalia, carry their young for a certain time within an organ destined for that purpose, from which they are excluded in the state of perfect animals.

The last function which we shall here notice, is sen-sensation. This appears to be less general than any which we have hitherto mentioned. It has indeed been supposed by many philosophers and naturalists, that plants possess a degree of sensibility; and this opinion has been lately avowed and strenuously supported by the elegant, but enthusiastic author of the Botanic Garden, and the Loves of the Plants. That plants possess a susceptibility of receiving impressions, and in consequence of that of being roused into action by external stimuli, we shall readily admit, and shall hereafter assign to this susceptibility its due importance; but as there is no reason to believe that it ever produces sensation, we must not confound it with the sensibility of animals; nor is the difficulty of explaining some of the functions of vegetables, without resorting to the hypothesis of a vegetable cerebrum, a sufficient reason for investing them with this faculty. It has even been doubted whether some of the inferior animals, as the zoophytes, possess this function, as nervous fibres have not yet been detected in their organization. It is probable, that there is a regular gradation in the tribes of organized beings with respect to sensation, as well as the other functions; and though we have not been able to discover all the links of this chain, these will probably, as our knowledge of nature increases, come more into view, and we shall then be able to reconcile many circumstances which we cannot at present comprehend.

With respect to the varieties that take place in the number and degree of the external senses, as possessed by the various classes of animals, we may refer the reader to what has been said on that subject in the first chapter of the comparative part of the article ANATOMY.

The duration of life is exceedingly various. We Duration know that there are animals which live but a few hours, as the insects called ephemerae; and that others, as the elephant, the raven, and the pike, may exist for a century. The term of life allotted to plants is also various; some live only for a year, and are hence called annual plants; others exist for two years, and are called biennial plants; while a few surpass in longevity anything with which we are acquainted in animated nature. Thus, the oak is said to require 100 years, in order to acquire its full maturity; to retain its perfect vigour for the like term, and to complete at least a third century before it entirely decays. The chestnut is still more remarkable instance of vegetable longevity. The account of the gigantic chestnut on Mount Etna, given by We have hitherto considered life as displayed in the exercise of functions; but it may exist independently of this exercise, or it may lie dormant for a considerable time, till called into action by particular circumstances. Every one knows how long a seed or an egg, when excluded from heat, air, and moisture, will retain the faculty of producing a plant or an animal. The only proof we have, that this faculty still exists, is, that when we place the seed or the egg in circumstances favourable to the development of the embryo which it contains, the process of generation goes on, till the plant or the animal is excluded. We know also, that after an organized being has commenced the exercise of its functions, this exercise may cease for a time, or at least become almost imperceptible, while yet the vital power shall remain susceptible of reviving its activity at a future period. We then say that the animal or vegetable is in a torpid state. On this part of the history of life we shall not enter at present, but shall defer the consideration of it till we come to treat of sleep and death.

The above is a hasty comparative sketch of the functions exercised by the various tribes of organized beings. It is sufficient to show, that there is in these beings a vital power which completely distinguishes them from brute or inorganic matter.

A question which naturally arises in every thinking mind is, What is the essence of life, or on what does it depend? Though we profess ourselves unable to answer this question in a satisfactory manner, and believe that all who have hitherto attempted to do so, have failed in their attempts, it may be acceptable to most of our readers to know the opinions of the most respectable writers on this abstruse subject. These, therefore, we shall briefly state.

These opinions have chiefly rested on the question, whether life be an independent, immaterial principle, or merely a physical or chemical modification of matter. We have already, in the historical view which we have given of the progress of physiology, mentioned some of the more remarkable doctrines respecting the principle of life that have been delivered prior to the 18th century; we shall here, therefore, only mention those which have been maintained since that time.

Mr John Hunter, in his valuable treatise on the blood, supposes the principle of vitality to exist in that fluid, or that the blood has life; and has founded this doctrine chiefly on the following proofs. First, It unites living parts, when it is effused between them. Secondly, It becomes vascular like other living parts; its temperature as it flows from the vessel, is always equal in the most opposite temperature in which the body can bear exposure. Thirdly, It is capable of being acted upon by a stimulus, as is the case when it coagulates. Fourth, Paralytic limbs, though deprived of motion and sensation, are yet nourished and preserved alive by the blood circulating through them.

Mr Hunter's idea of the vitality of the blood is merely the revival of one of the oldest physiological doctrines on record; namely, that delivered to the Israelites by Moses, that in the blood is the life of an animal.

Dr Goodwin, in his work on the connection of life with respiration, is of opinion, that the heart is the great seat of the principle of life in all the more perfect animals; and that the contractions of the heart with the ordinary stimulus is the only mark of the presence of this principle; that when the heart contracts under such circumstances, the body is alive; when not, it is dead. Life he therefore defines to be the faculty of propelling the fluids through the circulatory system. According to him, the external concomitant circumstances which operate upon the body in health are heat and respiration, which excite the vital principle to action; and whenever the functions of an animal are suddenly suspended, and the body puts on the appearance of death, it is always in our power to determine whether it be really dead, by restoring the temperature, and by inflating the lungs with proper air. He is of opinion, with some others, that there are no means of determining the absolute deprivation of the vital principle but by the presence of putrefaction.

It has lately become fashionable to consider life as the consequence of certain chemical changes, which are going on in the body; an opinion which is chiefly supported by Hufeland, Girtanner, and Humboldt.

According to Hufeland, life is a chemico-animal flame, to the production of which oxygen is absolutely necessary, and the vital power is the most general and powerful of all the powers of nature. He considers it as the cause of organization, and as possessing the following properties.

1. It has a greater affinity to some organized bodies than to others; thus, the polypus may be cut in pieces without being divested of it, and a decapitated tortoise or a frog deprived of its heart will live a long time after; whilst to the human body, or a quadruped, it would be instant death. According to this physiologist, it is a general rule, that the stronger the affinity between life and an organized being, the more imperfect is the animal; hence the zoophytes, whose whole organization consists in a mouth, a stomach, and a gut, have a life exceedingly tenacious, and difficult to destroyed. 2. It is in greater quantity in some organized bodies than in others. In general, cold-blooded animals live longer than those with warm blood. 3. It frees bodies from the chemical laws of inanimate matter, and transfers the component parts of a body from the physical or chemical to the organic or living world. 4. It prevents putrefaction, for no organized body can putrefy unless deprived of life.

Humboldt is of opinion, that the degree of vitality Humboldt depends upon the reciprocal balance of the chemical affinities of all the elementary parts of which the animal body is composed.

Some physiologists of the present day deny the existence of the vital principle altogether, "The idea of life (says Cuvier), is one of those general and obscure ideas produced in us by observing a certain series of phenomena, possessing mutual relations, and succeeding each other in a constant order. We know not indeed the nature of the link that unites these phenomena, but we are sensible that a connection must exist; and this conviction is sufficient to induce us to give it a name, which the vulgar are apt to regard as the sign of a particular principle, though in fact that name can only indicate the totality of the phenomena which have occasioned its formation."

Dr Ferriar, in his observations concerning the vital principle, thinks, that some direct arguments may be brought... brought against the general supposition of an independent living principle. These arguments he divides into two kinds, viz. refutations of the general proofs offered in support of the vital principle; and instances of the direct influence of the mind and brain over what is termed the independent living principle. The great proofs for the support of a vital principle, are the contraction of muscles separated from the body on the application of stimulants; the performance of the vital and involuntary motions without any exertion or even consciousness of the mind, and the birth of full-grown foetuses destitute of a brain. In all these cases, something is alleged to operate, independently of the mind, in producing muscular motion.

Dr Ferriar, in answer to the first argument drawn from the contraction of separated muscles, affirms, it may be said, i.e., That the power of contraction, in a separate muscle, is lost before putrefaction takes place, i.e. before the destruction of its texture; but if its vitality depended on its texture, this ought not to happen. 2dly, The power of contraction, in a separated muscle, is strongest upon its first separation, and becomes weaker by degrees; therefore, the contracting power seems to have been derived from some source from which it is detached by the excision of the part. 3dly, Irritation of the medulla oblongata, or of the nerves supplying particular muscles, occasions stronger contractions than irritation of the muscles themselves; and Dr Whytt furnishes an experiment on a frog, directly proving, that the action of separated muscles depends upon the nervous energy. 4thly, Dr Haller himself is obliged to make on this subject a concession sufficient to destroy his favourite hypothesis of the vis infita. 5thly, When a paralytic limb is convulsed by the electric shock, the motion never takes place without the patient's consciousness. In this case there is no distinction between the vital principle, and that exertion which in voluntary motion is always attributed to the mind. See Chap. iii.

In answer to the second argument, in favour of a vital principle, drawn from the performances of the vital and other involuntary motions, Dr Ferriar contents himself with only observing; that, allowing the organs of those motions to be supplied with nervous energy, their motions may be very well accounted for by the stimulus of their contained fluids.

The force of the third argument, drawn from the want of a brain in full-grown foetuses, is taken off by Dr Whytt, who remarks, that as the heart is sometimes wanting in full-grown foetuses, the argument would equally prove, that the heart is not necessary for the continuance of circulation, as that the brain is not necessary to the support of the system. Accordingly, foetuses born without a brain do not, in general, survive birth.

Besides the general supposition of an independent living principle, an inference has been drawn from facts, of a nervous energy independent of the brain. By this term is meant, that condition derived from the brain to different parts of the body, by means of which they become capable of motion. To show, by direct proof, that there is no independent vital principle, Dr Ferriar observes, 1. That it is justly urged by Dr Monro against the doctrine of the vis infita, that there is too much design in the actions of different muscles, affected by different stimuli, to be the effect of mere mechanism. General Thus, when the hand or foot is burnt, or otherwise sud, immediately injured, the muscles on the part immediately irritated are not thrown into action, nor the muscles on the side irritated, but their antagonists contract immediately and strongly. Now, if the instantaneous action be in this case chiefly produced by an effort of the mind, the supposition of a distinct vital principle is superfluous; if it be said to be produced by a living power independent of the mind, then there must be a rational power in the body independent of the mind, which is absurd. 2. The state of the vital and involuntary motions is considerably affected by the state of the mind, which equally disproves the existence of a separate vital principle; and proves the dependence of the nervous energy upon the brain. 3. Madness, it is well known, is frequently produced by causes purely mental, and in persons apparently in good health; and, as the patient's sensibility to very powerful stimuli is much diminished in maniacal cases, they afford another proof of the subordination of the nervous energy. 4. It has been observed, that in paralytic cases, motion is frequently destroyed, while life remains. As the cause of palsy almost always resides in the brain, this fact appears equally inexplicable on the opinion of a distinct living principle, or of a nervous energy, independent of the brain. 5. When nerves are regenerated, after being cut through, sensation and voluntary motion are not always restored to the parts beneath the division: the restoration was never made in Dr Monro's experiments. But, on the supposition of a distinct nervous power, the nerve, after its re-union ought to resume all its offices. 6. Dr Whytt affirms, that when the spinal marrow of a frog is destroyed, after decollation, no contraction can be excited in the limbs, by cutting or tearing the muscles. Such are the facts and arguments which Dr Ferriar brings against the opinion of a distinct living principle; and he thinks, that their investigation appears to lead us back to the brain as the source of sensibility and irritability.

In the life of Dr John Brown, we have given an account of the doctrine, of life being a forced state. This doctrine appears to have been first delivered by Dr Cullen, though he afterwards retracted it. Of late Dr Ruth of Philadelphia, in his Lectures on Animal Life, has advanced many arguments in favour of this doctrine. He includes, in animal life, three properties as applied to the human body, viz. motion, sensation, and thought; and these, when united, compose perfect life. It may exist without thought or sensation; but neither sensation nor thought can exist without motion. He affirms, that the lowest degree of life exists even in the absence of motion. He first considers animal life as it appears in the waking and sleeping state, in a healthy adult; and afterwards inquires into the modification of its causes in the foetal, infant, youthful and middle states of life, in certain diseases, in different states of society, and in different animals, and lays down the following propositions:

1. Every part of the human body, the nails and hair excepted, is endowed with sensibility or excitability, or with both.

2. The whole body is so formed and connected, that impressions made in the healthy state upon one part excite

3. Life is the effect of certain stimuli acting upon the sensibility and excitability, which are extended in different degrees to every external and internal part of the body; and these stimuli are as necessary to its existence as air is to flame.

He continues to observe, that the action of the brain, the diastole and systole of the heart, the pulsation of the arteries, the contraction of the muscles, the peristaltic motion of the bowels, the absorbing power of the lymphatics, secretion, excretion, hearing, seeing, smelling, taste, and the sense of touch, even thought itself, are all the effects of stimuli acting upon the organs of sense and motion.

These have been divided into external and internal.

1. The external are light, sound, colours, air, heat, exercise, and the pleasures of the senses.

2. The internal stimuli are food, drinks, chyle, blood, tension of the glands which contain secreted liquors, and the exercise of the faculties of the mind.

Life, therefore (according to the hypothesis of Ruff), even thought itself, is merely a quality residing in the component parts of a material system, dependent upon a peculiar organization, by which it is enabled to act, or in some ways to move on being stimulated or excited. Hence life can never be inherent in a simple uncompounded substance, nor in a particle of animal matter; and if the stimulus be withheld from a living system beyond a given time, all motion, sensation, and thought, must necessarily be extinguished.

Instead of one vital principle, some physiologists have supposed the existence of several in the same body; and from the phenomena that take place in some organized beings, as the reunion of parts that had been separated, the reproduction of others that have been lost, and the separate existence of the divided parts of some worms and zoophytes, it was formerly the opinion of a celebrated lecturer on anatomy, that the vital principle was really divided. From more considerate and extensive inquiry, however, he is now of opinion, that the irritability on which these phenomena depend, is never the direct or immediate operation of the vital principle, but only the consequence of its operation; and in no case exclusively the consequence, but the consequence likewise of other operations proceeding from a number of different causes; and hence it is that a vital principle may often exist where it cannot operate in a sensible manner, from the want of auxiliaries; and hence it is, likewise, that its effects may often be continued, at least for a while after its departure.

With regard to the portions of plants and polypi that continue to live in a separate state, assume the form of their respective species, and propagate their kind, they will be found, on a close examination, to have been originally complete systems; many of the plants and many of the polypi that were usually considered as simple individuals, not constituting one animated system, but rather a congeries of animated systems,—a congeries, too, which after all is nothing more than a species of society, where animated beings are associated together for mutual protection; such as we see among men in a city; among bees in their cells, which, in point of form, are similar to plants.

CHAP. II. Of Sensation.

As all living beings are so related to each other, and to the inanimate objects of nature, as to be capable of deriving benefit, or receiving injuries, from the one or to organize from the other; it seems necessary that they should possess the faculty of perceiving the proximity of the beneficial or injurious object, that they may avail themselves of the benefit which it holds out, or avoid the danger which it threatens. Accordingly, we find that all organized beings enjoy in some degree the capacity of receiving impressions, which we think is proved by the motions which take place in them when affected by external agents. When a plant expands its flowers to the sun, or turns, as it were, its back to the blast; when it stretches out the fibres of its roots to imbibe the distant moisture, or directs its branches to the only chink by which it can receive the light of day; we think these motions are the consequence of that capacity of receiving impressions, or of being roused to action by stimuli; we think that this may be conceded, without having recourse to the influence of mind, or even the medium of a nervous system; we do not believe that the grass we crush beneath our feet is sensible of pain, nor do we suppose with the poet, that

"E'en the poor beetle that we tread on In mortal sufferance feels a pang as great As when a giant dies."

but we are of opinion, that even in the lowest tribes there is a degree of that faculty, which in the higher orders of animals we call sensibility, and which we shall hereafter, in a lecture on the animal economy in London, denominate sensitivity. This inferior degree of the sensitive faculty we shall suppose to be possessed by plants, zoophytes, and animalcules, or those organized beings in which we can perceive no marks of a nervous system; while we shall confine the term sensibility to all other classes of animals.

These faculties we consider as qualities of living bodies, while we regard sensation, like perception, as a quality of mind. We leave it to the metaphysician to mark the line of distinction between sensibility and sensation, and to show how the one arises from the other. See METAPHYSICS, Part I. chap. 1.

The organs of sensation consist of the brain properly so called, the cerebrum, the medulla oblongata, the spinal marrow, the nerves, and ganglia; together forming what is called the nervous system. These parts in the human body have been described under ANATOMY. For an account of these organs in the inferior animals, we must refer to the lectures of Cuvier, vol. ii. or the Comparative Anatomy of Blumenbach.

In respect of sensibility the animal is only passive; but when sensation is produced, he becomes active, in as much as the organs of the external senses are then brought into action. It is by means of these senses that the animal receives intelligence from without. We shall therefore examine these before we mention the phenomena of sensation in general.

1. Of Feeling.

The most general of all the senses, and the most widely ly diffused over the body of an animal is that of touch or feeling. Animals that possess scarcely any other sense seem always to have that of touch. It is doubtless by this that polypi, actinia, and other water animals, perceive the approach of their prey, or are warned of impending danger, from the agitation of the water that is communicated to their bodies. Indeed so general is this sense, that some physiologists think we may reduce all others to it as a genus; and suppose that smelling, tasting, hearing, and seeing, are only species of feeling. This reference is not uncommon in ordinary speech, as it is not unusual to talk of feeling a smell.

By touch, taken in its ordinary limited sense, we perceive the more striking external qualities of bodies, as figure, hardness, softness, roughness, smoothness, moisture, dryness, heat, cold; of all which, except figure, we could scarcely form any idea by the other senses. There is probably no sense that can so well supply the place of others as that of touch; and it is particularly acute in those who have lost their sight or hearing. See the article BLIND, especially the Appendix.

The organs of touch are the skin and its productions, or rather the nervous papillae (see ANATOMY, No. 76.) that form so large a part of the true skin. As many animals, however, have the body so enveloped in a scaly, shelly, or hairy covering, as to prevent the actual contact of the body by external objects, there are other organs that seem destined to fulfil this office. In man, the points of the fingers and the lips are the most delicate feeling organs; in many quadrupeds too, the lips seem to possess an exquisite sensibility, and in some, as the rhinoceros, the upper lip is lengthened out as if to serve the purpose of a hand. The prolonged snouts of the tapir, the ibex, the mole, and the hog, seem to answer the same purpose; and the exquisite sensibility and flexibility of the trunk of the elephant is well known to fit that organ for almost all the purposes to which the human hand can be applied. The tail, in some species of monkeys, apes, and ant-eaters, and in some reptiles, seems to possess a high degree of sensibility. In some animals, as the cat, the whiskers are employed as organs of feeling, as we know that these are erected when the animal is passing through a narrow hole. Several species of fishes have cirri and tentacula, which they seem to use as fingers in ascertaining the approach of their prey; and in insects, the antennae and the palpi are evidently organs of feeling, as are the arms, the tufts, and tentacula of sea-stars, sea-urchins, actinia, medusa, and many zoophytes.

Most of the actions of external bodies on the surface of the animal body, are merely mechanical, though the sensations which they communicate may often be the effects of a chemical change in some of the feeling organs, and this change can be produced only in consequence of the power of simple pressure, to form or destroy some of the combinations that take place in the animal system. The sensations which appear most evidently to arise from a chemical change in the organs, are those that give notice of a change of temperature. When a body that has a temperature below that of the animal, comes in contact with the surface of this latter, we know that it abstracts from that surface a part of its caloric, as by the contact it gradually acquires the temperature of the animal; unless, indeed, it be so large and so cold as altogether to destroy life. As, however, the resistance which the animal body gives to a too great change of temperature, generally confines this change to the surface of the body; there must be something more than a mechanical or a chemical action, or the sense of feeling must depend chiefly on the vital principle.

As the sense of feeling, from its general diffusion, may be considered as the most essential of all the senses; its degrees of perfection have considerable influence on the nature of different animals.

Of all vertebral animals, man seems to possess this sense in the most perfect degree; but among the invertebrate animals, the touch seems to improve as the other senses degenerate; and those animals which appear to have no other sense, possess this in so exquisite a degree that they seem to feel even the light.

Dr. Darwin thinks it probable, that the animal body is furnished with a distinct set of nerves for the sensation of heat and cold. We do not see the necessity of this, as we think that this sensation is very naturally reducible to that of feeling.

To this head naturally belongs the consideration of what parts of the human body possess sensibility, and what are insensible. This discussion is curious, and some time ago exercised the ingenuity of two very able physiologists, Haller and Whytt; between whom it gave rise to a long and warm dispute. We cannot pretend to enter into the merits of this controversy, for an account of which we refer our readers to Dr. Whytt's Physiological Essays, and to the Principes de Physiologie de Dumas, tom. ii. part iii. sect. i. chap. i.

The general result seems to be, that many parts will appear sensible or insensible, according to the nature of the stimuli applied to them, and that many of those parts which in their natural and healthy state appear insensible of pain, are when inflamed or otherwise altered by diseases, highly sensible; and that the brain, which is considered as the centre of all sensation, and the puncture or laceration of which is attended with most distressing symptoms in other parts, is to ordinary stimuli as insensible as the cuticle or the nails. See also on this subject, Bichat "Anatomie Generale," tom. i. p. 161—167.

The principal morbid affections of this sense, are pain, itching, and want of feeling; for an account of which see MEDICINE, No. 77. The functions of the skin, independently of its use as an organ of touch, will be considered in two of our succeeding chapters.

2. Of Tasting.

This sense is the most nearly allied to feeling of any of the other senses, and therefore very properly comes under our consideration after that sense.

The principal organ of taste is the tongue, especially at its upper surface, point, and edges; but it also extends to the lips, the palate, and the velum pendulum palati. The tongue is not absolutely necessary to taste, as appears from a case mentioned by Joffieu, of a person who had only a fleshy tubercle in place of a tongue, and yet possessed the sense sufficiently perfect.

The several parts of the organs of taste are not equally sensitive to every rapid body; the tongue seems to be more particularly affected by saline and saccharine substances, chiefly, however, at its upper surface; the lips are said to be most susceptible of the taste of hellebore, the palate of belladonna, and the gullet of wormwood. The momordica elaterium is said chiefly to affect the back of the tongue, and colocynth its middle.

The greater or less perfection of this sense depends much on the softness, flexibility, and moistness of these parts. As man seems to possess these qualifications in a more eminent degree than most other animals, so, in the natural unsophisticated state of the tongue, he probably enjoys the benefit of taste much more highly than they. Such is the case with all young children, and with the peasant, whose simple fare appears to be eaten with a much greater relish than all the delicacies of the voluptuary, who must have recourse to various stimuli to enable him to derive gratification from even the daintiest viands (A).

Taste seems to be more exquisite when the rapid body is strongly pressed between the tongue and the palate. Taste is also rendered more acute when the tongue is stimulated by various condiments, as pepper, mustard, which even, when not taken in such quantity as to be very perceptible themselves, evidently increase the relish of the dishes which they season. Much also depends on the nature and state of the bodies that are applied to the organs of taste. These must, in the first place, be either fluid, or capable of solution in the saliva. They must also possess some saline or acid quality, to render them capable of acting on the nervous papillae. It was formerly supposed, that saline bodies alone possessed the power of affecting the organs of taste; and it was conceived by Bellini, that the different flavours of saline bodies depended on the figure of their crystalline particles. M. Dumas has taken considerable pains, and has advanced several arguments, to show the absurdity of this hypothesis; and we think, has treated it with more ferocity than it deserves. That the different sensations which rapid bodies excite in our organs of taste, depend chiefly on a difference in their chemical nature, must, we think, be allowed, and some have gone so far as to suppose, that the sensation depends on some chemical affinity between the rapid body and the nervous fluid.

The impression which rapid bodies make on the organs of taste is modified by age, sex, temperament, and habit. We know that children are particularly pleased with sweet things, while high seasoned dishes and vinous liquors are more palatable to people of a more advanced age. Women, from various causes, especially during pregnancy, and when labouring under hysterical affections, have often very singular tastes. People of a warm and a mobile constitution are often affected by flavours that are almost insensible to others; and custom will render palatable many substances, which, when first tasted, are rejected with disgust.

Besides the gratification afforded to animals by the sense of taste, this is supposed to afford one of the principal means of distinguishing between wholesome and deleterious substances. Indeed, with respect to the inferior animals, this discriminating sense is seldom known to fail, and in this instance, they are superior to man, who is often deceived. There are many poisonous herbs, the fruits or roots of which have a taste not unpleasant, but which cannot be eaten with impunity.

On the morbid affections of taste, see Medicine, No 78.

3. Of Smelling.

The sense of smelling, like that of taste, is nearly allied to feeling, and is one of those by which we become acquainted with the mechanical and chemical properties of external bodies. It is caused by volatile particles flying off from odoriferous bodies, and diffused or dissolved in the atmosphere, in union with which they enter the nostrils and affect the nerves of the smelling organs.

It is difficult to ascertain what are the essential organs of smelling. We know that in most animals which breathe through lungs or gills, there is either a nose, or there are certain holes that serve the purpose of nostrils; but in many animals there is nothing similar to these, and yet there is every reason to believe that they possess the sense of smelling in an exquisite degree.

Insects discover their food at a distance. Butterflies seek their females, even when inclosed in boxes; and as they are liable to be deceived by resemblance of colour, it is evident that these insects are guided in many circumstances by the sense of smell. Thus the flesh-fly (Musca vomitoria) lays its eggs on plants that have a fetid smell, imagining that it places them on corrupted flesh, and the larvae which are thus produced perish for want of their necessary food.

As the organ of smell, in all animals which respire air, is situated at the entrance of the organs of respiration, the most probable conjecture that has been proposed respecting its seat in insects, is that of Baer, since revived by several naturalists, who placed it in the mouths of the tracheae or air tubes. Besides many other reasons that might be stated in support of this opinion, we may observe, that the internal membrane of the tracheae appears very well calculated to perform this office, being soft and moistened, and that the insects in which the tracheae enlarge, and form numerous or considerable vesicles, are those which seem to possess the most perfect sense of smelling. Such are all the scarabeet, the bees, flies, &c.

The antennae, which other anatomists have supposed to be the seat of smelling in insects, do not appear to Cuvier to possess any of the requisites for that organ.

The mollusca, which respire air, may also possess this sensation at the entrance of their pulmonary vessels; but it is not necessary to search for a particular organ of this sense in them, as their whole skin appears to resemble a pituitary membrane. It is everywhere soft, fungous, and is always moistened by a great quantity of mucous matter. Finally, it is supplied with numerous nerves, which animate every point of its surface.

(A) It is generally supposed, that the sense of tasting is more acute in some of the inferior animals than in man; an opinion which is founded chiefly on the greater size and number of the papillae of the tongue in those animals. It is scarcely possible to decide this point; but we should conceive, from the infinite variety of substances that are occasionally subjected to the human palate, and from the extreme delicacy of taste displayed by some individuals, that man has the advantage of his brute neighbours in this sense. The worms and soft zoophytes, and all the polypes, are probably in the same situation. It cannot be doubted but that these animals enjoy the sense of smell. It is chiefly by it that they discover their food, particularly the species that have no eyes. Aristotle remarked, that certain herbs, which have a strong odour, were avoided by cuttle-fish and the octopus.

Of all the substances which affect our organs of sensation, odours are the least understood, though the impressions which they make on the animal body appear to be most powerful and extensive. Some bodies are always odoriferous, because the whole or a part of their substance, being volatile, is constantly flying off; others become odoriferous only under certain circumstances; as when a body containing a volatile principle in its composition is decomposed by another that has a less affinity for that principle, e.g. when muriate of ammonia is decomposed by quicklime.

Odours seem to be propagated in the air, much in the same manner as one fluid is diffused through another. Their motion is not direct, like that of light, nor is it rapid or susceptible of reflection and refraction like light and caloric. The odoriferous particles of volatile bodies may enter into combination with different substances, by chemical affinity, and thus lose their original properties. In this way the effluvia of putrid meat are destroyed by fresh burnt charcoal, and the noxious exhalations from pestilential apartments are removed by the vapours of nitric or muriatic acid.

These circumstances seem to prove that each smell is occasioned by a particular substance floating in the atmosphere. There are others, however, which appear to indicate that odour is not always produced in this manner.

Several bodies yield a strong smell for a great length of time, without sustaining any sensible loss of substance; such, for example, is musk. Some odours are perceived when no evaporation can be observed, as the smell which arises from the friction of copper, that produced by the fusion of a great number of bodies, and even by the melting of common ice. In other cases, real evaporation produces no sensible odour; this may be remarked on the disengagement of several gases, and even on the ordinary evaporation of water. Perhaps these phenomena prove only that the force of sensation is not proportional to the quantity of the substance by which it is excited, but that it depends on the nature and degree of the affinity of that substance with the nervous fluid*. The action of the greater part of odoriferous substances on the nervous system, is rendered manifest by a number of other effects besides the sensation of smell; some produce faintings, others giddiness, or even convulsions. Some, on the contrary, serve to remove these disorders; indeed the greater part of medicines act in general rather by their volatile and odoriferous parts, than by their other principles; and afford new proofs of the influence exercised in the animal economy by the gaseous and impalpable substances, the greater part of which are doubtless still unknown to us.

We know not whether odours have a peculiar vehicle, besides the matter of heat, which is common to them all in their quality of vapours or elastic fluids. We cannot explain why odours are agreeable or disagreeable to us, nor why those that are disgusting to us appear pleasing to other animals, and vice versa. Though man and other animals are generally pleased with the odour of those substances which serve them as food; yet when their appetite is satisfied, this odour often becomes displeasing to them. On the contrary, some animals appear to have a passionate fondness for strong smelling substances which seem altogether useless to them. Thus cats are extremely fond of cat-mint, and the fresh roots of valerian. In general, those odours which are most disagreeable indicate that the substances from which they proceed are injurious. Thus venomous plants, putrid flesh, and poisonous minerals, have commonly an unpleasant odour. This rule, however, is not universal; and the sense of smell, like that of taste, is not an unerring guide to man, whatever it may be to other animals.

It appears that the effluvia of odoriferous bodies are capable of diffusing themselves through water as well as air; for when these substances are thrown into water as bait for fish, we find that these animals are attracted by the smell from a considerable distance.

The comparative physiology of this sense is very curious, though we cannot explain the reason of the differences that we find to take place in the various tribes of animals. Man in a state of civilized society, where he may have recourse to a great variety of means by which to distinguish the properties of bodies, has less occasion for acuteness of smell; but we know that savages are in that respect greatly his superiors. Their smell is so acute, that like a blood-hound, they can scent their enemy to a great distance, and pursue his track with almost certain success. Among birds and beasts of prey we also find that acuteness of smell is a very general property. Hyenas, wolves, vultures, and ravens, can distinguish the putrid carcases on which they feed many miles off; and it is asserted by naturalists, that jackals hunt in packs, and follow their game like hounds by the scent. There is a curious diversity in this respect among birds, some having this sense very acute, others very blunt. We are told by Gattoni*, that the cock is scarcely* strongly affected with the smell of ammonia or hartshorn, while the duck is said to avoid all powerful odours whether agreeable or otherwise. We are not sufficiently acquainted with the nature of the olfactory membrane, nor with that of the nerves distributed to it, to enable us to form an opinion respecting the degree and the kind of sensations they procure to different animals. It may, however, be at first sight presumed, that all things in other respects being equal, the animals in which the olfactory membrane is most extensive, enjoy the sensation of smell most exquisitely; and experience confirms this conjecture. It would be curious to learn why the animals which possess the sense of smell in the highest degree, are precisely those which feed on the most fetid substances, as we observe in dogs which eat carrion.

Of Hearing.

The sense of hearing is more important than any which we have yet noticed, but it appears to be less generally diffused.

By means of it we become acquainted with those properties of bodies which fit them for making sensible impressions on the air, as hardness, elasticity, &c.; and these impressions on the air, when communicated to the organs of hearing, convey to our mind the ideas of sound. By this sense we derive two of the highest gratifications that we are capable of enjoying, viz., the pleasures of conversation and of music; and in this way most animals hold intercourse with each other.

The organs of hearing differ exceedingly in the various classes of animals. The human ear and its appendages have been described in Anatomy, Part I., chap. vii., sect. 4.; and for an account of these organs in other animals, we must refer to Cuvier's Lectures, vol. ii., or the Comparative Anatomy of Blumenbach, chap. xx. Red-blooded animals without exception have evident auditory organs; and analogous parts are found in many of the white-blooded. In a great number of the inferior classes, however, no such parts have been ascertained, though it is certain that many of them do really hear. In all those in which these organs have been detected, there is always found a gelatinous pulp, covered with a fine, elastic membrane, and in this pulp the ramifications of the auditory nerve are lost. It is therefore highly probable that the seat of hearing resides in the minute nervous fibres that are distributed through the pulp, and that this latter is the medium by which sounds are communicated from the percussed air. We may form a tolerably just idea of the manner in which this pulpy substance is connected with the external movements that are the cause of sound; for this quivering jelly will readily receive the concussions of the air or water that are transmitted to it from the vibrations of sonorous bodies, and communicate them to the nervous filaments. Thus far only can we trace the motion of sound; but the steps by which this motion is carried on till the perception of sound is produced in the mind, are equally unknown to the anatomist and the metaphysician.

The philosophy of sound has already been treated of under Acoustics. It is necessary here to remark only, that the qualities of sound may be distinguished into force, depending on the extent of the vibrations of the body from which the sound proceeds; tone, depending on the velocity of the vibrations; resonance, arising from the intimate composition of the sonorous body; simple modulation of voice, and articulations.

The human ear can distinguish all these different qualities with relation to one sound; this distinction is made with wonderful accuracy, by persons who frequently exercise that faculty, and particularly by professional musicians. The other mammalia exhibit proofs that they are capable of distinguishing the qualities of sound which relate to speech, that is to say, simple vocal modulations and articulations; for we may observe daily, that they remember the sound and signification of several words. Some are strongly affected by certain sounds. Acute tones produce a painful sensation in dogs, and we also observe that these animals are terrified by violent noises; they therefore distinguish these two properties. Birds have a feeling, no less exquisite, of voice, tone, articulation, and even resonance, since they learn to sing with great correctness; and when their vocal organs permit them, can completely counterfeit the human speech, with all the modifications practised by the individuals they imitate.

As to cold-blooded animals, it is well known that several of them call each other by certain sounds, and that others, which are incapable of producing sounds, can at least understand them, as corps, which appear when the noise of a bell indicates to them that they are to be fed, &c.; but we know not what qualities of sound they distinguish, and how far, in this respect, the delicacy of their sense of hearing extends.

For the morbid affections of hearing, see Medicine, vol. ii., sect. No. 80.

5. Of Seeing.

As we ascend from the simpler to the more complex senses, we find a greater scope for description and observation; but we also find our physiological difficulties increased. The sense of touch being the most simple of all the senses, requires but a simple organization, and is the most widely diffused; that of vision, on the other hand, is the most complex, and requires for its mechanism, a more elaborate set of organs. There is not, in the whole animal structure, a more curious and admirable organ than the eye, whether we contemplate it in its most perfect state in the human body, or in its most simple conformation, as it appears in the horn of a snail.

The anatomy of the human eye has been sufficiently described in the article Anatomy, Part I., chap. vii., sect. 5.; and if our readers desire a fuller account of this organ, we may refer them to the elegant work of Professor Soemmerring. The structure of the eye in the inferior animals is well described in Cuvier's twelfth lecture, and in Blumenbach's Comparative Anatomy, chap. xvi. We shall extract from the former a description of the eyes of insects and crustaceous animals, as being among the most curious and least known subjects of comparative anatomy.

"The structure of the eye of insects is so very different from that of other animals, even the mollusca, that it would be difficult to believe it an organ of sight, had not experiments, purposely made, demonstrated its use. If we cut out, or cover with opaque matter, the eye of the dragon-fly, it will strike against walls in its flight. If we cover the compound eyes of the wasp, it ascends perpendicularly in the air, until it completely disappears; if we cover its simple eyes only, it will not attempt to fly, but will remain perfectly immovable.

"The surface of a compound eye, when viewed by the microscope, exhibits an innumerable multitude of hexagonal facets, slightly convex, and separated from one another by small furrows, which frequently contain fine hairs, more or less long.

"These facets form altogether a hard and elastic membrane, which, when freed of the substances that adhere to it posteriorly, is very transparent.

"Each of these small surfaces may be considered either as a cornea, or a crystalline; for it is convex externally, and concave internally, but thicker in the middle than at the edges, it is also the only transparent part in this singular eye.

"Immediately behind this transparent membrane there is an opaque substance, which varies greatly as to colour in different species, and which sometimes forms, even in the same eye, spots or bands of different colours. Its consistence is the same as that of the pigment of the choroides; it entirely covers the posterior part of the transparent facets, without leaving any aperture for the passage of the light.

"Behind this pigment we find some very short white filaments, in the form of hexagonal prisms, situated close to each other, like the stones of a pavement, and precisely." precisely equal in number to the facets of the cornea; each penetrates into the hollow part of one of these facets, and is separated from it only by the pigment mentioned above. If these filaments are nervous, as in my opinion they appear to be, we may consider each as the retina of the surface behind which it is placed; but it will always remain to be explained, how the light can act on this retina, through a coat of opaque pigment.

"This multitude of filaments, perpendicular to the cornea, have behind them a membrane which serves them all as a base, and which is consequently nearly parallel to the cornea; this membrane is very fine, and of a blackish colour, which is not caused by a pigment, but extends to its most intimate texture; we observe in it very fine whitish lines, which are tracheae, and will produce still finer branches, that penetrate between the hexagonal filaments, as far as the cornea. By analogy, we may name this membrane the choroides.

"A thin expansion of the optic nerve is applied to the posterior part of the choroides. This is a real nervous membrane, perfectly similar to the retina of red-blooded animals; it appears that the white filaments, which form the particular retinae of the different ocular surfaces, are productions of this general retina, which perforates the membrane I have named choroides, by a multitude of small and almost imperceptible holes.

"To obtain a distinct view of all these parts, it is necessary to cut off the head of an insect that has the eyes large, and dissect it posteriorly; each part will then be removed in order the reverse of that in which I have described them.

"In the cray fishes, in general, the eye is situated on a moveable tubercle. The extremity, which is rounded on every side, and sometimes elongated into a cone, when viewed by a glass, presents the same surfaces as the eyes of insects. When we cut this tubercle longitudinally, we observe that the optic nerve passes through it in a cylindrical canal, which occupies the place of its axis. Arrived at the centre of the concavity of the eye, it forms a small button, which detaches very fine filaments in every direction; at a certain distance these filaments meet the choroides, which is nearly concentrical with the cornea, and covers the spherical brush of the extremity of the nerve, like a hood. All the distance between the choroides and the cornea is occupied, as in insects, by white filaments, closely arranged in a perpendicular direction to each other, and which have the extremity next the cornea also coated with a black pigment.

"These filaments perforate the choroides, and are continuations of those produced by the button, which terminates the optic nerve."

The immediate seat of vision is still in dispute; but it appears to be the expansion of the optic nerve upon the inner coat of the eye. The other parts of that organ serve to collect, refract, absorb, and sometimes even reflect, the rays of light, according as these operations are required for the distinct vision of any particular animal. Those animals that seek for their prey during night, have a pupil that is very dilatable, and have very little of that dark substance called pigmentum nigerrimum, that lies between the retina and the choroid coat in diurnal animals. Thus, the former have their eyes better adapted to receive and to retain the feeble rays of light, and thus possess a great advantage over the animals which they pursue, whose eyes are calculated for seeing best in a strong light.

The subject of vision has been so fully considered under Optics, Part I., sect. 5, that it is unnecessary for us to give any detailed account of it here. We shall therefore merely enumerate the principal phenomena.

1. The rays of light proceeding from luminous bodies, are collected by the cornea; variously refracted by the aqueous, crystalline, and vitreous humours, till they meet in a point (in perfect vision) in the retina, from which the sensation conveyed to the brain, excites there the ideas of light, colour, and other qualities of extreme objects, of which the eye is capable of judging.

2. The image of the object thus pictured on the retina, is inverted, though the mind is habituated to perceive it as if it were erect.

3. There is a certain point within the eye where the retina is deficient, and here the luminous rays make no impression.

4. The eye is calculated to see objects most distinctly at certain distances or foci, though these distances vary considerably in different species, and different individuals. A person of ordinary sight can read a middle-sized print most distinctly at the distance of about eight inches. Those who require a less distance are near-sighted, or myopes, and in them the point of divergence of rays is before the retina. Those who require a greater distance are long-sighted, or presbyopes; and in these the point of divergence is behind the retina.

5. In those animals that have two eyes, an image of a luminous object is formed in each, though the mind is accustomed to unite both images into one. In strabismus or squinting, the two eyes not being similarly directed, do not concur in producing a single object.

6. Though the images of many objects are impressed on the retina at the same time, the mind can attend distinctly to only one of them.

7. In perfect vision, the pupil contracts or dilates according to the greater or less quantity of light that is present.

8. When the eye has looked steadily for some time on a circumfribrated space, of a particular colour, as a piece of red paper placed on a white ground, it perceives a border of a different colour surrounding the original spot. This surrounding colour is called the accidental colour of the former, and differs according to the colour of the original spot. In the present instance it is green, or bluish-green. The other natural colours are attended by the following accidental colours, viz. orange, by blue, with nearly an equal proportion of indigo; yellow by indigo, with a mixture of violet; green by violet, with a mixture of red; blue by red, with a mixture of orange; indigo by yellow, with a considerable mixture of orange; and violet by green, with a considerable mixture of blue (B).

(b) Dr Darwin, in his Zoonomia, vol. i. sect. 2, employs the phenomena of accidental colours to prove that the fibres... The exercise of distinct vision depends chiefly on the following circumstances: 1. The perfect transparency of the cornea, and the several humours of the eye; 2. On the just proportional distance between the cornea and the crystalline lens, and on their degree of convexity; 3. On the sensibility of the retina; 4. On the degree of illumination of the visible object; 5. On the colour of the pigmentum between the choroid and the retina; and, 6. On the contraction and dilatation of the pupil.

The action of light on living beings is not confined to its effects in producing vision. It seems to act on the system in general as a moderate but constant stimulus. When the light of day is vivid, as in bright sunshine, the body is more active, and the mind more vigorous, than under a cloudy sky. Those climates which are frequently obscured by clouds and vapours, are notoriously the birthplaces of ferocious and gloomy; and Borean dulness and English melancholy have long been proverbial; while on the contrary, the serene brightness of an eastern sky has been considered as peculiarly favourable to the exertions of imagination, and the flights of fancy. Mr Stuart, a famous pedestrian traveller, told Dr Rush, that during a summer which he passed in a high northern latitude, where the sun is visible for several months together, he enjoyed an uncommon share of health and spirits, which he attributed to the long continuance of the light of the sun. In a state of nature most animals retire to rest when the light fails, and few people can sleep soundly, unless light be excluded.

The stimulating effects of light are peculiarly evident on persons whose nervous system is unusually sensitive; they cannot bear strong light, which not only hurts their eyes, but produces considerable agitation on their whole frame. The same effects are produced on those who have been confined in a dark prison. The countenance of these unfortunate is pale and fallow. This latter effect of the absence of light is similar to what takes place in vegetables, as we know that the colour, taste, and smell of plants depend on their being exposed to a due degree of light.

It has been remarked, that those animals which have been long confined in a dark situation, are universally disposed to grow fat; and this has been found to take place even in condemned criminals, in whom we would least expect it. This obesity has been attributed chiefly to the absence of light. We are disposed to think that the absence of this stimulus can have no immediate effect, but that the disposition to obesity depends rather on the indolence of the confined animals, which is favoured by the absence of light.

For an account of the principal morbid affections of vision, see MEDICINE, No 81.

6. Is there in some animals a sixth sense?

From the experiments of Jurin and Spallanzani on the flight of bats that have been deprived of sight, (see MAMMALIA, No 38.) it has been supposed by some that the accuracy with which these animals in their flight avoided the obstacles that were placed in their way, is owing to some additional sense which they possess. Others have conceived that the sense of hearing, which appears to be very acute in the species on which these experiments were made, is sufficient to supply their want of sight. It is scarcely possible to ascertain which of these two opinions is the more probable; but the writer of this article is rather inclined to adopt the latter, from having observed that when he was walking in an un-frequented street, when it was very dark, he was enabled to avoid running against the common stairs that projected into the street, from a certain sensation that he perceived, when he approached the wall of the stair, which he cannot better describe than by saying that the air at these points appeared to be unusually still.

With respect to sensation in general, we may lay down the following laws, which are considered by Dumas as fundamental principles of this function.

1. As activity is an essential character of sensation, this cannot exist without a certain action of the organs, and must be proportioned to the degree of attention bestowed on the external objects, or ideas by which it is produced.

2. A repetition of the same sensations tends to render the sensibility less acute, and less capable of receiving new impressions. By repose its energy is restored.

3. As sensibility cannot be employed on two impressions at the same time, it must hold a certain balance throughout all the organs, and it cannot be acutely excited in one part, without being proportionably diminished in another.

4. Sensibility is a relative faculty, which is not equally obedient to all kinds of excitations, but only to those which have some relation to it in the different parts of the living body.

5. It is increased and accumulated in the direct proportion to the defect or weakness of stimulus.

6. It is not proportioned to the number, arrangement, or distribution of the nerves, and its changes of increase or diminution are not susceptible of calculation.

7. It is inconstant, variable in its progress, and unconfined.*

To these we may add the following facts respecting this function and its organs.

1. The nerves which are principally distributed to the organs of the external senses, arise from that part of the senforium that is within the head.

2. The sensations produced in any part by the contact of external bodies, are more perfect, according as the nerves which terminate in that part arise more immediately from the common senforium.

3. When a ligature is fastened on a nerve, the parts on which the nerve is distributed are deprived of sensation as far as depends on that nerve.

4. Compression of the brain diminishes general sensation in proportion to its intensity. Slight compression produces numbness.

5. Though sensation probably takes place only in the central parts of the senforium, it is commonly referred to the extremities of the nerves. Thus, a gouty person who has lost his leg, will suppose that he sometimes feels

* Dumas, Principes de Physiologie, tom. ii. p. 151.

fibres of the retina are thrown into contraction, like those of muscles, and that some of them act as antagonists to others; as he considers the accidental as the reverse of the natural colours. Of sensation. Of sensation.

6. A sympathy takes place between those parts which are supplied by branches of the same nerve. Thus, a violent scratching of the head often produces sneezing; powerful odours sniffed at the nose produce a flow of tears; the head sympathizes with the stomach; the mammae with the uterus, &c.

These are all the phenomena respecting sensation which we can at present notice; we shall mention others when we come to consider the relation between this function and those of motion, digestion, circulation, &c.

What have been called the internal senses, as memory, imagination, and judgment, are rather qualities of the mind, than operations of the brain; and the consideration of them belongs rather to metaphysics than physiology. To that article, therefore, we refer the reader; and we shall conclude this account of the phenomena of sensation with the following comparative view of that function in the inferior animals.

In all animals that have nerves, voluntary motions and direct sensations take place by the same means as in man. The differences in their motions depend partly on the intrinsic mobility of their fibres, and partly on the disposition of their muscles, and the parts to which they are attached.

The differences in their sensations depend on the number of their senses, and the perfection of the organs belonging to each sense. The animals that approach nearest to man have their senses equal in number to his. In certain species, some of these senses are even more perfect in the structure of their organs, and susceptible of more lively and delicate impressions than ours; on the contrary, in proportion as animals are removed from us, the number of their senses and the perfection of certain organs are diminished; but perhaps some animals, at the same time, possess senses of which we can form no idea.

We know not whether there are differences in the intrinsic sensibility of the nervous system of different animals, i.e., whether an equal impression made on an organ equally perfect, would affect every animal with the same force.

The animals next in order to man have, like him, spontaneous, or what we call internal, sensations. Images are excited in them at times, when they receive no immediate impression from external objects. Thus, dogs and parrots dream. We are not certain, indeed, that the more inferior animals experience similar sensations.

The passions produce effects in animals similar to those which they excite in man. Love is manifested in the same manner in all classes; fear occasions a discharge of excrements in quadrupeds and birds; it makes them tremble, and even renders insects immovable; but the other animals afford fewer examples of these kind of phenomena than man, because they are not matters of their imagination, cannot direct it towards certain objects, and create for themselves fictitious passions. We are even ignorant whether their imaginations can, like ours, be brought up to such a pitch as to make them experience emotions of anger, desire, or fear, from simple ideas or simple recollections; and whether the real presence of the objects which cause these passions, is not always necessary to excite them in the inferior animals; we know, however, that those which approach nearest to us, the mammalia and the birds, have their fores. The affliction they feel on the absence or loss of a companion, friend, or benefactor, is manifested by evident signs, in the same manner as they testify their attachment without any temporary inducement.

The same animals exhibit frequent proofs of a very perfect memory; some even appear to possess a certain degree of judgment. But does anything similar exist in the inferior classes, and particularly in the lowest? Of this we shall probably remain always ignorant.

With so much resemblance in the structure of the nervous system, in its mode of action, and in the number and structure of the principal external organs, why is there so vast a difference, as to the total result, between man and the most perfect animal?

Is this owing to a more accurate proportion in the relative perfection of the external organs, so that one does not so much surpass another? Or has the internal organ, in which are performed all the intermediate operations between the sensation received and the movement executed, that is to say, the organ of perception, memory and judgment, greater differences than we have yet observed? Or, finally, is the substance by which these processes are effected of a different nature? These, however, are not anatomical questions.

The sympathies or effects resulting from the connections of nerves with each other, and the influence of the nerves on vegetative functions, are subject to the same laws in man and the other animals.*

The theory of sensation is perhaps more imperfect than that of any other function. On this subject we can derive little light from the structure of the brain and nerves, accurately as this has been examined. Anatomy has taught us, that the principal part of these organs consists of very delicate fibres, intermixed with a medullary pulp, and incased in membranes; and that they are furnished with a great proportion of blood-vessels; but whether the seat of sensation resides in the fibrous or medullary part, we cannot ascertain.

It was formerly the opinion, that the nervous power was propagated between the brain and the external organs, by vibrations of the nerves; but as the structure of these chords, and their connection with surrounding parts, must wholly disqualify them for such vibrations, this theory has long been abandoned.

Another hypothesis that has been very generally received is, that the nervous fibres are the conductors of fluid, a very subtle fluid, called the nervous fluid, the motions of which are the cause of sensation. This was the opinion of Dr Haller, (First Lines, chap. x.) and was strenuously maintained by Dr Cullen. We shall present our readers with the following modification of it, as given by an able disciple of Cullen.

"It is probable, (says this writer), that in each nervous fibril, an elastic fluid is inherent, forming, from the moment of animation, a part of it; differing, however, according to the state of the constitution, in power, mobility, and, perhaps, in other qualities. Of this fluid the nerves are conductors, and are surrounded in their course by non-conducting membranes, while the same membrane lines every part of the brain, and is carried into the deepest cavities, guarding with particular attention the slightest aperture. In this view sanguineous vessels are chiefly useful in nourishing this medullary substance, and they appear to be necessary also in adapting..." the nerves to their office; for, when the circulation is greatly increased, the sensibility is more acute; and when it languishes, or is destroyed, the nervous energy soon shares the same fate.

"This fluid must necessarily be an elastic one; and impressions are apparently conveyed through it by vibrations. It does not follow hence, that the nerves vibrate like muscular cords; or that, in every slightest motion, a portion is conveyed from the brain. The elasticity of the fluid is proved from the momentary continuance of the impression after the cause is removed; and vibration is a term employed in many branches of philosophy as a means of communicating motion, without any distinct application. If we touch an object with a stick, or with a metallic rod, we perceive through it the impression, and, in a general way, the nature of the substance. The impression must be conveyed by something; and whatever that something is, it may as well convey impressions through the nerves as through the rod. But through the nerves only can it affect the brain, and produce an idea, or some change in the brain, or its fluid connected with the nature of the object, and which conveys to the mind some peculiar and discriminated impression which it afterwards retains."

A third hypothesis, which is at present very fashionable, is, that sensation is produced by a change in the substance of the brain and nerves. M. Cuvier is an advocate for this doctrine, which he illustrates in the following manner.

The nervous system is susceptible of two kinds of action; one which is confined to our sensitive faculty, and another which affects our vital and vegetative functions only. External sensations are produced by the impressions of external bodies, on our senses; internal sensations, by changes which take place, in the state of the internal parts of the body, to which the nerves are distributed; and spontaneous sensations are caused by a change in the nerves, or in the brain itself, without any external excitement.

These circumstances, added to the phenomena arising from the cutting or tying of nerves, show, that sensation does not reside in the external organs, but nearly in the centre of the nervous system, and that the external organs serve only to receive the action of the external bodies, and to convey it to the nerves, by which it is propagated to a greater distance. They also demonstrate, that this propagation is not produced by any matter or concussion, but by a change in the state of the nervous substance. This change may arise from internal causes, or it may be produced by external causes, different from those which usually occasion it. The nerves are not merely passive agents, nor the conductors or reservoirs of any particular matter; but it appears, that the substance which produces sensation, is liable to be consumed, or to lose its activity by exertion.

There are phenomena which show that the general susceptibility of the nerves, for receiving sensations, may vary in consequence of causes external to the nerves themselves, and which can operate only by altering their substance. Certain medicines weaken or revive that susceptibility;—inflammation frequently increases it to an excessive degree. Does this take place in consequence of an increased secretion of the nervous matter? The most remarkable change that occurs in the susceptibility of nerves, is sleep. It is not unnatural to suppose that this change may be occasioned by the temporary loss of the substance which is essentially sensitive. But how does it happen that sleep depends, in a certain degree, on the will? Why do we awake suddenly, or from causes which do not appear calculated to restore that substance? Why does cold produce sleep? From these observations, may it not rather be supposed that this state is the effect of a change in the chemical nature of the nervous substance?

But whether the substance contained in the nerves is exhausted by sensations, or whether it merely undergoes an alteration in its chemical composition, and becomes, as it were, neutralized, it must remain in the nerve throughout the whole of its course, and leave it only at one of its extremities. It does not, however, resemble the blood in the vessels, either as to the manner in which it is retained, or in which it moves in the nerve. There is no evidence of the nerves being tubular. No phenomena indicate that any matter escapes from them when they are divided. Besides, what vessels could have parietes sufficiently compact to retain so subtile a fluid as that of the nerves must be. It is far more probable that it is retained in the nerves, in the same manner as the electric matter is in electric bodies, by communication and inflation; and that the nervous system is its only conductor, while all the other parts of the animal body are, with respect to it, cohabitant substances.

The theory of sensorial power, brought forward by Dr Darwin, has already been noticed.

**CHAP. III. Of Irritability.**

1. When any part of a living animal body that contains muscular fibres, as a part of its composition, is touched with a sharp instrument, with a hot iron, or of irritability, with a corrosive liquor, or when a shock of electricity or galvanism is made to pass through it, a contraction takes place in the part; and this contraction is discontinued when the stimulus is removed, but is renewed on repeating the application.

2. The same contractions take place in certain parts of a living animal body, from an exertion of the will.

3. Many parts in which the presence of muscular fibres has not been ascertained, possess the same capacity of being excited to motion by stimuli. Such are the ureters, the biliary ducts, the small blood-vessels, and probably the lymphatics; all of which, though not evidently muscular, have a fibrous structure.

4. Some parts of the living animal body which appear rather nervous than muscular, possess a contractile power, as the retina.

5. When the nerves which form a communication between a contractile part and the brain, in the higher orders of animals, are divided or compressed, those parts which before contracted in obedience to the will, lose this power; but,

6. These parts, as well as every muscular part, still contract on the application of stimuli, particularly electricity and galvanism.

7. Such parts of an animal body as have muscular fibres, are thrown into contraction on the application of stimuli, for some time after having been separated from the living body, provided that nervous filaments remain connected with the muscular fibres.

8. It has been found, that the fibrine of the blood is susceptible of contraction on the application of the galvanic stimulus, after having been separated from the living body.

9. In some animals in which a nervous system has not been detected, as polypes, this contractile power seems to pervade every part of the animal.

10. Plants, in a greater or less degree, possess the power of moving on the application of stimuli; and in some species this motion is very remarkable. See No. 57.

The above are some of the principal phenomena which take place in organized beings with respect to irritability. They are so analogous, that we may attribute them to the same cause or the same vital power. This susceptibility of being thrown into contraction on the application of stimuli is called irritability; and it is possessed in a greater or less degree by every organized being with which we are acquainted.

We have restricted the term irritability to denote the susceptibility of the fibrous structure to contraction on the application of stimuli; but it is proper to remark that this term has not always been used in the same sense.

Irritability has long been employed in medicine, as in common language, in reference to the passions, especially that of anger; and this appears to have been the original meaning of the term.

Multa fero ut placem genus irritable vatum. Hor.

It is perhaps still more common to apply it to a morbid sensibility of the system; and we speak of a person being of a very irritable habit, or possessing a great degree of irritability, when we mean to say that he possesses a more than ordinary share of sensibility, liable to a more keen sensation of the same impressions.

"Or are your nerves too irritably strung."

ARMSTRONG.

Even the accurate Dr Whytt, to whom the proper distinction between irritability and sensibility must have been familiar, and by whom it is in general strictly regarded, sometimes falls into this inaccuracy. He speaks, in his work on nervous diseases, of "a delicate or easily irritable nervous system." In fact, this confusion of irritability with sensibility, appears to be a stumbling block to most physiological writers. We shall presently inquire how far they are independent of each other.

The term irritability, in its most received acceptation, as a property of the muscular fibre, seems to have been first employed by Giffon, about the middle of the 17th century. He distinguishes two kinds of irritability, primary or direct, and secondary or sympathetic *. Haller was, however, the first who treated of irritability with any degree of accuracy. He confines it to the muscular fibre; though at the same time he will not allow it to many parts, the muscularity of which has never been questioned, and which, since his time, have, by decisive experiments, been proved to possess a considerable degree of contractile power. He completely distinguishes irritability from sensibility, with which he will have it to be totally unconnected; and he attempts to make a distinction between the irritability of the living, and that of the dead fibre †.

Dr Whytt, whose controversy with Haller respecting the nature of irritability and sensibility is famous in the annals of medical warfare, admits three kinds of irritability: 1. That power of alternate contraction and dilatation which is peculiar to those organs we call muscles; 2. That uniform contraction which takes place in the darts (one of the coats of the scrotum) and the pores of the skin; and, 3. That reduces and inflammation, which is excited in every sensible part of the body, as often as acid things are applied to it; although this last is allowed by him to be only an effect of the first kind of irritability taking place in the small vessels of the part ‡. Thus, he reduces the three kinds to two, and we may perhaps consider his second kind only as a modification of the first.

Among those who seem to have a sufficiently just idea of the nature of irritability, the word itself is not unfrequently misapplied. Thus, Vicq d'Azyr ||, and Dumas §, in enumerating the functions of the animal body, called those of motion and sensation, irritability and sensibility. These latter are powers or capacities of living beings, and as such should be distinguished from the functions that depend on them.

In considering the phenomena of irritability, it is necessary to take notice of the several kinds of stimuli which excite it. There have been reduced by Cuvier to five orders, viz. volition; external actions operating on the nerves; external actions operating on the fibre itself; mixed actions operating on both the nerves and fibres, and certain diseases or violent emotions.

When the animal body is in a state of health, and volition awake, the will exercises a prompt and constant influence over the greater part of the muscles, which, on that account, are denominated voluntary muscles. A small number of muscles, viz. those which produce the internal movements necessary to life, and which cannot be interrupted, such as the heart and the alimentary canal, are not subject to the will. It must be observed, however, that some of the muscles, that in man and most other animals are involuntary, are subject to the will in others. This is the case with the stomach in ruminating animals, the movements of which may be exerted at pleasure in two different directions. In some muscles, as in those of respiration, there seems to be a mixed action with respect to the will, as this faculty can interrupt their motion for a time, though, in general, this is continued from habit, without the will, or even consciousness of the animal. Those muscles that are absolutely involuntary, are continually excited by an extraneous irritating cause; for the blood which is brought to the heart on every dilatation, determines that organ to contraction, and the alimentary canal is affected in the same manner by its contents. It seems, therefore, that the will is not essential to the action of these muscles, and that it cannot interrupt their motion (c).

(c) There are facts which show that the will has often considerable influence even on muscles that are universally styled involuntary. The abbe Fontana, when making experiments with wheel polypes, was led to believe that the heart, in these animalcules, is a voluntary muscle, and from this belief he learned, in some degree, to accelerate... A muscle laid bare, and exposed to an irritating cause, will contract itself, even in the living subject, without being influenced by the will. It should seem, therefore, that though the muscles which we call voluntary, are usually put in motion by the will, they may yet be excited to action in opposition to that faculty.

The will itself seems to act only through the medium of the nerves; and it is found that those nerves which supply the voluntary muscles, are generally the largest.

The external stimuli that act on the muscular fibre through the medium of the nerves, and on the fibre itself, are chiefly of a mechanical and chemical nature, as concussions, punctures, lacerations, all of which are capable of producing convulsive motions in all the muscular parts to which the nerves extend.

One of the most remarkable of these stimuli is the galvanic influence. It is well known that the experiments by which this influence is made to act on the muscular fibre, consist in establishing between a muscle and the trunk of the nerves which extend to it, an external communication with one, or a series of substances placed close to each other. Metals are not the only means that may be employed in this operation; and in general, the conductors are not the same as those of electricity. Experiments have sometimes been successfully performed, when an interval was left in the series of exciters: this circumstance, in the opinion of Cuvier, proves the existence of an atmosphere.

The moment the contact takes place, the muscle suffers violent convulsions. These experiments succeed on the living body, or animals recently dead, and even on parts separated from the body, precisely in the manner of those which Haller accounts for on the principle of irritability. Neither pointed instruments nor acrid liquors are necessary; and the galvanic experiments even succeed when these means have failed.

Diffusion has been observed to have a powerful effect in exciting irritability.

Violent passions may, to a certain degree, be considered as the acts of the will strongly excited. These, in some cases, have an influence even on the involuntary muscles; for it is no unusual thing for palpitation of the heart, and sometimes even a fulspection of its motion, to be the consequence of strong passions. These actions, however, are to be prevented by moderating the excess of sensibility by which they are occasioned. Even in nervous diseases, which appear to be the least connected with those passions whose influence is more immediately felt, the will is often capable of preventing or retarding the approach of nervous symptoms, when the patient is determined to resist the paroxysm.

From what has been said, it appears that, in the superior classes of animals, all the orders of stimuli, either act through the medium of the nerves, or that they are capable of being modified or controlled by the will, the exertion of which depends on nervous influence.

With respect to the immediate cause of irritability, there have been several opinions. One of those which has been most generally received is, that irritability is intimately connected with sensibility; or, that it is an immediate effect of the nervous power. This was the opinion of Whytt and Cullen, the former of whom endeavours to prove it by the following arguments.

1. We almost always observe the irritability of the Whytt's muscular organs of the human body to bear a proportion to their sensibility. Thus, children, and people of delicate nerves and very quick feelings, are most subject to convulsive and spasmodic difficulties, while on the other hand old people, and those of less delicate sensibility, have a muscular system that is not so irritable.

2. Whatever increases the sensibility of the muscles, also increases their irritability.

3. Whatever lessens or destroys the sensibility of the muscles, also lessens or destroys their irritability or power of motion.

4. That the motions of irritated muscles are owing to the sensation excited by the stimulus applied to them, Dr Whytt thinks highly probable, if it be considered that we are in fact conscious of many involuntary motions in our own bodies, proceeding from a particular sensation, either in the organs moved, or in the neighbouring parts.

Dr Cullen was so fully convinced of the necessity of a nervous influence to produce muscular contraction, that he considered the muscular fibre to be only a continuation of the nervous fibre. See MEDICINE, No. 73.

Haller, as we have said, strenuously maintained, that Haller's irritability was quite independent of the nerves, and was an inherent power or vis insita of the muscular fibre. Indeed there are several circumstances which would induce us to believe that irritability is at least, in some cases, independent of nervous influence. We have seen (No. 111.) that it takes place in those animals in which there is no appearance of nerves; and that it is very remarkable in some species of vegetables, in which none but the most fanciful physiologists have dreamed of finding a nervous system. Nay, it appears that the fibrine of the blood, which we can scarcely suppose to be affected by the nervous power, when taken out of the body, is still susceptible of irritation.

From a comparison of all these circumstances, we must either conclude, that the irritability of living muscles, and of the superior animals, is different from that of the fibrine, of polypes and plants; or, if we admit that nervous influence is essential to irritability, we must also allow that this influence descends to the latter class of organized bodies.

Before we quit the subject of irritability, we must notice the chemical hypotheses that have been lately proposed, to explain the immediate cause of this faculty.

The first of these is that of Girtanner, who considered oxygen as the principle of irritability. The Girtanner's arguments are opinions. arguments on which he founded this opinion are the following:

1. The irritability of organized bodies is always in a direct ratio to the quantity of oxygen they contain.

2. Every thing that augments the quantity of oxygen in organized bodies augments at the same time their irritability.

3. Every thing that diminishes the quantity of oxygen diminishes likewise their irritability.

He distinguishes the organized fibre by three different states:

1. A state of health, or the tone of the fibre, in which the oxygen exists in its proper quantity.

2. A state of accumulation, in which the fibre is overcharged with the oxygen or irritable principle.

3. A state of exhaustion, in which the fibre is more or less deprived of it.

He likewise arranges the substances, that are capable of coming into contact with the irritable fibre, into three classes.

The first comprehends those substances that have the same degree of affinity for the irritable principle or oxygen, as the organized fibre itself; hence the substances produce no effect upon it.

The second comprehends those substances that have a less degree of affinity for oxygen than the organized fibre has: hence these, when they come into contact with it, surcharge it with oxygen, and produce a state of accumulation. They are called negative stimuli.

The third comprehends substances for which oxygen has a greater affinity than it has for the organized fibre. These, therefore, deprive the fibre of its oxygen, and produce a state of exhaustion. They are called positive stimuli.

By way of answer to this fanciful doctrine, we may observe, that if oxygen were so essential to irritability as is supposed in Girnanner's positions, those animals which respire most oxygen should possess most irritability, and those which are capable of living for a long time in deoxygenated air, should have their irritability very low. Now, the reverse of this is found to take place. The muscular fibres of birds which respire more oxygen than most other animals, possess but little irritability, while reptiles and worms, which can live for a long time without oxygen, are universally and strongly irritable.

The other opinion is that of Humboldt, who considers the galvanic fluid as the source of nervous power, and the primary cause of irritability. He lays down three principles as necessary to excite irritability; viz.

1. Oxygen, which forms combinations with different acidifiable bases.

2. The acidifiable bases (carbon, hydrogen, azote, and phosphorus), of the fibre, with which the oxygen may combine. And, 3. The galvanic fluid.

The galvanic fluid produces, according to Humboldt, the same effect in the animal economy, as the electric fluid in the mixture of azote and oxygen. It is this galvanic fluid that, being conveyed by the nerves, brings about the combinations of the oxygen with the different acidifiable bases of the fibres; but when the nerve of a part is tied, it prevents the fluid from passing, which explains the reason of the irritability being destroyed.

The oxygen necessary for these unions is carried by the arterial blood in the course of circulation; and the acidifiable bases, which are to unite with it, are found to be already present in the fibre.

He found that every thing that augments too much the quantity of the acidifiable bases diminishes the irritability; and that every thing that increases too much the quantity of oxygen, likewise diminishes it; and he thinks it very probable, that the same takes place with respect to the proportion of the galvanic fluid.

It is therefore only in a just equilibrium of these principles that the necessary irritability of the parts results.

Upon these principles this philosopher thus explains the production of muscular motion. "In a state of repose, the nerve being inserted in the muscles, the galvanic fluid is put into equilibrium in organs that touch each other. The spontaneous motion is made by a surcharge of galvanic fluid into the nerve. It appears that the instant we wish to make a motion, the galvanic fluid produced in the brain, is carried en masse towards the part that ought to move, and surcharges the nervous fibres. A discharge from the nerve is then made into the muscles. The particles of these last, animated by increased affinities, approach each other, and it is this that constitutes the phenomena of muscular motion."

Dumas lays down the following fundamental laws respecting animal irritability.

1. The essential characteristic of irritability consists in a series of contractions and dilatations, determined either by the impression of an external stimulus, or by the simple exertion of the will.

2. Irritability is independent of the action of the nerves; and though generally diffused throughout the animal organization, it belongs rather to the muscular fibre than to any other structure. Its action is in proportion to the number of fibres upon which the irritating causes can exert their influence.

3. Irritability is a relative faculty which is not indiscriminately obedient to every species of excitation, but only to those which have some relation to it in the different parts of the living body.

4. There belongs to each organ a specific irritability which requires a peculiar stimulus, accommodated to its nature, and to the kind of functions which it exercises.

5. Irritability has certain vicissitudes of diminution and increase, which vary in the different species of animals, in the different organs of the same animals, and under the different circumstances that successively occur in the life of an individual.

6. Irritability is developed with most energy at the moment of death, and immediately after this has taken place.

7. It is multiplied and revived in proportion as the organ which has lost it is divided into a greater number of pieces.

8. It diffuses itself in each part with a velocity proportioned to the activity, number, and duration, of the irritations by which it is excited.

9. There exist mutual relations with respect to influence between sensibility and irritability, though each of them is essentially distinct from the other.

10. The exercise of this faculty supposes in the organs a moderate degree of cohesion, above or below which the action of this force is enfeebled, obstructed, or opposed." The organs of motion vary considerably in their nature and connection in the different classes of animals. In some tribes, as in the animalcules and polypes, no distinct organs can be observed. In all above these, however, there are evident muscular fibres, and in many there are hard parts or strong membranes, which serve as points of attachment and fulcrum of motion to these fibres. The muscular fibres are to be considered as the essential moving organs, while the parts to which they are attached are merely the passive functions of this organ. It would be out of place here to enter on a comparative account of the organs of motion; and there is the less occasion for it, as they have been more or less fully described in the former part of the work. The bones, ligaments, muscles, and tendons, with their appendages, as they appear in man, have been amply described in the first and second chapters of the First Part of Anatomy; and those of other animals have been briefly noticed in the Second Part of that article. Such of our readers as wish for a more particular account, may consult Cuvier's Lectures, vol. i. or Blumenbach's Comparative Anatomy, chap. 1, 2, 3, 4, 5, and 22.

Many of the phenomena of muscular motion, as they take place in man, have also been related under Anatomy, No. 85 and 86. We shall here therefore only enumerate and briefly illustrate these phenomena, and shall then proceed to consider a most interesting part of the physiology of motion, the progression of different animals.

Dr Barclay, in his late excellent work on the muscular motions of the human body, has considered the general subject of muscular action under the following heads, which may be considered as fundamental principles.

1. Fleshy fibres that are continued into tendon by a straight line, shorten the muscle which they compose, in the same degree in which they shorten themselves; those fibres which enter the tendon obliquely, shorter it more, and still more in proportion to their degree of contraction, as they deviate more from the line of the tendon, and approach nearer to the perpendicular, in which last direction they would shorten the muscle most with the least contraction.

This may be illustrated in the following manner. Let AB (fig. 1.) represent a tendon, and CD a fleshy fibre; and let us suppose that AB is the diameter, and CD the radius of the same circle ADB. It is evident that if the fibre CD should contract so as to bring the point C of the tendon to the point G in the straight line, the extremities of the tendon A, B, (which are supposed to be moveable) would come respectively to E and F; and the situation of the tendon itself would be represented by the angle EGF. If the fibre could be supposed to contract so as to bring the point C to D, the two parts of the tendon CA and CB, would come in contact. If, on the other hand, the fibre CH, which enters the tendon obliquely, were to contract to H, so as to bring the point C to H, the point A would be drawn but a little beyond the middle point C, so that although this latter fibre is contracted to as great an extent as the former, it has not brought the extremities of the tendon so near together.

2. When two fibres enter a tendon on opposite sides and contract at the same time, they will draw the tendon in the diagonal, and the more nearly the angles which they form with the tendon approach to right angles, the more will the length of the muscle be shortened in proportion to the degree of contraction of the fibres.

Let the fibres BC, BD, BE, BF, BG, (fig. 2.) be fleshy fibres, inserted into the tendon AB, at the point B, and let us suppose that all these fibres co-operate in bringing the point B to the point G, in the straight line BG. Now the straight fibre BG will be so much shortened when B comes to G, as to be obliterated, while the oblique fibres EB and FB will be shortened only to Ea and Fa, and the more oblique fibres CB and DB will remain of the length of Cc and Dd.

3. All muscles that are inserted into bones, are thereby furnished with levers, and as in the action of all levers there are also a fulcrum, a power, and a resistance, these in different cases will be differently situated with respect to one another.

a. In the motions of the head backward and forward on the atlas, the fulcrum is situated between the power and the resistance; or the lever is of what is called in mechanics, the first kind. See Mechanics, No. 33.

b. When the tibia rests upon the astragalus, and the heel is raised by the muscles of the calf of the leg acting on the tendo achillis, the resistance (which in this case is the prelude of the tibia) is situated between the power and the fulcrum, which are here respectively at the heel and at the toes; or the lever is of the second kind.

c. In raising a weight at the palm of the hand, and bending the arm at the joint of the elbow, the power of action in this joint is situated between the resistance and the fulcrum, which are here respectively at the palm of the hand and the distal extremity of the humerus (D), or the lever is of the third kind.

The shortness of the lever, and consequently the great force of the muscular power required to overcome the resistance in this last case, may be thus illustrated. Let AB (fig. 3.) represent the radius articulated at B with Fig. 3., the humerus BC; let DFE represent the biceps flexor muscle running along the humerus, and attached to the radius at E; and suppose a weight W hung to the distal extremity A of the radius. Now, BH will represent the lever of resistance, and BG perpendicular to it the lever of the muscle, which is in this case extremely short.

4. As, other things being equal, all muscles produce a greater extent of motion by a less proportional degree of contraction, and consequently a less proportional change in their fibres, than if they were shorter; those muscles which follow a direct course are seldom attached at the nearest points of the two bones with which

(D) In Dr Barclay's nomenclature, that extremity of a bone which is towards the trunk is called proximal, and that extremity which looks from the trunk is called distal. which they are connected. Hence, beside the advantages already mentioned, relations are thus formed between parts at a distance, and the mutual dependence of the functions and their organs is extended and strengthened. On the contrary, those muscles that are not extended along the surface of the bones to which they are attached, are observed to follow an oblique direction, by which they acquire not only contractibility and length, but at the same time a shorter lever than if they had been inserted at the same place with a less obliquity.

5. Of muscles attached to ribs that are parallel, equally moveable, and at right angles to the vertebral column, those that follow a direct course from one to the other, will act on each by equal levers, and make them approach with the same velocity; while those that observe an oblique course will act on each by different levers, and make them approach with different velocities.

Let AB and CD (fig. 4.) represent two parallel ribs, articulated with the vertebral column at A and C, where they are equally moveable; and let DB and DE be two muscles, the former observing a direct, and the latter an oblique course. The levers of DB will be AB and CD, which, as AC is parallel to BD, are evidently equal; but the levers of DE will be CF and AG, which being of different lengths, the muscle must act with different degrees of force on the different ribs, so that it will make CD, on which it acts with the longest lever, approach AB, faster than it will make this latter approach the former.

Corollary.—When bones are not parallel, the muscles that cross in the interval between them, must fall obliquely on both, as it is impossible for a straight line to be at the same time perpendicular to two other lines, unless these be parallel.

6. As all bones move on a centre or axis of motion, while the muscular attachments move in a circumference, the muscles, in changing the relative position of any two bones, must, at the same time, be changing the direction of their own action, and varying their lever.

Let AB and CD (fig. 5.) represent parts of two parallel ribs, and let AB be moveable on the centre A; and let CF and GE be two muscles inserted obliquely into AB at F and E. Now suppose that by the action of these muscles, AB is brought into the position A b. The points of attachment of the two muscles to AB, will now be f and e, and the muscles will be C f and G e, having changed their length, situation, obliquity, and lever.

7. All muscles where the points of attachment move in a circle, draw either towards the centre, or towards the circumference.

8. If any two bones could, by the action of their muscles, be made to approach in a parallel direction, the oblique muscles attached to their parallel and approaching surfaces, would perform a greater extent of motion with a less shortening of their fibres, than any straight muscle attached to the same parallel surfaces.

Let AB and CD (fig. 6. and 7.) be parts of two ribs that are parallel, and that will continue parallel till they are brought in contact by the action of the straight muscles AC, EF, and BD, or by the action of the oblique muscles CE and DE (fig. 7.) and FA and FB (fig. 6.). It is evident, that when the point E comes in contact with F, the length of the straight muscles must be obliterated, while that of the oblique muscles will only be shortened by c E and d E in fig. 7. and f A and g B in fig. 6.

9. As, however, no two bones can approach one another in a parallel direction, at least by the action of a single muscle, and as no muscle can continue to act in a direction perpendicular to their two approximating surfaces; a muscle entering them at right angles, when they are parallel, may be placed so near to the centre of motion as to carry the bones through a given space, with a less shortening of fibres than any oblique muscle that has the same origin, but is inserted at a distance, and acts through the medium of a longer lever. Further, a muscle with a less obliquity may be fitted as to carry the bones through a given space, with a less shortening of fibres than any other muscle of the same origin, but of a much greater obliquity.

Let AB and CD (fig. 8.) be two ribs, of which AB is moveable about the centre A; and suppose that by the shortening of the straight muscle EF, and of the two oblique muscles, EG and EH, AB is brought into the position A b. The points of attachment, after moving in the segments Ff, Gg, and Hh, will now be respectively at f, g, and h. Now, on the centre E, with the radii Ef, Eg, and Eh, describe three different circular segments. The difference between the present and former lengths of the most oblique muscle EH, will be eH, while the differences between the present and former lengths of the muscles EG and EF, will be only G and F respectively.

10. The shortenings which any muscle suffers in carrying round the point of its attachment through a given space, will partly depend on the length of its lever, partly upon its degree of obliquity, partly on its drawing peripheral or central, and partly on its acting without or with a pulley (E).

11. The lever of a muscle, which is varied with every degree of obliquity, is also varied by every change in the centre of motion. Where bones are connected by large surfaces, the centre of motion frequently shifts from one part to another; but in general it approaches towards that aspect whither the bone is moving at the time; and as it advances, the muscles recede, to increase their force.

a. The lever of resistance, as well as of the power, is varied by the several changes of position; is sometimes shortened at the time that the lever of the power is lengthened; and vice versa.

(e) The terms peripheral and central, are employed by Dr Barclay, to denote the aspects of any organ, according as they respect the circumference or the centre of the organ; and when the termination of these words is changed from l into d, they denote, like the other terms of his nomenclature, the direction in which the action of these parts is exerted. See Barclay's Anatomical Nomenclature. If \( AB \) (fig. 9.) represent the radius, \( BC \) the humerus, \( DE \) the biceps flexor muscle, and \( R \) the resistance hung to the distal extremity of the radius, it will be evident that, when \( BA \) is, by the action of the flexor muscle, brought into the position \( BA \), the lever of resistance will no longer be \( BA \), but \( BH \), equal to a perpendicular straight line drawn from \( B \), the centre of motion, to the plane of resistance; and, as the lever of resistance has been shortened, the lever of the muscle has been proportionally lengthened. Were the radius to resume its former position, the reverse of these circumstances would take place.

b. Sometimes again, the lever of the power and of the resistance are lengthened or shortened at the same time.

Let \( AB \) (fig. 10.) represent the tibia, \( BC \) the femur, and \( DEF \) the cruralis muscle; and that the femur, with the weight of the body, is to be raised to the situation \( BC \); the centre of motion will, during extension, approach towards the muscle at the rotular aspect, while the plane of resistance, as is evident from the figure, will be approaching to the centre of motion.

c. In the changes of attitude, while a bone is turning on its centre of motion, the centre itself is often at the same time describing, either the segment of a circle, or a line composed of circular segments.

Let \( AB \) (fig. 11.) represent the foot, \( BC \) the tibia, \( CD \) the thigh bone, and \( DE \) the trunk; and let us suppose that it is required to bring the three last, by the action of their muscles, to the perpendicular \( BF \), so that \( BC \) shall occupy the situation of \( BG \), \( CD \) the situation of \( GI \), and \( DE \) the situation of \( IF \); the point \( C \) on the centre \( B \) will move in the segment \( CG \), and as \( C \) is changing its position in \( CG \), the point \( D \), which moves round the point \( C \) as its centre, will, if the extensions be regularly performed in the same time, describe such a curve as \( DI \); for as the point \( D \) must necessarily move aftward and afterward, (\( F \)) in order to preserve the centre of gravity, the general direction of its course must be known; and if \( CG \) be divided into equal parts, and at each of the divisions a circle described with the radius \( CD \), the points in \( DI \) corresponding in number with the points in \( CG \), and at equal distances in the lateral direction, will each be found in the circumference of one of the circles described successively round the point \( C \) as it passes along the segment \( CG \).

In like manner, if the extensions of \( CD \) and \( DE \) be regularly performed in the same time, the point \( E \) will describe such a curve as \( EF \), the points in \( EF \) being in the circumferences of the several circles successively described round the point \( D \) as it moves along the curve \( DI \).

12. When we examine the structure of the animal system, we shall generally find that the motions of the bones, as produced by the muscles, are the combined effects of different forces, and hence that a small number of muscles is enabled to produce, with steadiness and accuracy, an almost infinite variety of changes.

For more on the general subject of muscular action, and for an account of the principal motions of the human body, we must refer to Dr Barclay's publication.

One of the most interesting enquiries respecting animal motion, is that of the progression of different animals, or of the powers of loco-motion.

Those animals which possess the faculty of changing their place, exercise this faculty by very different organs. Some can only creep, as worms, and many mollusca; others can only swim, as all fishes, many of the mollusca, and some of the telesacea. Most birds can both fly, walk, and run, while a few do not possess the power of exercising the first of these motions. All the mammalia, and most reptiles, properly so called, can walk, run, climb, leap, and perform a variety of other motions; and a few of the former class can imitate the flying of birds. We shall briefly examine the mechanism of these different actions, but by way of introduction, we shall first consider how the action of standing is performed.

Standing, in most animals, is solely the effect of the continued action of the extensor muscles of all the joints, as is evident from the circumstance, that if an animal, while standing, suddenly dies, or in consequence of some powerful cause, as a strong electric shock, ceases to make the necessary efforts for preserving the upright position, all the articulations of the legs yield to the weight of the body, and bend under it. In some animals, however, the extension of the muscles is so much assisted by powerful ligaments attached to the articulations of the legs, that they are enabled to continue standing for a much longer time, and with much less fatigue than most others. This is the case with birds that perch, and it is particularly remarkable in the stork, which by means of this peculiar mechanism is able to stand on one foot for several days together.

The action of standing is somewhat different, according as the animal stands on two feet or on four.

That a body may be supported in a vertical position, standing on equilibrium, or that it be so disposed as to be in a state of two feet.

See Mechanics, No. 193, et seq. It is evident that the more extensive the base is on which the body stands, the less is the danger of its losing its balance. Man can very easily preserve himself in the vertical position, from the broad basis formed by his feet, and from the great power he possesses of separating these to a considerable distance. This latter depends chiefly on the greater weight of his pelvis, and the length and obliquity of the neck of the thigh bone, by which this bone is carried more outward, and removed farther in its articulation, than in any other animal. In man, too, the foot is peculiarly adapted to stand firmly on the ground, from the flatness of its inferior surface, and from having the heel bone so formed as to come in perfect contact with the ground. The muscles that move the foot are also very advantageously inserted, and the extensor muscles of the neck are proportionably thicker than in most of the mammalia.

The thigh of man, when in the erect posture, is in a straight line with the trunk and the leg, whereas in quadrupeds, it is situated close upon the flank, and forms an acute angle with the spine. On this account, the thigh bone of quadrupeds is flat, and proportionally weaker than that of man. The extensor muscles of the thigh are proportionally stronger in man than in the other animals; and as the thigh bone moves upon the pelvis in every direction, these extensors are in man so considerable, that he is the only animal that possesses what are properly called hips.

In consequence of this structure, the human facial extremities are furnished with a sufficient base, and form Of Animal very solid bodies for supporting the trunk. Man also Motion.

possesses several advantages for maintaining the general equilibrium of the body, especially the facility with which he holds his head in the erect posture, owing to the position of the occipital bone, and the horizontal direction of the eyes and mouth. See the article MAN, N° 5, and 6.

The quadrupeds that sometimes try to stand on their hind feet only in order that they may either employ their fore feet in taking hold of some object, or avoid keeping their head too low, seem rather to fit than to stand. Their trunk rests at the same time on their hind feet, as far as the heel, and on the buttocks; it is still necessary, however, that their head and neck should be proportionally small, as in monkeys, squirrels, opossums, &c., otherwise the weight of those parts would be too great for the force employed in their elevation; but even when seated, the animal is generally obliged to rest on the fore feet, as may be observed in dogs, cats, &c.

Some quadrupeds use their tail as a third foot, to enlarge the base of the body: and when it is strong, it is capable of contributing to their support for some time. We find examples of this in the kangaroos and jerboas.

We have already noticed the mechanism in the feet of birds, which enables these animals to support themselves on two legs, though they do not stand in a vertical position, and though the atlantal part of their bodies is advanced more beyond the centre of gravity than the sacral part. Other advantages possessed by birds in this respect are, the great flexion of the thigh bone and tarsus; the length of the anterior toes, and the length and flexibility of the neck.

An animal which stands on four feet is supported on a very considerable base; but from the great weight of the head and neck in these animals, their centre of gravity is nearer to the atlantal than to the sacral extremities (f). It is evident from this, that in quadrupeds, the former must sustain almost the whole weight of the body; and we find, accordingly, that they are furnished with very strong muscles. In short, all that the sacral extremities seem to want in muscular force, appears to be transferred to the atlantal.

As in most quadrupeds the head inclines towards the horizon, and the neck is often very long, very powerful means are required to sustain the former. These means are furnished by the great size, and extensive attachments of the muscles of the neck, and especially in many quadrupeds by the cervical ligament. In the mole, which employs its head to raise considerable burdens of earth, the cervical muscles are peculiarly strong, and the ligament is converted into bone.

The body of a quadruped hangs between the four legs, and by its weight tends to draw the spine downwards. This is counteracted by the abdominal muscles, especially by the straight muscles, which produce a curvature in the opposite direction. The abdominal muscles act with peculiar force in arching the spine upwards in those mammalia that are covered with scales or spines, and are accustomed to roll themselves upon the approach of danger, as the hedgehog, the armadillos, and the pangolins.

Oviparous quadrupeds or reptiles, have their thighs directed outward, and the inflections of the limbs take place in planes that are perpendicular to the spine. In these, therefore, the weight of the body must act with a much longer lever, in opposing the extension of the knee-joints; and accordingly they have the knees always bent, and the belly dragging on the ground between their legs, whence the name of reptiles.

In walking on a fixed surface, the centre of gravity Walking is alternately moved by one part of the extremities, and sustained by the other, the body never being at any time completely suspended over the ground.

Animals which can stand erect on two legs, such as man and birds, walk also on two legs. But several quadrupeds that cannot stand on two feet but with great difficulty, may yet move in that posture for some time with sufficient ease. This arises from its being in general less painful to walk than to stand, the same muscles not being continued so long in action. And also it is less difficult to correct the unsteady motions by contrary and alternate vacillations (a thing easy in walking), than it is to prevent them altogether.

When man intends to walk on even ground, he first advances one foot; his body then rests equally on both legs, the advanced leg making an obtuse angle with the tarsus, and the other an acute one. The ground not yielding to the point of the foot, the heel and the rest of the leg must of necessity be raised, otherwise the heel could not be extended. The pelvis and trunk are consequently thrown upward, forward, and somewhat in a lateral direction. In this manner they move round the fixed foot as a centre, with a radius consisting of a leg belonging to that foot, which, during this operation, continually diminishes the angle formed with the tarsus. The leg which communicated this impulse is then thrown forward, and rests its foot upon the ground; while the other, which now forms an acute angle with its foot, has the heel extended in its turn, and in like manner makes the pelvis and trunk turn round upon the former leg.

As each leg supports the body in its turn, as in standing on one foot, the extensor muscles of the thigh and knee are brought into action, to prevent these articulations from yielding; and the flexors act immediately after, when the leg having thrown the weight of the body on its fellow must be raised before it can again be carried forward. As the undulatory motion that necessarily attends a man's walking, cannot be perfectly regulated on both sides, he cannot walk in a perfect straight line, nor can he walk in a direct course with his eyes shut.

In walking down an inclined plane, or descending a staircase, as the advanced leg is placed lower than that which remains behind, the extensors of the leg must act more powerfully to prevent the body from falling backwards. Again, on ascending such situations it is requisite at each step, not only to transport the body horizontally,

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(f) These terms signify the same as superior and inferior in man, anterior and posterior in quadrupeds; but are more convenient, as applying indiscriminately to both. Atlantal denotes what is next the atlas; sacral what is next the sacrum. See Barclay's Nomenclature. If Animal zontally, as on walking on level ground, but to bear it up against its own weight, by means of the extensors of the knee of the advanced leg, and those of the heel of that which is behind; this is the reason of the knee and calf of the leg being fatigued in ascending; and the fatigue is relieved by inclining the body forward, because then the lever by which its weight acts on the knee is shortened.

Running is only a succession of short leaps, and it will be understood from what we shall presently say of leaping.

When a quadruped walks, he first slightly bends the articulations of the hind legs, and then extends them, in order to carry forward the body, which motion is considerably aided by the extensors of the knee and the heel. The breast being thus thrown forward, the fore legs incline backward, and the animal would fall, did it not instantly throw them forward in order to support itself. It then draws up the trunk upon the fore legs, and renews its former efforts.

In this walking, each step is performed by two legs, one belonging to the fore, and the other to the hind pair. Sometimes these are of the same side, and sometimes those of opposite sides. The motion of a horse who steps forward in the latter way, is termed a pace.

In the animals that have the fore feet longer than the hind, and have their strength chiefly in the anterior part of the body, the principal impulse is given by extending the fore foot. The hind foot then rises to follow it, and it is not until the moment that the latter extends itself in its turn, that the fore foot is raised. This is the manner in which the giraffe is said to move.

But when the fore legs are considerably disproportioned to the others, and particularly when the posterior extremities are feebly and badly articulated, as in the sloths, the animal is obliged to drag itself forward, by first extending the fore legs, and then bending them so as to draw the body after them. Hence the progression of the sloth is laborious.

Those animals which have their fore legs very short in proportion to their hind legs, would be incapable of sufficiently supporting their bodies, and must fall forward on each impulse of the latter, had they not the precaution to make a prancing movement; that is, to raise the anterior extremities entirely off the ground, previously to their being impelled onward by means of the hind feet. Accordingly, such animals cannot in propriety of language be said to walk; they only move forward by leaps. This is the case with hares, rats, and particularly jerboas. Indeed, these animals cannot be said to walk at all, except in the action of ascending. When they attempt to walk slowly on level ground, they are obliged to move themselves by the fore feet, and merely to drag after them the hind pair. This may be observed in rabbits, and still more distinctly in frogs.

In leaping, the body rises completely from the earth, and remains without any support for a short period, the duration of which depends on the force with which the leap has been made. This action is performed by a sudden extension of all the muscles belonging to the sacral articulations, immediately after they have undergone an unusual degree of flexion. By this general extension these articulations receive a violent motion, the impulse of which is communicated to the centre of gravity of the body, and it is thus projected with a determined velocity, which is more or less in opposition to its weight.

The projectile force and extent of the leap depend on the proportional length of the bones, and strength of the muscles. Those animals, therefore, leap best that have the sacral extremities longer and thicker than the atlantal; as the kangaroo, jerboas, frogs, allice, grylli, fleurs, &c.

Small animals leap proportionally much farther than the larger species; and we know of none whose muscular strength, in this way, can be put in competition with that of a flea, which in a moderate computation is known to leap to a distance of at least 200 times its own length. The direction of a leap depends on the situation of the centre of gravity with respect to the member by which the impulse is given. Hence, only man and birds can leap vertically, because they alone have the trunk situated above the members by which the leap is effected. Quadrupeds, and most insects, can only leap forward; but spiders, which have several long feet on each side of their body, can also leap sideways.

Running consists of a series of low leaps performed alternately by each leg. It differs from walking, in the body being projected forward at each step, and in the hind foot being raised before the anterior touches the ground. It is more rapid than the quickest walk, because the acquired velocity is preserved, and increased at each bound by a new velocity. Running, therefore, cannot be instantaneously suspended, though a stop may be put to walking at each step.

In running, the animal inclines its body forward, that the centre of gravity may be in a proper position for receiving an impulse in that direction from the hind leg; and it is obliged to move the fore leg rapidly forward, to guard against falling.

Man varies his manner of running, only by taking longer or shorter steps, or giving to this motion a greater or lesser degree of rapidity; but quadrupeds vary this motion by the different order in which they raise each foot, or bring it to the ground.

Trotting is a mode of running in which the feet diagonally opposite rise at once, and fall at once, each pair alternately, but in such a manner, that for a moment all the four feet are off the ground. This produces a regular motion, and the sound of the animal's steps are heard two and two in succession.

Galloping is a running motion in which the animal raises the anterior feet at each step, and throws the body forward by the extension of the posterior feet. When the two fore-feet descend at the same time, and are followed by the two hind feet also descending together, the motion is called a full gallop, which is the most rapid a horse can perform, and the only mode of running in dogs, hares, &c. In this kind of gallop the steps of the horse are likewise heard by two beats at a time. The common gallop is when the two fore-feet are lifted unequally, and fall one after another. This may be divided into gallops in which the horse's footsteps are heard by a series of three or four beats, because the posterior feet may fall to the ground either both together, or one after the other.

There are several kinds of animals which leap by the means of organs different from feet, but always by a sudden extension of several articulations.

Serpents leap by folding their bodies into several undulations. Of Animal-dulations, which they unbend all at once, according as Motion they wish to give more or less velocity to their motion; some may be afflitted by the scales of their belly, which they can elevate and depress, but only a few genera are capable of employing this means.

Some fishes also leap to the tops of cataracts by bending their bodies strongly, and afterwards unbending them with an elastic spring.

The long-tailed cray-fisher, particularly the trumps, leap by extending the tail after it has been previously bent under the body.

The larva of the fly, vulgarly called the maggot, forms itself into a circle, contracts itself as much as possible, then suddenly unbending, darts forward to a considerable distance.

The motion of climbing, so useful to many of the inferior animals, consists in hanging from, and strongly grasping any object susceptible of being seized by the fingers, toes, or tail, and thus rising, by successive efforts, in a direction opposite to the animal's weight. From this explanation, it is evident that those animals which have the divisions of their extremities most distinct and flexible, will be the best climbers; and accordingly we find that the animals called quadrumanous, as the apes, lemurs, and a few others, perform this action in the most perfect manner. Man is but an indifferent climber, as he can only grasp with his hands. In opposums, ant-eaters, and sloths, one of the toes is distinct, like the thumb in man, apes, and lemurs; or else they have a considerable protuberance on the heel, which has the same effect. Many animals, as some of the monkeys, some species of opposum and ant-eater, the manis, &c., have a very flexible prehensile tail, which assists them in climbing. The animals of the cat genus have very sharp talons, by which they are materially assisted in this kind of progression, as they enable them to adhere firmly to the bark of trees, &c. Creepers, nut-hatches, woodpeckers, and other climbing birds, support themselves in a similar manner.

The motion of flying, by which an animal can support itself for some considerable time in the air, can properly be said to be performed only by birds: for though bats can imitate this motion with tolerable success, and the gallinithescus, flying squirrels, and flying-opposums, appear to fly from one tree to another, the motion of the former cannot be supported for so long a time as that of birds; and the motion of the latter animals can be considered only as a leap, assisted and prolonged by the opposition given to the air, by the membranous expansion between their limbs.

When a bird designs to fly, it first darts into the air, either by leaping from the ground, or by throwing itself from some height. In the mean time it raises the whole of the wings which had till then remained folded, and which it unfolds in a horizontal direction by extending the bones. When the wings have thus acquired all the superficial extent of which they are susceptible, they are suddenly depressed, till they form, with the vertical plane of the body, an angle that is obtuse upward, and acute downward. The resistance which the air gives to this motion suddenly performed in it, produces a reaction on the body of the bird, and thus moves it forward as in ordinary leaps. This impulse once given, the bird refolds the wings by bending the joints, and repeats its efforts by another stroke. As the velocity of animals thus acquired in ascending is gradually diminished by the effect of gravitation, a moment occurs in which it ceases, and in which the bird tends neither to ascend nor descend. If at this moment it gives a new stroke with the wings, it acquires a new ascending velocity, by which it will be carried as far as before, and by repeating these efforts, it will ascend in a uniform manner. If this second stroke be made before the velocity first acquired is lost, an additional impulse will be received; and by a continuance of this action the bird will ascend with an accelerated motion. If the wings do not vibrate when the ascending velocity is lost, the bird will begin to descend; and if it allow itself to fall down to the point from which it set out, it cannot ascend as high as at first, but by a much stronger exertion of the wings; but if it seizes in the fall a point so situated that the acquired descending velocity, and the small space which it has to fall down reciprocally balance each other, it may, by a series of equal vibrations, keep itself at the same height.

When a bird wishes to descend rapidly, as when it darts upon its prey, it altogether suppresses the vibration of its wings, and thus falls by its own gravity. While descending, however, it may suddenly break its fall by extending its wings, and this suspension is called a recover.

We have as yet considered only the vertical flight of a bird. To fly horizontally, it must rise in an oblique direction, and make a new movement of its wings, when it is ready to descend below the point from which it departed; but in this way it will not fly in a straight line, but will describe a series of curves to very much depress, that the horizontal will overcome the vertical motion. In order to ascend obliquely, the bird must make quicker vibrations of its wings, and to ascend in a similar direction, the vibrations must be slower.

The deviations of flight to the right or left are chiefly produced by the unequal vibrations of the opposite wing; those of the left wing carrying the bird to the right, and vice versa. The more rapid the flight is forward, the greater is the difficulty of one wing surpassing the other in the velocity of its vibrations, and of course the deviation sideways is the more difficult. Hence birds which fly with the greatest velocity make large circles in turning.

The tail, when spread out, contributes to sustain the posterior part of the body. If it is depressed when the bird has acquired a progressive velocity, it presents an obstacle which elevates the posterior part of the body, and depresses the anterior. If it is turned up, the contrary effect is produced. Some birds incline to one side, to assist them like a rudder, when they wish to change their horizontal direction.

The structure of most birds peculiarly adapts them for rapid motion through the air, and for sustaining themselves in this element with the greatest facility.

See Ornithology, No. 37.

The action of swimming, like that of flying, nearly resembles leaping, except that, like flying, the leap does not take place on a fixed surface. A great variety of animals, besides fish, and most of the other inhabitants of the waters, are capable of swimming. This action is performed with considerable ease by several of the mammalia; The organs employed by fishes, in making their way through the water, are their fins, tail, and air-bladder; the two former exerting the necessary motions like the wings of birds, while the latter, by being compressed or expanded, causes the necessary changes in the specific gravity of the body, and thereby renders the animal more or less buoyant. The swimming of fishes has been treated of with sufficient minuteness under Ichthyology, chap. iii. sect. 3., to which we refer the reader.

The cetacea employ much the same means as fishes; but in them the principal efforts of the tail are made in a vertical direction, and the use of the air-bag is supplied by lungs, which they can compress and dilate at pleasure, by the action of the diaphragm, or the intercostal muscles. See Cetology.

The swimming of mammalia, and of water birds, is performed by means of the legs and feet, which are used like oars, to propel the body forward by the resistance which they make to the water in the contrary direction. Hence those quadrupeds and birds that have flat or webbed feet, swim most easily, as the resisting surface is the greatest. Of all the mammalia, man has the most occasion to use his hands in swimming, on account of the greater proportional weight of his head.

Serphants, and the larvae of such insects as sometimes inhabit the waters, perform the action of swimming by rapid inflections of the body like an eel or a leech. The larvae that are most commonly found in the waters are those of the water beetle, the hydrophilus, the dog fish, the aquatic tipulae, and gnats.

No animal walks without legs, or flies without wings (if we except the flying fish, whose fins enable it rather to spring than fly); but there are many that swim without fins, and that leap and creep without any legs. The rapidity of movement is not proportioned to the number of instruments that are employed: if the trout-fish be observed to move slowly with one leg, the sea-urchin moves still more slowly with many thousands; the oyster moves by quitting out water; the scallop by the jerk of its shell, and when in the water it rises to the surface and sails before the wind.

Many animals are formed by nature to fly, walk, leap, and swim: the fate of those is rather uncommon whole muscles or feet are by nature attached to their integuments; the lobster is obliged to throw off its shell, and the caterpillar all its feet, with the skins, and in that situation to remain stationary till it receive new instruments of motion.

Whoever has read the celebrated work De Motu Animalium, needs not to be told that, besides the organs which are here mentioned, the form, the structure, and even the specific gravity of the body, as depending on the nature of the bones and muscles, or as varied by air-vehicles and bubbles, with a great variety of other circumstances, are necessary to explain the different phenomena of locomotion.

As to vegetable motions, they evidently depend on external agents: the wings of seeds only fit them to be carried by the wind, their specific gravity to float in the water, and their legs or tentacula to adhere to bodies that are in motion; the singular motions which have been ascribed to sleeping, to waking, to sensation, and volition, in the vegetable kingdom, seem only the consequence of light, heat, moisture, and such stimulants, acting invisibly or with secret influence: the opening and closing of the meteoric flowers are always correspondent to the states of the atmosphere; and the opening and closing of the equinoctial and tropic flowers, to the light, the length or shortness of the day.

The principal intentions of locomotion are to get food, to turn danger, to promote intercourse and diffusion of the species.

There is perhaps no part of physiology which is more important than the relations which subsist between the different functions of the living body; but it is a part of the subject which is as yet but little understood. We regret that our limits will not permit us to pay all the attention to it which we could wish. We shall, however, briefly notice under each function, the principal relations that are found to take place between it and those which have been previously considered.

Besides the dependence which animal motion has, in mutual relations, on the nervous system, (see No. 111.) we likewise find an evident sympathy between these two functions between sensation and motion.

A violent emotion or impression on the nerves often throws the limbs into convulsive agitations; spasmodic affections are relieved, or sometimes removed, by the coming on of delirium; and these symptoms will alternate with each other: a compression of the brain, or of some large nervous trunk, produces general or partial want of motion, and when this compression is removed, the muscles for the most part recover their usual action; an attack of epilepsy is often preceded by the sensation of a stream of vapour commencing in some external part, and rising to the brain. These, and many other phenomena that might be mentioned, fully prove the sympathy between the nervous and muscular systems; and with this enumeration we must dismiss the subject.

CHAP. V. Of Digestion.

The necessity of repairing the waste of the body is announced in all animals by the feelings of hunger and thirst; the former of which intimates the occasion for solid, the latter for liquid food. This imperious necessity overrules all the other affections of the vital principle, and every other appetite often remains suspended till that necessity be satisfied. It is difficult to assign the final cause of these singular sensations, but probably our researches on that subject are rather curious than useful. Whatever be the ultimate end of these appetites, we readily perceive how much they are influenced by habit. We find that when we are accustomed to take food at particular times, the appetite, under ordinary circumstances, always reminds us at these times, of the occasion, whether real or apparent, for receiving a new supply. By this influence of habit some animals, especially man, are accustomed to take several meals in a day, while others can fast for days, or even weeks, together. The appetite for food also varies considerably at different ages. It is more lively and more imperious in infancy and early childhood, and in general in those animals who have not yet acquired their full growth; it is on the contrary weaker in advanced age, and when the body ceases to increase... increase in size. It is more frequently renewed in the strong and healthy, and those who are accustomed to laborious occupations or active exercises.

We know that in the natural state of the animal body, the appetite for food is influenced by the nature of the aliment on which the animal is accustomed to subsist. Many animals live entirely on vegetable food, and these have no appetite for animal substances, and even reject these when offered to them. On the other hand, many tribes live entirely on animal food, and either refuse vegetable, or, if obliged by necessity to employ it as food, do not appear to derive nourishment from it. We find, however, that it is in the power of habit to remove these appetites; that a horse or a sheep may be taught to live on animal food, while a dog or a cat may be supported entirely on vegetable substances. A few animals are capable of subsisting on almost every kind of animal or vegetable substances, or are omnivorous.

Many animals are capable of being supported by water and air alone. We know that several fishes, as the minnow, the gold and silver fish, &c., will live for a long time in a vessel containing pure water, and freely exposed to the air. Rondelet (a celebrated writer on fishes in the 16th century) relates a remarkable instance of this. He kept a fish during three years in a vessel that was constantly full of very pure water. It grew to such a size, that at the end of that time the vessel could no longer contain it. Leeches are often kept for several years with no other nutriment but water, and that not very often changed. There is good reason to believe that the sole food of plants consists of water and air, and that the soil in which they grow answers scarcely any other purpose than that of preserving and conducting those necessary aliments.

It has been supposed that some animals are capable of subsisting on matters that appear to contain no nutritious principles, such as sand, hair, and wool. Borelli long ago conceived this opinion, from observing that in many testaceous animals which he dissected, the alimentary tube contained nothing but sand. It has often been remarked, that horses, cows, and sheep, when deprived of their usual nourishment, will lick their bodies, and swallow down the hair, or, in the case of sheep, will tear off and swallow each other's wool. If we consider the nature of these substances, we think there is no reason to suppose that they answer any other purpose than attending the alimentary canal or stomach, and thus in some measure counteracting the effect of hunger.

The subject of food in general has been already treated of, under Aliment, and in Materia Medica, Part I. No 17; and the function of digestion, as far as it relates to man, has been considered under Anatomy, No 106, 107, and under Chemistry, No 2548. It remains for us here only to make a few observations on the comparative physiology of this function.

Digestion differs considerably in the various classes of animals, both as to the organs by which it is performed, and as to the simplicity or complex nature of the operation itself. The general variations that take place in the organs of digestion, have been mentioned under the comparative part of Anatomy, No 152, and are fully treated of by Cuvier, in his Leçons d'Anatomie Comparée, tom iii. and Blumenbach, in his Comparative Anatomy, chap. 6. and 7.

In the more perfect animals, digestion supposes a series of operations, from the time that the food enters the mouth, till the nutritious parts of it are taken into the circulating system. These operations are, mastication, infalivation, deglutition, chymification, and chylification.

Mastication is performed by means of teeth, and therefore can scarcely be said to take place in those animals that are not furnished with these organs. We know that all mammalia, except those which Cuvier calls edentata, as the ant-eaters, pangolins, and platypus, have teeth, fitted both for dividing and chewing their food; but here an important difference takes place. Those animals which live chiefly on animal food, have most of their teeth sharp and pointed, for the purpose of seizing and tearing their prey, while the granivorous and graminivorous animals have very large and strong grinders, in which the hard substance commonly called enamel (or what Blake calls corpus striatum,) forms alternate layers with the bony part. Such are also found in most reptiles and serpents, and in many fishes; but in some of these they seem less to serve the purpose of dividing the food, than to seize and retain it till swallowed. Birds have no teeth, though some of them have the mandibles of the bill so formed as to divide and cut in pieces their food.

During mastication the food is mixed with the saliva, and is thus better fitted for easy solution in the stomach. This infalivation of the food may, however, take place without previous mastication. It is common for serpents to swallow their food whole; but in order to facilitate its passage down the throat, they first besmear it all over with their mucous saliva. In many animals, a process similar to infalivation takes place, while the food remains in the mouth. In several species of the ape tribe there is a pouch situated on each side of the jaw, and in these pouches the greater part of the food is retained, not merely as some suppose, to serve as a future meal, but to undergo a dilution by the fluids that are there secreted. In granivorous birds, the food is first received into a membranous bag, formed by a dilatation of the gullet, and commonly called the crop, where it is macerated by the fluids that are there separated by means of glands or exhaling veils, and passes down, as the animal requires, to be further prepared by the stomach. The bustard, indeed, though a granivorous bird, has no proper crop, but the gullet is furnished with numerous and large glands.

For an account of the chemical nature and properties of saliva, see Chemistry, No 2723.

The operation of deglutition depends chiefly on the action of the tongue, and on that of the muscles which surround the pharynx and gullet. It is more or less speedy in proportion as these are more or less active and vigorous. Most animals, after having once swallowed their food, do not receive it again into the mouth; but this takes place in several tribes, and is called rumination, or chewing the cud.

Rumination takes place chiefly in those animals that feed on herbage, and have not a muscular stomach; such as all the tribes that Linnaeus has ranked under the order pecora. In these the food, after being slightly chewed, is received into the first stomach, and after remaining there for a short time, it is gradually brought by a retrograde action of the gullet into the mouth, where it undergoes a complete trituration and infalivation. Some of those birds which have a diluting sac or ingluvies, seem likewise to ruminate. This in the parrot was observed by the gentlemen of the French academy. It has since been observed in rooks, macaws, cockatoos, and others; and Mr Hunter, to whom physiology is so much indebted, discovered that the male and the female pigeon secrete in their ingluvies a certain liquor for feeding their young; and that most kinds of what have been thought ruminating birds do very often in expressing their fondness regurgitate their food. Yet both this and another species of regurgitation which is very common with those animals that swallow indigestible substances with their food, should be carefully distinguished from rumination. For a farther account of rumination, and of the digestive organs of ruminating animals, see Comparative Anatomy, No. 228—234, and Phil. Trans., 1807, Part ii.

The food having entered the stomach, undergoes in that organ processes that are partly mechanical, or rather organic, and partly chemical, depending on the structure of the stomach, and the nature of the juices secreted into its cavity. By these actions it is reduced into a pulpy substance commonly called chyme.

The organic action of the stomach is greater or less, according as this organ is more or less muscular. There are many animals, chiefly birds of the granivorous tribes, that have a very muscular stomach, commonly called gizzard, capable of grinding, not only the grains received into it, but even of reducing to powder small pieces of glass, and of blunting the points of needles and lancets. These facts were first proved by Borelli, who introduced into the gizzards of fowls, nuts, filberts, hollow spheres of glass, hollow cubes of lead, small pyramids of wood, and several other substances, which he found were either crushed together, or broken to pieces. He computes the power exerted by the stomach of the Indian cock as equal to the pressure of 1350 pounds weight. These experiments were repeated and verified by Spallanzani.

Some animals that are not possessed of a muscular stomach have, within that organ, teeth, or other hard bodies, for the purpose of breaking or grinding their food. This is the case with many of the crustacea, as crabs and lobsters.

A great many animals have what Spallanzani calls intermediate stomachs, i.e. not so muscular as the gizzard of fowls, nor so membranous as the stomachs of ruminating animals; this is the case with many birds, as ravens, crows, herons, &c. The stomachs of these animals are possessed of considerable force, though this is not nearly equal to that exerted by the gizzard. These animals possess the power of rejecting by the mouth the substances that are incapable of digestion in the stomach, every nine, or sometimes every three hours.

The animals with membranous stomachs are very numerous, comprehending man, most beasts and birds of prey; many reptiles, snakes, fish, &c. The stomachs of these animals are susceptible of but little muscular action, though in many species they both contract upon the food, and reject it through the gullet, on various occasions. Birds of prey, like the ravens, crows, &c. possess the power of rejecting, in the form of pellets, the indigestible parts of their food, which usually takes place every 24 hours.

A most interesting paper by Mr Everard Home is published in the Philosophical Transactions for 1807, Discoveries part ii. on the structure and functions of the stomachs of various animals. We regret that we can here give Home little more than the results of his inquiries.

From previous investigations respecting the stomachs of ruminating animals, Mr Home was led to believe that the fourth stomach in these tribes was either always, or during digestion, divided into two portions, each performing a different office in the digestive process; and he even conjectured, that a similar division might take place in other animals.

Mr Home has examined the stomachs of a great variety of animals, and investigated the progress of digestion in ruminants, the hare tribe, which occasionally ruminate, the beaver, dormouse, water-rat, common rat, mouse, horse, and afa, kangaroo, racoon, hippopotamus, elephant, the cetacea, fowl, and lastly in man.

The human stomach appears to be the uniting link between those that are fitted only to digest vegetable matter, and those that are entirely carnivorous; and yet we find, that in its internal structure it is in every material respect similar both to those of the phytomastic monkey and squirrel, which usually digest only vegetable food, and to those of carnivorous animals.

The human stomach is occasionally divided into a cardiac and pyloric portion, by a muscular contraction similar to that of other animals; and as this circumstance has not before been noticed, it is proper to be more particular in describing it.

The first instance in which Mr Home observed this muscular contraction in the human stomach, was in a woman who died in consequence of being burnt, and who had been unable to take much nourishment for several days before her death. The stomach was found empty, and was taken out of the body at a very early period after death. It was carefully inverted to expose its internal surface, and gently distended with air. The contraction was so permanent, that after the stomach had been kept in water, in an inverted state, for several days, and at different times distended with air, the appearance was not altogether destroyed.

Since that time, Mr Home has taken every opportunity of examining the human stomach shortly after death; and he finds that this contraction, in a greater or lesser degree, is very generally met with. He of opinion that this effect is not produced by a peculiar band of muscular fibres, but that it arises from the muscular coat, in the middle part of the stomach, being thrown into action to a greater or lesser extent according to circumstances. When this part of the stomach is examined by dissection, its muscular fibres are not to be distinguished from the rest. If the body be examined too late as 24 hours after death, this appearance is rarely met with; a circumstance which accounts for its not having before been particularly noticed.

That the food is dissolved in the cardiac portion of the stomach, is proved by this part only being dissolved in found digested after death; the instances of which are sufficiently numerous to require no addition being made to them. This could not take place unless the solvent liquor was deposited there. Mr Hunter goes so far as to say in his paper on this subject, "there are few dead bodies in which the stomach at its great end is not in some degree digested."

That the chyle is not formed there, and that it is commonly formed before the food passes through the pylorus, is proved by the result of some experiments made by Mr Hunter upon dogs, in the year 1760. The dogs were killed while digestion was going on; and in all, the food was least dissolved, or even mixed, towards the great end of the stomach, but became more and more so towards the pylorus, just within which it was mixed with a whitish fluid like cream.

From the result of these experiments, as well as from the analogy of other animals, it is reasonable to believe, that the glands situated at the termination of the cuticular lining of the esophagus, which are described by Mr Home, secrete the solvent liquor, which is occasionally poured on the food, so as to be intimately mixed with it before it is removed from the cardiac portion; and the muscular contraction retains it there, till this takes place.

Such contraction being occasionally required in the stomach, accounts for its being more or less bent upon itself, as by this structure it is more readily divided into two portions, by the action of the muscular fibres at that part where the angle is formed.

This contraction also explains why the contents of the stomach are not completely discharged from the first effect of an emetic; and by it Mr Home thinks we may explain the cramp of the stomach, and some kinds of indigestion.

After comparing the stomachs of several carnivorous animals with that of man; in tracing the gradation from carnivorous beasts through the bat tribe to birds of prey, Mr Home remarks that "the only real link between the stomachs of quadrupeds and birds is that of the ornithorhynchus (or platypus), which, however, is more an approach to the gizzard, being lined with a cuticle containing sand, and having the same relative situation to the esophagus and duodenum. The food of this animal is not known; it is probably of both kinds; the papillae at the pylorus, which appear to be the secretory ducts of glands, are peculiar to it.

From the facts and observations brought forward in this valuable paper, Mr Home deduces the following general conclusions. "That the solvent liquor is secreted from glands of a somewhat similar structure in all animals, but much larger and more conspicuous in some than in others.

"That these glands are always situated near the orifice of the cavity, the contents of which are exposed to their secretion.

"That the viscid substance found on the internal membrane of all the stomachs that were examined recently after death, is reduced to this state by a secretion from the whole surface of the stomach, which coagulates albumen. This appears to be proved, by every part of the fourth cavity of the calf's stomach having the property of coagulating milk.

"This property in the general secretion of the stomach leads to an opinion, that the coagulation of fluid substances is necessary for their being acted on by the solvent liquor; and a practical observation of the late Mr Hunter, that weak stomachs can digest only solid food, is in confirmation of it.

"That in converting animal and vegetable substances into chyle, the food is first intimately mixed with the general secretions of the stomach, and after it has been acted on by them, the solvent liquor is poured upon it, by which the nutritious part is dissolved. This solution is afterwards conveyed into the pyloric portion, where it is mixed with the secretions peculiar to that cavity, and converted into chyle.

"The great strength of the muscles of the pyloric portion of some stomachs will, by their action, compress the contents, and separate the chyle from the indigestible part of the food.

"In animals whose food is easy of digestion, the stomach consists of a cardiac and pyloric portion only; but in those whose food is difficult of digestion, other parts are superadded, in which it undergoes a preparation before it is submitted to that process."

The action of the juices of the stomach, or of what we call the gastric juice, appears to have much more effect in the process of chymification, than the muscular action of the stomach, though the dissolving power of this fluid seems to be proportionally less in those animals that have the most muscular stomachs. The gastric juice of granivorous birds is capable of dissolving flesh; but when this is entire, it requires four or five days for solution; whereas when bruised, half that time is sufficient. Even grain is not dissolved in it except when bruised. The gastric juice of animals with intermediate stomachs dissolves flesh and cartilage, but not bone. It is incapable of dissolving entire seeds. In animals with membranous stomachs, the gastric juice is extremely active, and seems to be almost the only agent in the digestive process. In some of these animals, however, as the ruminating tribes, this fluid has no effect on the food, unless it be bruised, or thoroughly masticated. Spallanzani found, that owls digest flesh and bones, but not grain;—that the gastric juice of the eagle dissolves bread and bone, and even animal and vegetable matters, when it is taken out of the body;—that a wood pigeon may be gradually brought to live on flesh;—that the owl and falcon do not digest bread;—that the gastric juice of the dog dissolves even the enamel of the teeth.

Hence, in every order of animals, the gastric juice is the principal cause of digestion, and it agrees in all many properties, and differs in others. In the frog, the newt, fleshy fishes, and other cold-blooded animals, it produces digestion in a temperature nearly equal to that of the atmosphere. In warm-blooded animals it is capable of dissolving the aliment in a degree of heat lower than that of these animals. In them too the food is digested in a few hours, whereas in the opposite kind it requires several days, and even weeks, particularly in serpents; likewise, the gastric juice of the gallinaceous class can dissolve only bodies of a soft and yielding texture, and previously triturated: whilst in others, as serpents, the heron, birds of prey, and the dog, it decomposes substances of great tenacity, as ligaments and tendons; and even of considerable hardness, as the most compact bone. Man belongs to this class, but his gastric juice seems to have no action on the hardest kinds of bones. Some species, like the pig, are incapable of digesting vegetables, as birds of prey; but man, the dog, cat, crows, &c., dissolve the individuals of both kingdoms alike, and are omnivorous, and in general their gastric juices produce these effects out of the body. For an account of the chemical nature and properties of the gastric juice, see Chemistry, No. 2551.

The process of chymification depends also, in a great measure, on the nature of the substances employed as food, as some of these are much more soluble than others. On this subject much information may be derived by consulting the experiments of Dr Stark *, and those of M. Goffe of Geneva, an abstract of which is given in Johnston's Animal Chemistry, vol. i. p. 207.

From the latter experiments it appears, 1/3, That the following substances are either insoluble, or are not digested in the usual time in the stomach.

Animal Substances. 1. Tendinous parts. 2. Bones. 3. Oily or fatty parts. 4. Indurated white of egg.

Vegetable Substances. 1. Oily or emulsive seeds. 2. Expressly oils of different nuts and kernels. 3. Dried grapes. 4. Rind of farinaceous substances. 5. Pods of beans and peas. 6. Skins of stone fruits. 7. Husks of fruits, with grains or seeds. 8. Capsules of fruit, with grains. 9. Ligneous stones of fruits. 10. The gastric juice does not destroy the life of some foods; hence bitter-sweet, hemp, mifletto, and other plants which sometimes grow upon trees, are produced by the means of the excrements of birds, the kernels of seeds being defended from the menstruum by their exterior covering.

2d, That the following are partly soluble, viz.

Animal Substances. 1. Pork dressed various ways. 2. Black puddings. 3. Fritters of eggs, fried eggs and bacon.

Vegetable Substances.—1. Salads of different kinds, rendered more so when dressed. 2. White of cabbage less soluble than red. 3. Beet, cardoons, onions, and leeks. 4. Root of scurvy-grafts, red and yellow carrots, succory, are more insoluble in the form of salad than any other way. 5. Pulp of fruit with acids, when not fluid. 6. Warm bread and sweet pastry, from their producing acidity. 7. Fresh and dry figs. By frying all these substances in butter or oil they become still less soluble. If they are not dissolved in the stomach, they are, however, in the course of their passage through the intestines.

3d, That the following are soluble, or easy of digestion, being generally reduced to chyme in an hour, or an hour and a half.

Animal Substances.—1. Veal, lamb, and in general the flesh of young animals, are sooner dissolved than that of old. 2. Fresh eggs. 3. Cows milk. 4. Perch boiled with a little salt and parsley. When fried or seasoned with oil, wine, and white sauce, it is not so soluble.

Vegetable Substances.—1. Herbs, as spinach, mixed with fennel, are less soluble. Celery. Tops of asparagus, hops, and the ornithogalos of the Pyrenees. 2. Bottom of artichokes. 3. Boiled pulp of fruits, seasoned with sugar. 4. Pulp or meal of farinaceous seeds. 5. Different sorts of wheaten bread, without butter, the second day after baking; the crust more so than the crumb. Salted bread of Geneva more so than that of Paris without salt; brown bread in proportion as it contains more bran is less soluble. 6. Rapes, turnips, potatoes, parsnips, not too old. 7. Gum arabic, but its acid is soon felt. The Arabians use it as food.

The solvent power of the gastric juice is increased by various stimulants, especially by those called condiments, as sea salt, spices, mustard, vinegar, as well as by various and spirituous liquors and old cheese in small quantities, and by various bitters. It is retarded by large quantities of diluting liquors, especially when taken hot; by acids and astringents taken a short time after eating; by unctuous substances; by mental employment, or severe bodily exercise, too soon after a meal; and by leaning with the breast against a table.

It may be proper here to notice the various opinions various that have been entertained respecting the immediate theories of cause of digestion. The principal of these opinions are, digestion, that it is produced by coction or heat; by trituration in the stomach; by fermentation, or by putrefaction.

That it is not brought about by heat alone, will appear from the circumstance, that many cold-blooded animals digest their food as completely, though not so expeditiously, as warm-blooded animals.

That it is not effected by trituration in the stomach Trituration, alone, is evident from the experiments that have been made by Spallanzani, Stevens, and others, of giving to animals food enclosed in hollow perforated balls, sufficiently strong to resist the muscular power of the stomach; as the balls have been found empty, and not compressed.

That it is not owing to fermentation is proved by the Fermentation circumstance, that the more perfectly digestion proceeds, the less is the evolution of gases in the stomach; the contrary of which would be the case, if digestion consisted in a fermentation of the aliment.

That it does not depend on putridity, is evidenced by Putridity, the observations that have been made on putrid food given to dogs, and examined some time after, when it was found perfectly fresh.

On the whole it appears, that in most animals the digestion of food in the stomach depends partly on a due degree of heat, partly on the vital action of the stomach, but chiefly on the action of the gastric juice.

When the aliments have been converted in the stomach to the crude pulp called chyme, they are gradually propelled through the pylorus into the duodenum, where they are mixed with the bile, the pancreatic juice, and the fluids that are separated by the mucous coat of that intestine, and are thus reduced to a still finer pulp, containing, as one of its principal ingredients, the nutritious fluid called chyle, the nature and properties of which, as they have been but slightly mentioned in the former parts of this work, fall to be noticed here.

The properties of chyle have not been minutely investigated; but according to Fordyce, as far as experiment has been carried, the chyle of quadrupeds is so similar to that of man, and of each other, as hardly to be distinguished, even in tribes the most opposite to each other in their structure, food, and habits of life. As far as we can perceive, the chyle of a dog or a wolf differs in nothing from that of a sheep or an ox.

The chyle consists of three parts; one part which is fluid, and contained in the lacteals, but coagulates on extravasation.

The second part consists of a fluid, which is coagulable by heat, and in all its properties hitherto observed, it is similar to the serum of the blood.

The third part consists of globules, which render the whole white and opake. These globules have been supposed by many to be an expressed oil; but this has not been proved. Neither has it been perfectly demonstrated that sugar is contained in the chyle, although it has been been made very probable. The difficulty of determining these points arises from the small quantity that can be collected, the largest animals not supplying more than one ounce or two, at the most. However, the part coagulating on extravasation; the part agreeing with serum in its qualities; the globular part, which in some animals, but not in quadrupeds, exists without giving whiteness to the chyle—alone, or along with sugar, form the essential parts of the chyle.

The compound pulpy matter containing the chyle is carried forward from the duodenum through the whole course of the intestines, where it is subjected to the continual action of the internal wrinkled membrane of the bowels, and its nutritious particles, or chyle, selected and absorbed by the lacteals that are abundantly distributed there, and open their mouths directly within the cavity.

As to the movements of the alimentary canal, the direction of hairs found in the stomach, and the balls of hairs which are thrown up, would appear to indicate a circular motion. The intestinal part has a motion similar to that of a worm, and is called the vermicular or peristaltic. Here every portion retains its own motion, although it be separated from the rest by ligatures. The stomach of the polype, the gullets of the ruminating kinds, and the ceca, have the motion in different directions at different times; and that observed in the alimentary canal of a loupe is, when viewed through a microscope in the time of action, amazingly rapid; the stimulating causes employed are the food, the different liquors with which it is mixed, the air, the nerves where they exist, and a portion of heat. Some degree of heat is necessary to every process of digestion, both in the animal and vegetable kingdom; what that degree is depends on the nature of the living body; and is various according to its age, its health, its employments, and habits.

With respect to the function of digestion in the lower classes of animals, we can say but little. We know that their food is dissolved in the stomachs of the crustacea, of mollusca, and of polypes; but whether this process in most insects and worms is anything more than inhibition, or taking in aliment, which is to undergo little change, we are uncertain. We know, indeed, that many insects live on substances which must be dissolved before they enter into the pores of their bodies, and that many of them abound in acid juices, which are well fitted for this solution. It does not appear that plants possess what may be called the faculty of digestion.

The relations between digestion and the functions we have already considered, especially sensation, are various and important. The sympathies that exist between the head and the stomach, have been long acknowledged. Several affections of the brain are accompanied with sickness at the stomach, loss of appetite, and indigestion; while, on the other hand, the deranged state of the digestive organs seldom fails to produce giddiness, headache, ringing in the ears, confusion or depravation of sight, &c.; and if the former symptoms arise to a great height, as in the case of overloaded stomach or surfeit, coma, or even apoplexy, is frequently produced. In many nervous affections, particularly hysteria and hypochondriasis, in which there frequently takes place astonishing accumulations of air in the stomach and bowels, the affections of the head, such as stupor, confusion of thought, partial blindness, &c., sometimes proceed to such a height, as to threaten, or even sometimes to produce, an apoplectic paroxysm. In many cases these affections are referred immediately to the head; but are proved, in most instances, to depend on the disordered state of the alimentary canal, from the immediate relief procured by those remedies which promote the discharge of air, or produce copious evacuation from the bowels. On the other hand, in some diseases, where the head is primarily affected, as in phrenitis hydrocephalica (water in the head), the complaint is referred to the bowels, from the costiveness or other disordered state of these. The daily experience of literary men show how much intense thought diminishes the digestive powers, and how imperfectly studious occupations can be carried on after a full meal. The action of the digestive organs is also considerably influenced by the mind, or the passions. We know how readily the appetite may be diminished or destroyed by sudden anger or affliction.

The action of the stomach may even be influenced by the will. We have known a person who could vomit whenever he pleased; and Dr Darwin speaks of another who had acquired this voluntary command over the inverted motions of the stomach and throat, to such a degree, as to gain a subsistence by exhibiting these unnatural powers to the public. At these exhibitions he was accustomed to swallow a pint of red rough gooseberries, and a pint of white smooth ones; to bring them up in small parcels into his mouth, and restore them separately to the spectators, who called for red or white, as they pleased, till the whole were redelivered.

The sympathies that take place between the brain and the digestive organs, are easily explained, from considering the distribution of the great sympathetic nerve, to illustrate which we have given a figure (fig. i.) showing its course and distribution from the head through the chest, as far as the stomach.

The relations between the digestive and the locomotive functions, are not less obvious. Experience shows how much digestion depends on regular exercise, and how imperfectly it is carried on in the stomach of the indolent and sedentary; while, on the other hand, when the stomach is overloaded, voluntary motion becomes difficult and fatiguing. Spasmotic contractions of the muscles, twitchings of the limbs, and similar affections, are the common attendants of indigestion, though these may perhaps be referred equally to the nervous as to the muscular system.

The principal morbid affections of digestion are, nausea, flatulence, eructation, rumination (c), vomiting, heartburn, digestion.

(c) That ruminating power which is natural to the quadrupeds of the order Pecora, is sometimes met with in man. We have heard of persons who regularly brought up their food into the mouth soon after eating, chewed it over again, swallowed the juices with the saliva, and spat out the more solid parts. In these cases, the rumination Mr John Hunter made several experiments to show that the veins do not absorb. He conveyed milk, coloured with various dyeing substances, or perfumed with musk, into the small intestines of an ass, which was soon after killed. On opening the veins of the intestines, and veins allowing the blood to separate into serum and coagulum, the serum was found neither to be tinged with the colouring matters, nor scented by the perfume, while the coloured milk was evident in the lacteals. That the veins, however, do in some cases perform the office of absorbers, is evident from the speedy depletion of the corpora cavernosa penis, after having been distended with arterious blood; and from a similar depletion that takes place in the nipple of the female breast.

The principal object in dispute respecting the function of absorption in man and the higher classes of animals, is whether the skin possesses the power of absorption. This question, as it is both curious and important, we shall examine pretty much at large; and for this purpose we shall avail ourselves of an able paper on the subject, by Dr George Kellie.

It had long been received as an established truth, that the skin was an inhaling or an absorbing organ, and that sometimes the inhalation balanced, or even surpassed, the exhalation of the cutaneous surface; but of late this doctrine of inhalation has been called in question, and, in the opinion of many, entirely overthrown. It has been said, that this absorption neither does nor can take place on the outside of the cuticle; that in every case of apparent absorption, the epidermis had been injured, or that the matter absorbed had been mechanically forced through it, and brought into immediate contact with the skin.

Haller had asserted*, on the authority of Deffault, *Elementa that the body acquired an increase of weight in the Physiolo warm bath; and this augmentation of weight was esteemed an experimentum crucis in favour of cutaneous absorption.

Experiments, however, have since been made with every necessary care, which seem to contradict the position, and to prove, that the body acquires no additional weight in the warm bath.

Seguin, from a great many experiments of this description, concludes, that there is no inhalation, because the body, far from gaining, always lost some part of its weight during immersion, although much less than in the air in equal times†.

In other experiments again, as in those of Gerard and Eclairce, Currie, there was no increase of weight; but the body

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is to be considered as a disease, depending on the inability of the stomach to propel the solid food into the duodenum.

Mr Home, in the paper we have already quoted, (N° 161—169), relates a curious instance of habitual rumination in a man 19 years of age, who is blind, and has been an idiot from his birth. He has a very ravenous appetite, and it is necessary to restrict him in the quantity of his food, since, if he eats too much, it disorders his bowels. Fluid food does not remain on his stomach, but comes up again. He swallows his dinner, which consists of a pound and a half of meat and vegetables in two minutes, and in about 15 minutes he begins to chew the cud. Mr Home was once present on this occasion. The morsel is brought up from the stomach with apparently a very slight effort, and the muscles of the throat are seen in action when it comes into the mouth; he chews it three or four times, and swallows it; there is then a pause, and another morsel is brought up. This process is continued for about half an hour, and he appears to be more quiet at that time than at any other. Whether the regurgitation of the food is voluntary or involuntary, cannot be ascertained, the man being too deficient in understanding, to give any information on the subject. was not observed to have lost anything during immersion in the warm bath.

Now, during these experiments, the body was doubtless wafting, by the pulmonary and cutaneous discharges, and yet the weight of the body either continued unchanged; or where a loss of weight was observed, this was constantly less, greatly less, than is experienced during the same interval in air. And we might be inclined to infer, from a truth so general, and so well ascertained, an argument in favour of absorption.

It might be argued, that the loss of weight amounts to little or nothing, because, during immersion, the body acquires more by inhalation than it does or can do in the air; that the loss by the pulmonary and cutaneous discharges is counterbalanced, or nearly counterbalanced, by the increased absorption.

Those, however, who deny absorption, will not allow us the advantage of this argument. They tell us, that the exhalation by the skin and lungs is diminished, which sufficiently explains why the body loses less in the warm bath than in air. But that the accustomed discharges are suppressed or diminished in the warm or tepid bath, is, we apprehend, far from being proved; and, till this proposition is made good, the argument against cutaneous inhalation cannot be securely maintained.

One of Dr Currie's cases deserves farther consideration. We allude to the case of dysphagia, published by this gentleman*, in which Mr M. the subject of the case, was several times immersed in a warm bath of milk and water, and was weighed when taken out. Mr M. it is true, gained no weight while in the warm bath; but the loss continually going on in the air was, as in other trials, suspended during the immersions. Besides, he always expressed great comfort from the bath, with abatement of thirst; and, subsequent to the daily use of it, the urine flowed more plentifully, and became less pungent. An observation, precisely similar, is made by Mr Cruickshank. "A patient of mine (says Mr Cruickshank), with a stricture of the oesophagus, received nothing, either solid or liquid, into the stomach for two months; he was exceedingly thirsty, and complained of making no water. I ordered him the warm bath for an hour, evening and morning, for a month; his thirst vanished, and he made water in the same manner as when he used to drink by the mouth (H).†

But to return to the case of Mr M.—Dr Currie himself remarks, that the discharge by urine alone exceeded much in weight the waste of his whole body; and it cannot be doubted that the discharge by stool and perspiration exceeded the weight of the clysters.—Thus it appears, that the egesta exceeded the ingesta in a proportion much greater than the waste of his body will explain. How is this accounted for, Dr Currie asks, unless by cutaneous absorption?

That the excess of these discharges above the ingesta and total waste, can be accounted for by absorption only, was indeed an irresistible conclusion. Still, however, cutaneous absorption is denied; and, when forced to confess, that there are cases where the egesta exceed the ingesta in a much greater proportion than the waste of the body will explain, and which can only be accounted for by absorption, they refuse this function to the skin, and bestow it most liberally, and, in so far as we know, most gratuitously, on the lungs. We are not entitled, in return, to deny the reality of pulmonary absorption, but we may surely be allowed to urge, that there is no proof that the only inhaling organ is in the lungs; and there is none against the possibility of cutaneous absorption.

Is it not, on the other hand, proved, by the experiments of Seguin and Lavoisier, that the exhalation greatly exceeds the absorption by the pulmonary system? And if this is always the case, we cannot explain by pulmonary inhalation alone, why the egesta should, in some cases, exceed the ingesta in a much greater proportion than the waste of body will account for.

We now proceed to examine another class of experiments, much insisted on by those who deny cutaneous absorption; we mean those experiments performed by immersing a part of the body in solutions of active drugs, the absorption of which should be indicated by their usual effect on the system.

Seguin made numerous experiments of this kind with solutions of muriate of mercury (corrosive sublimate), on phthisic patients. And we are informed, that in cases where the epidermis was perfectly found, neither the known effects of mercury on the body, nor any amelioration of the venereal symptoms, was ever observed.

He also immersed his own arm in a solution of two drams of the mercurial muriate in ten pounds of water. At the temperature of 12° and 26° Reaumur, no part of the salt was missing at the end of the experiment; but when the bath was at 18° of the same scale there was a loss of one or two grains of the muriate in the hour, though the quantity of fluid was not diminished.

The explanation given by Seguin of this unexpected result is curiously ingenious, but embarrassed, and inconsistent.

At the temperature of 12°, he observes, the exhalants are in a state of contraction, and their orifices nearly closed. When the heat again is raised to 26°, the exhalation is so rapid, that nothing can enter the vessels from without; but at 18° of temperature, the orifices of the exhalants are sufficiently relaxed, and the exhalation at the same time so conveniently languid, that the solution rests quietly in contact with the matter of perspiration in the mouths of the exhalants, where it is somehow or other decomposed; a part of the salt leaving the water of solution, and combining with the perspirative matter, with which it is carried into the system*. Carried into the circulation by the exhalants! Is not this a plain acknowledgment of the reality of inhalation? But if in tom. iii., one case substances may thus be carried into the circula-

(h) That thirst may be allayed by immersion in water, is fully proved by the experience of shipwrecked mariners, who, when obliged to take to their boats with very little fresh water, frequently have recourse to bathing in the sea, or covering themselves with a shirt wetted in salt water, and thus quench their thirst, nearly as well as if they had drunk fresh water. circumstances where the body is continually losing, we may infer, that something has been gained by absorption. And where the ingesta exceed the ingesta in a proportion much greater than the waste of the body will explain, there absorption must have been going on.

The case of Mr M—, published by Dr Currie, is not singular. The writings of physicians abound in similar examples. They had often occurred to that excellent clinical practitioner De Haen, who was therefore persuaded that water was imbibed. Haller too, with his usual industry, has collected a great many observations of the same kind.

Again, when physicians were engaged in their extensive statical experiments, weighing themselves, their ingesta and egesta, for many months, nay for years together; they sometimes observed, that so far from losing, they had gained weight, especially during cold and moist weather. Thus, Rye, under a cold and humid atmosphere, gained 13 ounces. Linnings, during two hours exposed to cold, acquired 8½ ounces. The abbé Fontana, after two hours exposure to a moist atmosphere, returned home some ounces heavier than he went out. De Gorter gained 6 ounces in one night; and on other occasions, two ounces and four ounces. These observations are confirmed also by the experience of Dr Francis Home, professor of Materia Medica in the university of Edinburgh. "Having fatigued myself pretty much (says he) in the afternoon, I went to bed without supper, and was so hungry that I could not fall asleep for some time. Betwixt eleven at night and seven next morning I had gained two ounces."

Here then are examples of the body gaining considerably more than the ingesta will account for, acquiring weight when neither food nor drink had been swallowed. And we have the concurring testimony of the most respectable writers supporting the same truth.

How can this increase of weight be accounted for, unless by absorption? In such experiments, the loss of weight, which cannot be accounted for by the sensible egesta, is attributed to the exhalation; the increased weight sometimes observed, and which cannot be explained by the sensible ingesta, must in like manner be referred to the inhalation.

That the system may be affected by active medicines introduced and absorbed by the skin, cannot be denied. And were proofs still wanting to establish the doctrine of cutaneous absorption, this argument might be insisted on. It is true, that friction is commonly employed when we wish to introduce medicines by the skin, by which, it is said, the substance is mechanically forced through the cuticle, and brought into contact with the absorbents of the true skin. The system, however, may be affected without friction, for example, by wearing a mercurial plaster, and more certainly by mercurial fumigation, as practiced by Lalonette and others.

It might even be concluded from an examination of the structure of the skin, that absorption must take place at its surface. We know that the cuticle is porous, and is penetrated by exhaling vessels; we know that lymphatics commence immediately below it; and we know that when certain substances are applied to the cuticle, especially when this application is aided by moderate friction, as in the case of applying garlic poultices to the feet, and the more familiar instance of mercurial imunion, that these substances are taken up by the absorbents, and conveyed into the circulation.

From a consideration of all these circumstances, we think it fully proved, that the skin is an absorbing or inhaling organ. For further proofs we may refer our readers to Bichat's Anat. Gener. tom. iv. p. 691.

Mr Charles Darwin, son of the late Dr Darwin, published in 1780, a Latin thesis, which is translated in the retrograde 29th sect. of his father's Zoonomia, vol. i. in which he attempts to prove, that the valves of the lymphatics are formed as in particular cases to admit of the regurgitation of the absorbed fluids. The arguments on which he finds this opinion (beside the difficulty of accounting for the phenomena of several diseases on any other principle), are chiefly the following:

First, The mouths of the lymphatics seem to allow water to pass through them after death, the inverted way, more easily than in the natural direction.

Secondly, In some diseases, as diabetes and scrofula, it is probable the valves themselves are diseased, and are thence incapable of preventing the return of the fluids they should support.

Thirdly, There are valves in other parts of the body, analogous to those of the absorbent system, which are liable, when diseased, to regurgitate their contents.

Fourthly, The capillary vessels, which must be considered as analogous to absorbents, may be seen, in animals submitted to the microscope, to regurgitate their contents into the arteries, during the struggles of the dying animal.

By means of this hypothesis (for notwithstanding the arguments adduced in its favour, we can call it no better) Mr Darwin explained the speedy passage of watery fluids from the stomach to the urinary bladder; the phenomena of diabetes; diarrhoea, dropsy, cold sweats; translations of matter, chyle, milk, and urine; the operation of external remedies, &c.

In all those classes of animals that possess a complete absorbent system, the phenomena of absorption seem to proceed much in the same manner as in man; but in some of the inferior animals, especially in mollusca and worms, this function seems to be performed by the veins. In the echinodermata, however, especially in the sea-urchin (echinus esculentus), lymphatics have been demonstrated by the second Monro. In insects and polyps there is no proper absorption.

The absorbing vessels of plants are chiefly the fibrils of the roots, which evidently imbibe moisture, and per-ov plants, perhaps gaseous fluids, from the earth; but there have also been demonstrated vessels opening beneath the outer bark, which botanical physiologists consider as lymphatics. "Lymphatic vessels (says Wildenow), are found in the epidermis of plants, and are of great minuteness, anastomosing in various ways through small intermediate branches." They surround the apertures of the cuticle, by which the inhalation and exhalation of vegetables are carried on; but they are so minute as not to have yet been filled with coloured liquids. Round each opening, which is commonly shut by a moveable valve, they form a circle, rarely a rhombus, as in the sea murex. In the lilium calcedonicum, those vessels run obliquely, and somewhat in an irregular undulating manner, fig. 2. In the common onion (allium cepa), they run in a straight, though oblique and regular form, fig. 3. In the pink, (dianthus). quantity of purulent matter from an abscess, in consequence of violent vomiting.

**Chap. VII. Of Circulation.**

We have traced the nutritious particles of the food from the intestines through the lacteals, the mesenteric glands, and the thoracic duct, into the left subclavian vein, where we find the chyle is mixed with the venous blood, and carried to the right auricle of the heart. We must now consider how the fluids are conveyed to every part of the body, or we must examine the function of circulation.

This function takes place in all the vertebral animals, and in mollusca, worms, and crustacea; but there appears to be no real circulation in insects, zoophytes, and plants.

The organs by which circulation is performed differ essentially in the several classes of animals. Those of the human system have been described in the first part of Anatomy, sect. x. and xi. and a brief comparative view of the organs in the inferior animals, has been given in the second part of that article, No. 154, 201-204, 300, &c., and in the articles Cytology, Erpetology, Ichthyology, and Ornithology. For a fuller account of these latter, we may refer to Cuvier's Lecons, lec. xxiv. and xxv. and to the Comparative Anatomy of Blumenbach, chap. xii.

It is well known that, in the red-blooded animals, the pulmonary blood is not of the same shade of red in every part of the body; but that what has passed through the lungs, and mic parts, is circulating through the arteries that proceed from the aorta, is of a florid red colour, while that which is sent to the lungs by the pulmonary artery, as well as that which is returning from the extremities of the arteries through the veins, is of a purple or crimson colour. As one fet of organs always contains florid, and another fet always crimson blood, it is convenient to distinguish each fet by an appropriate name. Dr. Barclay has done this, and he calls that fet of organs which are employed to convey the blood from the arteries, and distributed to the lungs, pulmonic, comprising the pulmonic veins, viz., the vena cava and its branches, pulmonic auricle, pulmonic ventricle, and pulmonic arteries; while he designates that fet of organs which return the blood from the lungs, and distribute it to the system, systemic, comprehending the systemic veins (pulmonary veins), systemic auricle, systemic ventricle, and systemic arteries, (the aorta and its ramifications). One great advantage of this nomenclature, is that it prevents the ambiguity of the expressions right and left, anterior and posterior, applied to the auricles and ventricles of the heart. We shall therefore employ them in the subsequent part of this chapter.

For an account of the nature and properties of the clay on blood, see Anatomy, Part I. sect. 14. and Chemistry, Medicinal Materia, No. 2642.

It may not be improper, in this chapter, to notice the principal arguments that have been used to prove the proofs of circulation of the blood. They are as follow:

1. When an artery is tied, the part of the artery that is betwixt the ligament and the heart, swells; but that part of it which is betwixt the ligature and the remote branches, becomes more flaccid than before. On the other hand, when a vein is tied, the part between the ligature... 2. The valves placed at the mouths of the aorta and pulmonary artery, must prevent the blood from regurgitating into the ventricles, while they permit it to flow forward through the arteries, into the capillary branches of the veins. Again, the valves situated in the course of the veins prevent the blood from flowing back into the arteries, while they permit it to proceed forward through the venous trunks into the heart.

3. By the assistance of a microscope, the blood may be seen in the pellucid parts of animals, as the feet of frogs, flowing from the arteries into the veins.

4. When an artery of moderate size is wounded, and not secured by ligature or compression in proper time, the blood flows out till the animal be dead.

5. Any thin liquid, when injected into an artery, does not pass backwards into the heart, but flows forward into the inoculating veins. On the other hand, such a liquid thrown into a vein, flows towards the heart, and not into the smaller branches of the vein.

The phenomena of circulation in the human body have been already mentioned under Anatomy, sect. xii. and xiii. and under Medicine, No. 95. We shall here only offer a compendious view of the course of the blood in the adult and in the fetal state.

I. After birth, the blood coming from every part of the body through the numerous ramifications of the vena cava, is poured into the right or pulmonic auricle of the heart, by the contraction of which it is thrown into the right or pulmonic ventricle, which contracting, throws it into the pulmonary artery, being prevented from regurgitating into the auricle by the action of the tricuspid valves. It is now, by means of the ramifications of the pulmonary artery, distributed through the lungs, from which it is brought back by four principal pulmonary veins, and poured into the left or systemic ventricle, which contracting with great force, propels it into the aorta, it being prevented from regurgitating into the auricle by the action of the mitral valves. The blood being propelled into the aorta, is by its trunks and branches, distributed to every part of the body, and brought back as before by the ramifications and trunks of the vena cava.

II. In the fetal state, the blood being brought back from every part of the body by the ramifications and trunks of the vena cava, is poured into the pulmonic auricle of the heart, where it is mixed with the blood brought from the placenta. A part of this blood is conveyed from the sinus of the auricle while in a state of dilatation, through the oval hole, into the systemic auricle; while another part, by the contraction of the auricle, is thrown into the pulmonic ventricle, which contracting, propels it into the root of the pulmonary artery. From this the greater part of the blood passes through the arterial canal, that in the fetus joins the pulmonary artery to the aorta, into this latter; while the remainder is distributed by the ramifications of the pulmonary artery to the lungs, and brought back by the pulmonary veins to the systemic auricle of the heart. By the contraction of this auricle, the blood is thrown into the systemic ventricle, which contracting, propels it into the aorta. Now, while one part of the blood is distributed by the numerous ramifications of the aorta to every part of the body of the fetus, another part is carried from the internal iliac arteries, through the two umbilical arteries, into the placenta, from which it is conveyed through the umbilical vein to the sinus of the liver. Hence, one part of the blood, without entering the liver, is transmitted by a branch of the umbilical vein, called venous duct, into the left branch of the vena cava hepatica; thence into the inferior great vena cava; while another part, by another branch of the umbilical vein, flows into the left branch of the vena porta, by the numerous ramifications of which it is distributed to the liver. From the liver it is carried by the vena cava hepatica into the inferior great vena cava, whence it is conveyed with the rest of the blood, to the pulmonic auricle of the heart, to be distributed as before.

We must now briefly consider the powers by which Powers carry the blood is made to circulate through such a multitude of vessels, so infinitely ramified, and differing so much in their diameter. It seems generally allowed at the present day, that these powers are chiefly the immediate muscular action of the heart, the action of the arteries, the valves of the veins, and the pressure produced on some of the veins by the action of the muscles that lie contiguous to them.

1. That the heart must possess very considerable force Action of in propelling the blood through the arteries, may be the heart, supposed from the great muscularity of its ventricles; and this force has been proved by experiment and observation. From experiments made by Hales, viz. that of inserting a glass tube into a large artery, and measuring the height to which the blood ascends at each pulsation, it has been calculated, that the human carotid artery is capable of projecting its blood to a perpendicular height of seven feet and a half; and if we estimate the surface of the systemic ventricle at 15 square inches, we shall find that it sustains a pressure of 7350 cubic inches, equal to 51 pounds weight, which it has to overcome by its contracting force*. This is a moderate * Hales's computation of the force of the heart, for Borelli estimates it at 180,000 pounds, while Keill diminishes it to eight ounces. Senac again, from having observed, that if a weight of 50 pounds be attached to the foot, with the knee of that side placed over the opposite knee, the weight will be raised at each pulsation, and allowing for the distance at which the weight is placed from the centre of motion, computes the force of the heart at 400 pounds†. Blumenbach has seen the blood projected from the carotid of an adult more than five feet‡. On a medium calculation, estimating the quantity of blood contained in the body at 30 pounds, the number of pulsations in each minute at 75, and the quantity of blood ejected from the systemic ventricle at each contraction at two ounces and a half, we shall find that the whole 30 pounds of blood will be carried through the whole body no less than 23 times in an hour, or the circulation will be completed in less than three minutes. From these circumstances we must infer that the impelling power of the heart is very great, and fully adequate to the office which it has to perform.

Various hypotheses have been formed to explain how the heart and arteries are excited to motion, but our limits will not permit us to detail them. Our readers will find them related at considerable length, and fully examined in the Principes de Physiologie of Dumas, tom. The general opinion at present entertained on this subject is, that the heart is excited to action by the stimulus of the blood.

2. Although it is more than probable that the action of the arteries of the heart is the principal instrument in carrying on the circulation, there can be no doubt that the arteries contribute essentially to this office. They are evidently muscular, and are possessed of considerable irritability; are supplied with numerous nervous filaments, and are nourished by small arteriæ branches, commonly called vaginae variorum. Nay, we know that they are susceptible of contraction; and when we divide an artery in the living body, the divided extremities gradually contract, till, if the animal is not killed by the experiment, the aperture is at length obliterated. Lastly, there have been instances of foetuses without a heart; and as we must suppose that, during the life of the foetus, the circulation was going on, it is a natural inference that this was chiefly effected by the contraction of the arteries, and not entirely by the impelling power of the circulating system of the mother.

3. It cannot be supposed that the veins have any intermediate action on the blood, as they exhibit no circular fibres like the arteries, except in the immediate vicinity of the heart; but their valvular structure must contribute to the carrying on of the circulation, from the opposition it gives to the return of the blood, so that what is called the vis à tergo, or impelling power from behind, aided by the conical form of the veins, may have its full effect.

4. That the action of the muscles has considerable influence in propelling the blood towards the heart, in those veins that lie in their neighbourhood, is evident from the effect that bodily exercise produces in accelerating the circulation, and from the efficacy of friction in removing congestions of blood in the veins of the extremities, and in the more familiar instance of promoting the swelling of the veins of the arm by the same means in the operation of bleeding.

The circulation in those animals whose structure approaches to that of man, differs little from what is above described. There are indeed some peculiarities, a few of which we shall presently notice. An account of the circulation in the cetacea will be found in the article CETACEA, No. 140—145, that of the reptiles is described under REPTILIA, p. 309.

The mollusca possess an evident and powerful circulation. Most of them have a simple heart, consisting of one auricle and one ventricle; and in these the vena cava performs the office of an artery, carrying the returning blood to the gills, whence it passes to the auricle, and is afterwards thrown into the aorta. There is a peculiarity in the cuttle fish, which has a heart consisting of three ventricles, without any part that can properly be called an auricle. Two of the ventricles are placed at the roots of the two bronchiae, and have each a branch of the vena cava, by which they receive the blood from the body, and propel it into the bronchiae. The returning veins open into the middle ventricle, and from this the aorta proceeds.

Some of the vermes, as the leech, and the tribes of the naias, nemertes, and aphrodites, and some species of lumbricus, have no heart, but they have circulating vessels with evident contraction and dilatation.

In the crustacea, the circulation is performed by a single ventricle, expelling the blood into the arteries of the body, and receiving it again after it has passed through the gills, in a manner very similar to the circulation of fishes.

There is no circulation in insects; but these animals have running along the back a membranous tube, in which alternate contractions and dilatations are perceptible. This tube, however, is closed at both ends, and has no vessels proceeding from it.

From the researches which evince circulation to be a function so general among animals, some are disposed to think it takes place in all living bodies. But notwithstanding the fashionable language of circulating fluids, of veins, arteries, and even of valves, in the vegetable structure; yet nothing performing the office of a heart, and nothing that seems to conduct fluids in a circular course, has been found in plants. In the vegetable kingdom, the chyle is distributed to all the parts, from the numerous vessels which convey the sap; and these vessels, being fitted by their structure to carry the sap either downwards or upwards, from the branches to the roots, or from the roots to the branches, is the reason why plants inverted in the ground will send forth roots from the place of their branches, and send forth branches from the place of their roots. Even a similar distribution of the chyle takes place in some animals. In the human tenia, in the fasciola hepatica (fluke) of sheep, and in most polypes, the chyle, without a circulating system, is conveyed directly to the different parts from the alimentary canal.

For an account of the motion of sap in plants, see Darwin's Phytologia, a paper by Mr Knight in the Philosophical Transactions for 1801, Wildenow's Principles of Botany, sect. 276, and the article PLANT in this Encyclopedia.

The relations that subsist between the function of circulation, and those which we have already considered, are very important. We shall begin with those of circulation and sensation. That the functions of the nervous system must be considerably influenced by the circulation of the blood, may be supposed a priori, from the large quantity of blood sent to the head, this being, on a moderate calculation, about one-tenth of the whole. A certain quantity of blood in the vessels of the brain seems essential to the due performance of the functions of that organ; and those animals, which, like man, have the blood sent in greatest quantity, and with greatest impulse, seem to possess the faculties of the brain in the greatest perfection, while those in whom the motions of the blood towards the head is much retarded, as in the sheep and cow, are remarkable for mildness and fluidity. When, however, the quantity of blood becomes too great, or its impetus too violent, the faculties of the brain are impaired, or altogether destroyed. No man, and very few other animals, can remain suspended with the head downwards for any long time, without dangerous, and commonly fatal consequences. The bat, indeed, is a remarkable exception to this rule, for this animal can hang by its hinder feet for days or weeks together, with perfect safety, a circumstance that may be accounted for from the very small quantity of blood contained in its circulating vessels. Again, the brain exerts an evident influence on the circulation. It is well known how much the action of the heart and arteries is quickened, impeded, or rendered irregular, by the The passions of the mind. Cases are recorded in which these passions, carried to excess, have altogether stopped the circulation, and produced infant death. Fainting is often brought on by the sight of a disgusting or terrifying object, or by the odour of perfumes, or of substances to which the person has a particular antipathy. The sympathy between the heart and the nervous system is further shown by the violent pain below the sternum, and sometimes in the arm, in cases of organic disease of the heart.

The circulation is even affected by intense thought; and we have heard of a bleeding from the nose being brought on by long and deep study, while the body was in a reclining posture, namely in bed.

The functions of circulation and motion are intimately related. It is scarcely necessary to notice the acceleration of the pulse, in consequence of exercise and labour, or to remark that in indolent and sedentary people the circulation is generally slow and languid. In general, too, the blood circulates with most rapidity in those animals who are formed for quick motion, though the instance above quoted, of the bat, shows that the quickness of motion does not depend on the quantity of blood. Several curious anatomical facts have rendered it probable, that the production of quick or slow motion depends in a great measure on the mode in which the arterial branches are distributed to the moving organs. The arterial branches that supply the organs of voluntary motion, are divided in such a manner as to impede the motion of the blood towards these organs as little as possible: their ramifications are therefore few, and they go off from the trunk at very acute angles; whereas those that supply the viscera are at nearly right angles, are often tortuous, and are otherwise so constructed as frequently to impede the flow of blood. Physiologists have even explained the greater power that is generally found in the right arm, and the greater readiness with which most people use that arm, by the manner in which the right subclavian artery comes off from the aorta. This indeed we are disposed to consider as fanciful, and to attribute the more ready use of the right arm solely to habit and early instruction not to employ the left.

In some animals that are remarkable for slowness of motion, as the lemurs tardigradus, or slow lemur, and the Bradypus tridactylus, or common sloth, there is a curious construction of the arteries that are distributed to the limbs of these slow-moving animals, which must have the effect of breaking the force, and impeding the velocity of the blood towards these limbs. In the lemurs tardigradus, a specimen of which was dissected by Mr Carlile, it was found that the subclavian artery and the external iliac were, soon after rising from the general trunk, divided into a great number of equal-sized cylinders surrounding the principal artery, now diminished to a very small size, and that each of these branches was sent to each of the principal muscles of the limbs, while the other arteries that supplied the other parts of the limbs were divided in the usual arborifcent form. Struck with this appearance, Dr Shaw and Mr Carlile afterwards examined several specimens of the sloth, and found a similar conformation, while in other species of bradyptus, not remarkable for slow motion, no such appearance took place*. Fig. 5 & 6. Plate CCCCXVIII. represent the division of the arteries in the slow lemur above described.

Vol. XVI. Part II.

The relations between the circulating and digestive organs are proved by the sudden acceleration of the pulse from stimuli received into the stomach; from the diminished circulation or sudden cessation of the heart's motion from powerful sedatives received into the same or of circulation, especially the prunus lauro-cerasus; from the irregularity produced in the circulation in consequence of dyspepsia, and many other considerations.

That there is an intimate relation between circulation and absorption, cannot be doubted, though the nature and effects of this relation are not yet well understood. We know that the vessels sympathize with the absorbents in their activity or languor; that when the absorbents are languid in their action, the blood-vessels, especially the exhalants, are in a feeble or relaxed state, and that the absorbents are often roused to greater action by remedies that first act on the circulation.

Numerous experiments have shown how much the colour and consistence of the blood are altered by the mere action of the vessels; and this discovery has enabled us to conjecture with more probability than we did formerly, why in infants and phlegmatic persons the blood is paler, in the choleric more yellow, and in the fanguine of vermilion red. It explains likewise in some measure, why the blood varies in the same individual, not only with regard to the state of health, but likewise at the same instant; and why the blood which circulates through the veins has not the same intensity of colour, nor the same consistence, as that of the arteries; and why the blood which flows through the organs of the breast differs from that which passes languidly through the viscera of the lower belly. This power of the vessels over the blood will bring us also to the true cause why the vessels vary in the density of their coats and in their diameters; why they are sometimes convoluted in a gland, and why they sometimes deposit their contents in a follicle; why they are sometimes of a spiral form; why the branches strike off at various angles; why they are variously anastomosed; why they sometimes carry the blood with dispatch, and sometimes slowly through a thousand windings. By these means their action is varied, and the blood prepared numerous ways to answer the ends of nutrition and secretion.

On the varieties of the pulse, and the morbid affections of circulation, see Medicine, No 96—104.

Chap. VIII. Of Respiration.

While the fluids are passing through the body, by necessity what is called the greater circulation, they give out certain parts or principles, partly for the purpose of nutrition, and partly to free the system from noxious matters. In order to regain some of the principles which they have lost, it is necessary that they should be exposed to the influence of atmospheric air; for which purpose they are made to pass through appropriate organs, which, as we have already observed, are in general either lungs or gills. The action by which the fluids and the air are made to act on each other, is called respiration, and consists of two kinds, inspiration, by which the air is received into the body, and expiration, by which it is again thrown out.

So essential is respiration to the system, that snails, chameleons, and some other animals, can live for years without any apparent nourishment, provided they be not excluded. excluded from air. We have seen a chameleon that lived, and was vigorous, for 22 months, without any food, and which might have continued to live much longer, but for an unfortunate bruise by a fall.

Other phenomena equally demonstrate the importance of air to the living body. The frog leaps away wanting its heart; it survives the loss of the greatest part of its spinal marrow; without its head, it lives for some days, and its heart continues to circulate its blood. Borelli found, that eels and serpents, though their bodies be opened, and the whole of their bowels be taken out, are able to move for a day after, and yet, in all these animals, the life is observed to be suddenly extinguished when the all-vivifying air is excluded. Even the smallest insect has died, and the plant lost its vegetative power, when retained for any considerable time in a vacuum. Fishes themselves, when placed under an exhausted receiver, have started anxiously to the surface of the water in quest of fresh air; and finding none, have sunk to the bottom and expired in convulsions. It will presently appear that this necessity of air to life is general in all the classes of organized beings (k).

The organs of respiration belonging to the human system, viz. the larynx, windpipe, lungs, diaphragm, ribs, and numerous muscles, have been sufficiently described in various parts of the article ANATOMY; and some account of these organs in the inferior animals has been given in the second part of that article, No 155, 156, 206, 208, 271—274, and in the articles CETOLOGY, EREPTOLOGY, and ICHTHYOLOGY. For a more complete account of the respiratory organs in the inferior animals, the reader is referred to Cuvier's Leçons d'Anatomie Comparée, Lec. xxvi. et xxvii., and to Blumenbach's Comparative Anatomy, chap. xiv. and xv.

In examining the function of respiration in warm-blooded animals, two circumstances are principally to be considered; the mechanism of respiration, or the mechanical means by which the organs are enabled to receive and expel the air, and the effects produced by respiration on the circulating fluids, and on the system at large. Our principal object in this article is briefly to explain the mechanism of respiration; to notice the effects produced on the air by the respiration of different animals, with the effects produced on them, and the relations that take place between respiration and the preceding functions of sensation, motion, digestion, and circulation.

In order to make an inspiration, the intercostal muscles, and the muscular fibres of the diaphragm, are thrown into contraction, while at the same time the abdominal muscles, and the muscular fibres of the windpipe, are relaxed. By these means the diaphragm being drawn towards the sacrum, and rendered less convex towards the chest, and the ribs being drawn upwards (or forwards in quadrupeds), the cavity of the chest is enlarged, and the air remaining in the lungs being raised, the external air rushes in through the windpipe by its own gravity, and distends the lungs. In making respiration, what is called a very deep inspiration, other muscles that are connected with the atlantal ribs, viz. those called scaleni, trapezius, cervicales descendentes, serrati superiores, and pectoral muscles, assist to elevate the ribs more than in an ordinary inspiration. By the action of the intercostal muscles the ribs are drawn atlanted (upwards in man, and forwards in quadrupeds), because the most atlantal rib on each side of the thorax is fixed, and therefore all the other ribs are drawn towards it; and they are also drawn peripherad (outwards), because their greater curvature is in the direction of the sacrum, and because they turn on their vertebral extremities as on a fulcrum.

In order to make an expiration, the abdominal muscles, and the muscular fibres of the windpipe, are contracted, while the intercostal muscles, and the muscular fibres of the diaphragm are at the same time relaxed. By these means, aided by the elasticity of the cartilages of the ribs, and perhaps of the mediastinum, the ribs are drawn facrad (towards the sacrum), while the diaphragm, partly by its own muscular action, and partly in consequence of the pressure of the bowels, is rendered convex towards the chest. Thus, this cavity is considerably diminished, the lungs are compressed, and part of the air is expelled through the windpipe. In making a strong expiration, little more is necessary than a more powerful contraction of the abdominal muscles.

The mechanism of respiration in mammalia is so similar to that of man, that we need not enter into it; that of cetacea has been sufficiently explained under CETOLOGY, No 146—153; that of birds may be gathered from what has been said on their structure, in the comparative part of ANATOMY, No 271, and in ORNITHOLOGY, No 37; that of reptiles has been fully explained under EREPTOLOGY, page 311; and that of fishes under ICHTHYOLOGY, page 73. The mechanism of this function in the classes of animals below these is so simple, that a consideration of it is unnecessary. In insects the air enters the numerous ramifications of the tracheæ; is carried by them into every part of the body, and is then returned by the same passages. In some of the mollusca, respiration proceeds in a similar manner, by the ramifications of the pulmonary vessels that enter by the neck; but in most of these, and in all the lowest tribes, such as worms and zoophytes, respiration seems to be carried on entirely by the pores of the skin.

Many attempts have been made to ascertain the quantity of air received and emitted in a single inspiration or expiration. See CHEMISTRY, No 2535. This point is not yet fully ascertained, but we may probably estimate the quantity expelled by each ordinary expiration at one-seventh part of the whole contents of the lungs, and that of the most violent expiration at about four-sevenths.

(k) It was long ago observed by Pliny, that if the bodies of insects are besmeared with oil, they soon perish: "oleo illis infecta omnia exanimantur." The same observation was afterwards made by Ray, who explained it by showing that in this way the pores through which the animals breathe are stopped. Mayow also found, that if the oil be applied only to some of these pores, the neighbouring parts become paralytic, while the rest of the body continued found. See Ray's Wisdom of God, and Mayow's Tractatus. sevenths of that quantity; or that the medium quantity of air consumed during a common inspiration or expiration is about 40 inches.*

Another circumstance respecting the mechanism of respiration merits notice, viz., the ordinary number of respirations made in a minute by a healthy person. This varies considerably in different subjects, being in general greatest in children, and least in old persons. Dr Hales estimated the number of 30 in a minute; the subject of Dr Menzies' experiments respired only 14 times in the same period, while Mr Davy reckons his respirations at between 26 and 27; the subject of the experiments made by Messrs Allen and Pepys, breathed about 19 times in a minute, and with this Dr Thomson's experience agrees. The average of all these is about 25, which we may probably consider as a tolerably just estimate. Much, however, will depend on the circumstances in which the person is placed; on his habits of activity or idleness, of temperance or intemperance; on the state of the atmosphere, &c.

The chemical changes produced on the air by the respiration of animals have been described, so far as they were then known, under Chemistry, No. 2536. Since that article was written, however, several valuable observations have been published, and the most important of these must be here noticed; but as we cannot, in this place, give anything more than a summary sketch of these observations, we shall here enumerate the principal works that have appeared on this experimental part of our subject. The chemical physiologists who have been most conspicuous in these researches are, Mayow (in his Tractatus de Respiratione, or the Analysis of his works by Beddoes); Priestley (Experiments and Observations on Air); Lavoisier (Traité Élémentaire de Chimie, Physical and Chemical Effects, and a work on Atmospheric Air); Goodwyn (On the Connection of Life with Respiration); Coleman (On Suspended Respiration); Menzies (On Respiration); Spallanzani (Memoires sur la Respiration), translated into French by Senebier, and since into English, and Rapports de l'Air avec les Étres Organisés, &c., also collected by Senebier; Davy (Recherches Philosophiques et Chémiques, into the Nature of Nitrous Oxide); Ellis (Inquiry into the Effects produced on Atmospheric Air by Respiration), and Allen and Pepys (Philosophical Transactions, 1828, Part II., or Phil. Mag. vol. xxxvii.). Most of the facts observed by these experimentalists, except those of the three last mentioned, have been collected by Dr Bostock, in his Essay on Respiration, by Dr Thomson in his System of Chemistry, vol. v., third edition, and by Mr Johnson in his Animal Chemistry, vol. iii.

The following changes produced on air that has been respired by warm-blooded animals, seem to have been ascertained; viz. 1. That the respired air generally suffers a sensible diminution of bulk; 2. That it loses a part of its oxygenous portion; 3. That it acquires an additional quantity of carbonic acid gas; 4. That it is charged with watery vapour. We shall resume these facts, and consider the additional information which has been acquired respecting them since the publication of our article Chemistry.

1. Atmospheric air generally suffers, by the respiration of warm-blooded animals, a sensible diminution in its bulk.—The results of the experiments made by different chemists on the diminished volume of respired air, are exceedingly various. Mr Davy makes the diminution amount to one-eighth of the whole air inspired; Lavoisier and Goodwyn estimate it at no more than one-twentieth, and Dr Bostock so low as one-eightieth; while Crawford and some later experimentalists could perceive no diminution. Dr Thomson states the results of his experiments upon this subject to be, that in some cases he could perceive no diminution at all, while in others it was perceptible. It was greatest when the animal was taken out repeatedly during the experiment, or when he employed air purer than that of the atmosphere. He is disposed to consider the diminution as accidental, and as owing to some abstraction of air, altogether independent of respiration, and exceedingly various in different circumstances. In the experiments of Messieurs Allen and Pepys, the general average, third rate of the deficiency in the total amount of common air inspired, appeared to be very small, amounting to about six parts in 1000, and they are inclined to attribute it, in a great measure, to the difficulty of exhausting the lungs so completely after an experiment as before it.

2. Atmospheric air, by the respiration of warm-blooded animals, loses a part of its oxygen.—From a comparison of the experiments of Mr Davy, with those made by Bostock, just before his death, Dr Thomson estimates the quantity of oxygen consumed in a minute, by respiration, at 31.6 cubic inches, making in 24 hours 43,504 cubic inches; and he concludes that in a day a man consumes rather more than 25 cubic feet of oxygen, and that he renders unfit in the same time for supporting combustion and respiration, no less than 125 cubic feet of air.

3. Atmospheric air acquires, by respiration, an additional quantity of carbonic acid gas.—The opinion of Dr Menzies, that the bulk of carbonic acid gas produced by respiration, is precisely equal to that of the oxygen lost, appears now to be fully confirmed. In Mr Davy's experiments they corresponded very nearly; and in those of Mr Dalton and Dr Thomson, they corresponded exactly. The latter chemist found, on the whole, that the bulk of oxygen which disappeared was somewhat greater than that of the carbonic acid generated; but the difference varied considerably, and kept pace with the diminution of the bulk of air respired. Hence he considers it as owing to the abstraction of part of the air by some other way than respiration, and allowing for this abstraction, he has no doubt that the bulk of the carbonic acid formed is precisely equal to that of the oxygen that has disappeared. He is disposed to consider the absolute quantity of carbonic acid generated in 24 hours, as something less than 40,000 cubic inches on an average. The following results of the experiments made by Messieurs Allen and Pepys, as they were made on a large scale, may be considered as quite satisfactory on this head. 1. It appears that the quantity of carbonic acid gas emitted is exactly equal, bulk for bulk, to the oxygen consumed. 2. Atmospheric air once entering the lungs, returns charged with from 8 to 8.5 per cent. carbonic acid gas, and when the contacts are repeated almost as frequently as possible, only 10 per cent is emitted. When the inspirations and expirations are more rapid than usual, a larger quantity of carbonic acid is emitted in a given time; but the proportion is nearly the same, or about eight eight per cent. The proportions of carbonic acid gas, in the first and last portions of a deep inspiration, differ as widely as from 3.5 to 9.5 per cent. It appears that a middle-sized man, aged about 38 years, and whose pulse is 70 on an average, gives off 302 cubic inches of carbonic acid gas from his lungs in 11 minutes; supposing the production uniform for 24 hours, the total quantity in that period would be 39,534 cubic inches (agreeing almost exactly with Dr Thomson's estimate), weighing 18,683 grains, the carbure in which is 5363 grains, or rather more than 11 ounces troy; the oxygen consumed in the same time will be equal in volume to the carbonic acid gas; but it is evident, that the quantity of carbonic acid gas emitted in a given time, must depend very much upon the circumstances under which respiration is performed; and here it may be proper to notice, that all these experiments were made between breakfast and dinner.

A larger proportion of carbonic acid gas is formed by the human subject, from oxygen than from atmospheric air.

4. Atmospheric air returns from the lungs charged with aqueous vapour.—Of this circumstance there is no doubt, but the quantity of water contained in the expired air, and the sources from which it is derived, are still in dispute. Dr Thomson estimates the former at about 19 ounces per day; but he does not lay much stress on the results of his own experiments, as they were not sufficiently varied to give a fair average. As to the sources of this watery vapour, it has been generally supposed, that the water is formed in the lungs by a combination of part of the oxygen consumed with hydrogen evolved from the venous blood. This, however, is mere hypothesis. It has not been proved that hydrogen is evolved from the blood; and as the quantity of oxygen consumed appears to be taken up in forming the carbonic acid gas that is expired, there is none left to form water.

There is another change supposed by most chemical physiologists, to be produced on the air by respiration, namely the loss of part of its azote; but this is still disputed. Dr Bostock concludes it to be probable, that a small portion of azote is lost, which he estimates on an average at \( \frac{1}{20} \) of the air respired, making in 24 hours, about 4.5 ounces, or four cubic feet. Mr Davy found the consumption of azote to amount to about one seventh of that of oxygen; and some late experiments of Dr Henderson afford a similar result, though in these the proportion is rather less. Dr Thomson also found a loss of azote, but it was extremely inconstant, sometimes being scarcely perceptible, at others considerable. It kept pace with the diminution of the bulk of the air respired, and with the difference between the bulk of the oxygen consumed, and the carbonic acid formed. He conceives that a portion of the air respired disappears without undergoing any change, and that this portion occasions the diminution of the azote, and the difference between the bulk of the carbonic acid formed, and that of the oxygen consumed. He thinks it conceivable, that the disappearance of such a portion may be confined to the unnatural circumstances occasioned by the experiment; that the difficulty of throwing out the air from the lungs in these circumstances, may be such as to induce absorbents to act, and remove a portion which in the ordinary state of the lungs would have been thrown out by expiration.

Experiments on the changes produced on atmospheric air and oxygenous gas, by the respiration of the inferior animals, have been made chiefly by Vauquelin, Spallanzani, and Mr Davy, and some of them have been repeated and varied by Mr D. Ellis. From all these experiments we find, that by the respiration of amphibia, of fishes, of insects, of mollusca, and of worms, the air in which they have been confined suffers changes analogous to those produced in it by the respiration of the warm-blooded animals; that the oxygenous part is diminished, and that this diminution is most complete when insects and worms have been confined in it; that carbonic acid gas is in all cases produced, but that the quantity produced varies in different animals; that fishes live for the shortest time, and amphibia and worms for the longest, when confined in a certain quantity.

From the latest experiments made by Spallanzani, on the effects both of living and dead animals, on atmospheric air, as collected by Senebier, that experimentalist has drawn the following conclusions. 1. In beginning with worms, and rising up to man, there is no species of animal which does not destroy the oxygen of the atmosphere after death, and destroy it entirely if it be kept inclosed in it, provided the quantity be not too great in proportion to the size of the animal; because a considerable time is required when the volume of air is large, and a less time when the quantity is small.

2. This destruction of oxygen by dead animals, is under similar circumstances slower than that effected by living animals; if we regard merely the effects produced by the cutaneous organ, independent of the action of the lungs.

3. He thought he had legitimately proved, that the destruction of oxygen by the cutaneous organ, is not occasioned by the combination of this gas with the carbure of the animal; but that it is a true absorption of that element, by the body of the animal deprived of life. It does not give out carbure, but carbonic acid, as he believed he had proved by unanswerable experiments.

4. The absorption of oxygen by animals cut into small pieces, is greater than that occasioned by animals entire in similar circumstances.

5. A cold-blooded animal of the same bulk, and in the same circumstances as a warm-blooded animal, absorbs more oxygen than the latter after death.

6. The skin is not the only part of an animal which absorbs oxygen; all the parts, solid, fluid, and soft, not excepting the driest horny parts, as the nails of quadrupeds, the bill and feet of birds, produce the same effect.

It has long been known that plants would not vegetate, if excluded from atmospheric air. Papin confined vegetation an entire plant in the exhausted receiver of an air-pump, on the air, and it soon perished; but on keeping a similar plant in this vacuum, with only its leaves exposed to the air, it continued to live for a long time. When the leaves of a plant are stript off, or blighted by insects; when they have the upper surface smeared with oil, with varnish, or laid upon water, the plant dies in a few days. Hence it is evident that the leaves of a plant are necessary organs, and that there is produced on the air in which which the plant vegetates, some change essential to the healthy action of the plant. What this change is, has not been fully ascertained. It is the general opinion, that the leaves absorb a portion of the atmosphere, and give out certain gaseous products; and it is generally believed, that most plants have the property of giving out oxygenous gas during their exposure to the light, and azotic gas or some other irrefrangible air in the dark. That oxygenous gas is necessary to vegetation, is fully proved; and it seems certain, especially from the experiments of Mr Ellis, that under ordinary circumstances, carbonic acid gas is generated during their vegetation*. Mr Ellis, who is not satisfied with the accuracy of the experiments of Scheele and Priestley, seems to doubt whether plants at any time give out oxygenous gas; and thinks that the principal use of oxygen to them, is to combine with the superfluous carbure produced by vegetation, and thus form the carbonic acid evolved.

Having considered the effects produced on the air by the respiration of animals, and the vegetation of plants, we must now notice the effects produced by the exercise of the same function on the animal body, as the comparison of these effects with the changes produced on the air itself, affords us the only clue to a rational theory of respiration.

We have already stated (Chemistry, 254), that during respiration, the blood changes from the dark colour which it has in the veins, to the bright scarlet of arterial blood. It has been found, that a clot of venous blood, when out of the body, assumes the bright tinge, when exposed to the action of oxygenous gas; and that venous blood confined within a bladder, undergoes a similar change, when the bladder is immersed in oxygenous gas. It has been also found, that when arterial blood out of the body is exposed to the action of irrefrangible gases, it loses its bright colour, and assumes the purple hue of venous blood.

It is fully ascertained, that the heat of the body is chiefly kept up by respiration. See Chemistry, No 254.

Let us now consider the most probable theories of respiration, chiefly as they are applicable to the human system. Without staying to notice the older hypotheses that have been advanced to explain this function, we shall only state the present most received doctrine, and mention the objections that have been lately made to it.

This doctrine is stated in the following manner by Dr Bostock, one of its most strenuous defenders. "The blood arises at the right side of the heart, in a venalized state, loaded with a quantity of the oxide of carbure; as it passes through the pulmonary vessels, it becomes subjected to the action of the air contained in the bronchial cells; a portion of the oxygen is removed from the air, part of which, forming an intimate union with the oxide of carbure, is expelled in the form of carbonic acid gas, while the remainder is dissolved in the blood." It is here necessary to remark, that it is not oxygenous gas, but oxygen, which is supposed to be mixed with the blood. The caloric thus set at liberty is employed, part of it in maintaining the temperature of the lungs, which would otherwise be cooled by the admission of the external air; part of it in carrying off the aqueous vapour; and another portion in converting the carbonic acid into carbonic acid gas; but the greatest part of it is united, in the form of specific heat, to the arterial blood, which, by becoming arterialized, has its capacity for heat increased. The arterial blood is poured into the left cavity of the heart, and propelled through the arteries into the extreme parts of the body. The oxygen which was dissolved in the whole mass of blood, during the circulation, gradually unites itself more intimately to a portion of the carbure in it, which it converts into the oxide of carbure, and thus the blood acquires the venous state. By this change, its capacity for caloric is diminished; the specific heat which it obtained in the lungs, is given out in the capillary vessels, to keep up the temperature of the body, and the blood returns to the right side of the heart completely venalized. This hypothesis is nearly similar to the one which was proposed by M. M. La Grange and Hassenfratz; it received some modifications from Mr Allen of Edinburgh, and was delivered by him nearly in the form which I have stated above, in his admirable course of physiological lectures. It was, I believe, first published in my Essay on Respiration."

This doctrine, if sufficiently established, would explain the manner in which the blood becomes arterialized in the course of the lesser circulation. It would also show how, under particular circumstances, the arterial blood may be venalized without leaving the arteries, and the venous blood arterialized without leaving the veins. It accounts for the gradual evolution of caloric in the capillary vessels, during the course of the circulation, by the union which takes place between the oxygen and the carbure; whereas in the other hypotheses, (see No 241.) this union is entirely completed in the lungs. It would allow a considerable time in which this union might be accomplished, and would likewise suppose the constituent parts to remain in perfect contact for an indefinite period. This hypothesis would also explain how the oxygen is diluted of, which is supposed not to be concerned in the formation of carbonic acid, and would likewise possess the advantage of supposing the existence of a surplus quantity of oxygen, which being carried along the circulation, might be expended in a variety of useful purposes in the different parts of the animal economy. It would show how the supply of matter which is poured into the blood by the absorbents, is gradually incorporated with the cells; and after the separation of that portion, which is necessary for repairing the waste of the different organs, the remainder is united to oxygen, and keeps up the temperature of the body; and, having afterwards no farther useful purpose to serve, it is discharged by the lungs†.

Several objections, however, may be made to this theory. It is not proved that there is in natural respiration any absorption of oxygen by the blood; for though much of the oxygenous portion of the atmosphere is lost, the quantity of carbonic acid generated is sufficient to account for it. Mr Ellis has lately made very strong objections to this supposed absorption of oxygen, drawn chiefly from the anatomical structure of the blood vessels, and of the bronchial cells. He contends that the coats of the former, and the membranes bounding the latter, can scarcely admit the passage of air through them, much less that of the solid basis of oxygenous gas, which basis Dr Bostock supposes to be the principle absorbed. It is still more improbable, according to Mr Ellis, that two solid bases, namely those of oxygenous gas, and of oxide of carbure, should be at the same time passing through... through these resisting membranes; a supposition that is necessary in the hypothesis just stated. Again, supposing that this absorption of oxygen should take place, from the affinity of the blood for this principle, it is not easy to conceive why this affinity should so soon cease, and why the blood should again part with the oxygen, to the sake of carbonic oxide. These objections are certainly very forcible; let us see how they have been answered.

Dr Bostock, in an ingenious reply to Mr Ellis's objections, in invalidation of the first objection, quotes the well-known experiment of Dr Priestley, mentioned in No. 235, that venous blood becomes changed when exposed to oxygenuous gas, even though a bladder be interposed between them; and in controverting of the rest, he seems chiefly to rely on the supposition that a greater quantity of oxygen is consumed than is taken up in the formation of the carbonic acid. He also does not consider it as necessary to suppose that either the oxygenuous gas, or the oxygen itself, should enter the blood vessels, and should afterwards be expelled from them; but only that a part of the oxygen should be attracted by the blood, and after entering into a variety of new combinations, should be discharged as a constituent of some of these new compounds. Without inquiring in what way the action between the blood and oxygen takes place; whether it be in consequence of the mechanical structure of the membranes, which permits the oxygen to pass through their pores, or whether it be owing to the affinity of the blood for oxygen, which causes it as it were to become saturated with this substance before it transmits it; it appears to him sufficient to state, that oxygen and blood can act on each other, through a membrane which is very much thicker, and probably much denser, than that which separates the blood in the lungs from the air in the bronchial cells.

From a consideration of the principal experiments on respiration that have been made by the ablest chemical physiologists, and a comparison of these with what he has himself made, Mr Ellis contends that no part of the air enters into the blood, but all the oxygen which disappears is to be found in the carbonic acid produced; and that this carbonic acid is formed by the union of carburetted air with part of the oxygenuous portion of the inspired air.

To this opinion Dr Bostock, in the paper already referred to, makes the following objections; that this opinion does not explain how the regular supply of carburetted air, at each successive circulation, brought to the lungs in a state proper to be discharged; and that it does not explain in what way the oxygen is employed, which is consumed in respiration.

To the first of these objections (which, if it be proved that the whole of the oxygen is taken up in forming carbonic acid, is the only objection that can properly lie against his opinion), Mr Ellis replies, that the supply of carburetted air is derived from the digestive organs; but he does not conceive, as Dr Bostock seems to imagine, that this is no sooner received into the blood than excreted, or that the first operation which takes place in the sanguiferous system after it has received the substance which is to afford nutriment to the body, is to discharge the greatest part of it. He regards carburetted air as a constituent part of the animal fluids, and he has endeavoured to show, that it is emitted by the exhalants of the skin and intestines, as well as by those of the lungs, producing in all cases similar changes on the air. Digestion he holds to be in no other way the source of the carburetted fluids, than as it is the source of all the other principles which they contain. We know that all the phenomena of respiration are often exhibited for long periods where no digestive process is carried on; but the functions of life must sooner or later come to an end, if the various means of exhaustion be not recruited by supplies through the digestive organs. It is only in this dilatant view that he considers digestion as the source of carburetted air; its immediate sources are the exhalant functions of the body, which will afford carburetted air as long as they are supported by the motion of the blood, and will no longer yield it when the motion of the blood has ceased. But whether the exhalant functions continue or cease, he considers that carburetted air exists abundantly, if not equally, in the ferum and crassifemurum, in arterial and venal blood.

On the whole, though the final causes of respiration, or the uses to which it is subservient in the animal economy, are now pretty well understood, we must acknowledge that the mode in which these beneficial effects are produced, has not yet been satisfactorily explained.

The principal uses of respiration appear to be, 1. To bring about some beneficial change in the fluids of the body, and through them on the solids; 2. To preserve the equable temperature of the body; and, 3. In all those animals that breathe by lungs to produce those sounds which arise from what we call voice.

That animal heat is kept up chiefly by respiration, requires we think, no particular proof. It is well known, that those animals which consume most air during respiration, have the highest temperature. Birds in particular have the most extensive breathing organs, and the temperature of these animals is higher than that of any other class. The respiration of reptiles, fishes, and most of the lower classes, is slow and languid, and the temperature of these animals is proportionally low. The heat of each species is, however, pretty uniform under ordinary circumstances. That of the human body is generally about 98° of Fahrenheit. This however depends on the circumstances in which it is placed. When much chilled by the action of cold, the temperature of the human body falls a degree or two below the ordinary height; and under the influence of violent fever, it rises several degrees above it. The temperature is generally highest in children; and instances are recorded of these having survived, while their mothers, to whose breasts they clung, have perished from the severity of cold.

One of the most interesting facts relating to the subject of animal heat, is the capacity of preserving the equable temperature of their bodies, possessed by most animals. Man himself can live with little inconvenience in the frozen regions of Spitzbergen, and under the equatorial heats of Africa. He can even support a greater degree of heat than is perhaps ever known to take place from the rays of the sun, as is proved by the experiment of Drs Blagden and Fordyce in heated rooms; these gentlemen having remained for fifteen minutes together in a heat exceeding 130°. The heat supported by some of the inferior animals is still more extraordinary. A dog has been known to live for a considerable time in air heated to 260°, and still the heat of his body was not raised more than 2° above its natural standard. A frog has lived for more than twenty-five minutes when laid on flannel, nel, heated from 95° to 106°. Fishes live very well at 72°; and Lucas, in his history of mineral waters, speaks of carp that were living in a hot bath, whose temperature was at least equal to that of the human body.

How this equable temperature is preserved, cannot be completely explained. We know that the heat of the human body is commonly moderated by perspiration; but in some cases, as in that of Dr Fordyce alluded to above, where the heated atmosphere was filled with watery vapour, this could have little effect. We can ascribe it only to the action of the living principle.

It was long ago observed by Aristotle, that those animals only who possess lungs, have a true voice, and this opinion is confirmed by the experience of modern naturalists. We find, that only mammalia, cetacea, birds, reptiles and serpents, can utter vocal sounds. Several tribes below these do indeed emit certain sounds, especially insects; but these are owing to vibrations of the air in consequence of the agitation of their external organs. It is only in mammalia and birds that the voice becomes an interesting object of enquiry; for that of the cetacea is little more than blowing and grunting, and that of the other two classes is either hissing or croaking.

Nothing can exceed, in variety and execution, the human voice; as will readily be allowed, if we consider the complicated structure of the human vocal organs, and the almost infinite variety of changes of which they are susceptible. Dr Barclay has calculated them with great accuracy, proceeding on the principle, that where a number of moveable parts constitute an organ destined to some particular function, and where this function is varied and modified by every change in the relative situation of the moveable parts, the number of changes producible on the organ must at least equal the number of muscles employed, together with all the combinations into which they can enter. Now, the muscles proper to the five cartilages of the larynx, are at least seven pairs; and fourteen muscles that can act separately or in pairs, in combination with the whole, or with any two or more of the rest, are capable of producing 16,383 different movements; not reckoning as changes the various degrees of force and velocity, nor the infinitely varied order of succession by which they may occasionally be brought into action. The number appears almost incredible; but to lessen the surprise, it must be recollected that we speak not here of the powers possessed by any individual, which will depend on habits and circumstances, but of the powers of the vocal organs, considered in the abstract, free from all the influence of custom, equally indifferent, and equally disposed to act in any order of succession, in any combination, and with any degree of force and velocity of which their original powers were susceptible.

If the powers we have mentioned appear astonishing, and able to account for many thousands of these varieties observed among the voices of the human species, we have further to add, that the muscles alluded to are only the proper muscles of the larynx, or the muscles restricted in their attachments to its five cartilages. These are but a few of the muscles of voice. In speaking we use a great many more. Fifteen pairs of different muscles, attached to the cartilages, or os hyoides, and acting as agents, antagonists, or directors, are constantly employed in preserving the cartilages of the larynx steady, in regulating the place of their situation, or moving them as occasion requires, upwards and downwards, backwards and forwards, and in every way, directly and obliquely, according to the course of the muscular fibres, or in the diagonal between different forces. These muscles, independent of the former, are susceptible of 1,073,741,823 different combinations; and co-operating with the seven pairs of the larynx, of 17,592,186,044,415, exclusive of the changes which must arise from the different degrees of force and velocity, and the infinitely varied order of succession in which they may be brought into action.

But these are not all that cooperate with the larynx, either in forming or changing the voice; the diaphragm, the abdominal muscles, the intercostals, and all that directly or indirectly act on the air, or on the parts to which the chondral and hyoidal muscles are attached, contribute their share. The os hyoides could not be raised unless the inferior jaw-bone were previously fixed by the temporals, mastoids, and internal pterygoids; and a similar assistance is likewise furnished by several other auxiliary muscles that fix the head, sternum, and scapula; to these we must add some pairs belonging to the pharynx and illium faucium, and some also belonging to the tongue, which, combining with others, give to that organ an inconceivable variety of movement; and so quickly, that, in rapid utterance, they change its state 3000 times in a minute. Thus Haller could articulate 1500 letters in a minute, which required 1500 contractions, Nomen, and as many relaxations of the lingual muscles.

The principal organ of voice is the larynx, which is improved by the circumstance that, when this is injured, of voice the voice is either lost, or rendered very indistinct. In speech, ordinary respiration the chink of the glottis seems to be in a relaxed state, and when this chink is contracted, voice is produced, and the sound of the voice is more or less shrill, according as the glottis is more or less contracted. By this contraction of the glottis alone we can produce only inarticulate sounds, varied indeed almost infinitely with respect to intensity and tone, by the action of the muscles. The production of speech requires the action of the tongue, the lips, the palate, and the teeth; and the articulations are most complete, when all these parts are most perfect in their structure, and in the most healthy condition. Too great length or shortness of the tongue, swelling of this organ in consequence of inflammation, &c., imperfection of the palate, loss of the teeth, swelling of the lips, all serve to render speech imperfect and inarticulate. The strength of the voice depends on the quantity of air expired, and on the contraction of the glottis; and consequently those animals who have the most capacious and most dilatable lungs, together with an ample cartilaginous and elastic larynx, will, other things being equal, have the strongest voice.

Among the various effects of the human voice, there is none more calculated to produce surprize in the hearers, than that extraordinary talent which some men possess of deceiving their hearers into a belief, that the sounds which they utter do not proceed from the real speaker, but from situations at a distance. This talent has been termed ventriloquism, from an idea that the voice of the speaker proceeded not from the mouth, but from the belly. The most remarkable instance of this rare talent of which we have heard, is that of M. Fitz-James, who was formerly at Paris, and exhibited in London in the year 1803. Mr Nicholson has given an amusing, amusing account of the performance of this ventriloquist, and we shall present part of it to our readers.

After some remarks on the nature of ventriloquism, which we shall notice presently, and on the difficulty of ascertaining the direction of the sound, Mr Nicholson thus proceeds:—“We should scarcely be disposed to ascribe any definite direction to it; and consequently are readily led to suppose it to come from the place best adapted to what was said. So that when he went to the door, and asked in French (in which the whole performance was carried on), ‘are you there?’ to a person supposed to be in the passage, the answer in the unusual voice was immediately ascribed by the audience to a person actually in the passage; and upon quitting the door and withdrawing from it, when he turned round, directing his voice to the door, and said, ‘stay there till I call you,’ the answer which was lower, and well adapted to the supposed distance, and obstacle interposed, appeared still more strikingly to be out of the room. He then looked up to the ceiling and called out in his own voice, ‘what are you doing above?’ ‘do you intend to come down?’ to which an immediate answer was given, which seemed to be in the room above, ‘I am coming down directly.’ The same deception was practised on the supposition of a person being under the floor, who answered in the unusual, but a very different voice from the other, that he was down in the cellar putting away some wine. An excellent deception of the watchman crying the hour in the street, and approaching nearer the house, till he came opposite the window, was practised. Our attention was directed to the street by the marked attention which Fitz-James himself appeared to pay to the sound. He threw up the sash and asked the hour, which was immediately answered in the same tone, but clearer and louder; but on his shutting the window down again, the watchman proceeded less audibly, and all at once the voice became very faint, and Fitz-James in his natural voice said, ‘he has turned the corner.’ In all these instances as well as others which were exhibited to the very great entertainment and surprise of the audience, the acute observer will perceive that the direction of the sound was imaginary, and arose entirely from the well-studied and skilful combinations of the performer. Other scenes which were to follow required the imagination to be too completely misled to admit of the actor being seen. He went behind a folding screen in one corner of the room, when he counterfeited the knocking at a door. One person called from within, and was answered by a different person from without, who was admitted, and we found from the conversation of the parties, that the latter was in pain, and furious of having a tooth extracted. The dialogue, and all the particulars of the operation that followed, would require a long discourse if I were to attempt to describe them to the reader. The imitation of the natural and modulated voice of the operator, encouraging, soothing, and talking with the patient; the confusion, terror, and apprehension of the sufferer; the inarticulate noises produced by the chairs and apparatus, upon the whole, constituted a mass of sound which produced a strange but comic effect. Some observers would not have hesitated to assert, that they heard more than one voice at a time; and though this certainly could not be the case, and it did not appear so to me, yet the transitions were so instantaneous, without the least pause between them, that the notion might very easily be generated. The removal of the screen satisfied the audience that one performer had effected the whole.

His principal performance, however, consisted in the debates at the meeting of Nauterre, in which there were twenty different speakers, and certainly the number of different voices was very great. Much entertainment was afforded by the subject, which was taken from the late times of anarchy and convulsion in France; when the lowest, the most ignorant part of society, was called upon to decide the fate of a whole people by the energies of folly and brute violence. The same remark may be applied to this debate, as to the other scene respecting tooth-drawing; namely, that the quick and sudden transitions, and the great differences in the voices, gave the audience various notions, as well with regard to the number of speakers, as to their positions and the direction of their voices.”

Various explanations of this peculiar modification of how each voice have been given. From the report of Fitz-James himself, it appeared to Mr Nicholson, that by long practice he had acquired the faculty of speaking during the inspiration of the breath, with nearly the same articulation, though not so loud, nor so variously modulated, as the ordinary voice, formed by expiration of the air. M. Richerand, who heard Fitz-James at Paris, gives a different account of the matter. He says that every time the ventriloquist exerted this unusual peculiarity, he suffered distention in the epiglottic region; that sometimes he perceived the wind rolling even lower, and that he could not long continue the exertion without fatigue. Richerand believes that the whole mechanism of this art consists in a slow, gradual expiration, drawn in such a way, that the artist either makes use of the influence exerted by volition over the muscles of the parietes of the thorax, or that he keeps the epiglottis down by the base of the tongue, the apex of which is not carried beyond the dental arches.

He always made a strong inspiration just before this long expiration, and thus conveyed into the lungs a considerable mass of air, the exit of which he afterwards managed with such address. Therefore repletion of the stomach greatly incommode the talent of M. Fitz-James, by preventing the diaphragm from descending sufficiently to admit of a dilatation of the thorax, in proportion to the quantity of air that the lungs should receive. By accelerating or retarding the exit of the air, he can imitate different voices, and induce his auditors to a belief, that the interlocutors of a dialogue kept up by himself alone, are placed at different distances.

Mr Gough in an ingenious paper, containing an investigation of the method whereby men judge, by the ear, of the position of sonorous bodies, relative to their own persons, explains the phenomena of ventriloquism on the principles of reverberated sound, and considers it as consisting in the talent of making the voice issue only from the mouth; whereas he thinks that in ordinary cases the different vibrations which are excited by the joint functions of the several vocal organs in action, pass along the bones and cartilages from the parts in motion, to the external teguments of the head, face, neck, and chest, from which a succession of similar vibrations is imparted to the contiguous air, thereby converting the upper half of the speaker’s body into an extensive seat of sound. He thinks that the sounds proceeding from the mouth Of mouth of a ventriloquist are uttered in such a direction that the hearers may receive the impression of some echo with much more force than they can receive the original sounds*. It may be doubted whether such echoes can take place in an ordinary room filled with a large assembly; and on the whole we are inclined to consider this phenomenon as being effected partly by the gradual emission and a skilful management of a large quantity of air taken in by a full inspiration, and partly by the influence which the performer is capable of exerting over the imagination of his hearers.

Several of the mammalia have a characteristic voice, which is formed by particular organs. These are in some animals tense membranes; in others peculiar cavities opening into the larynx, and sometimes appearing like continuations of the laryngeal ventricles. Thus the neighing of the horse is effected by a delicate, and nearly falciform, membrane, which is attached by its middle to the thyroid cartilage, and has its extremities running along the outer margins of the opening of the glottis. The braying of the ass is produced by means of a similar membrane, under which there is an excavation in the thyroid cartilage. In this animal there are also two large membranous sacs opening into the larynx. The purring of the cat seems to be owing to two delicate membranes that lie below the ligaments of the glottis. Some of the monkey tribe, especially the *femin semiculus* and beelzebul, have the middle and fore part of the os hyoideum formed into a spherical bony cavity, by which these animals are enabled to produce those horrible and penetrating tones, which can be heard at vast distances, and have gained them the name of howling apes†. See MAMMALIA, No 33.

The simplest vocal organ seems to be that of birds. These animals have, on the sides of the windpipe next the lungs, and at the opening of the bronchia, two membranous folds which partly close the pulmonary aperture of the windpipe, and the aperture next the head is susceptible of great contraction and dilatation. In short, the vocal organ of birds may be considered as one of the most perfect wind instruments, very much resembling, both in its structure and effect, a clarinet or hautboy, the opening next the lungs being similar to the reed of these instruments. For some remarks on the song of birds, see ORNITHOLOGY, No 42; and for farther observations on the voice, see ANATOMY, Part I, No 122.

In tracing the relations of respiration with the preceding functions, we must deviate a little from our usual order, and begin with those between respiration and circulation, as it seems to be through the medium of the circulating system that respiration principally acts on the other functions. The relations between respiration and circulation are the most immediate and the most obvious. When the breathing is most free and rapid, the circulation is most vigorous and active; while in labored or interrupted respiration, the action of the heart and arteries becomes slow, feeble, irregular; and where the lungs are deprived of oxygenous gas, the arteries gradually cease to pulsate, and soon after the motion of the heart ceases. If the stimulus of oxygen be not too long withheld, so that the lungs can again be excited to action, first the heart, and then the arteries, gradually renew the exercise of their functions, and the circulation proceeds as before. On the application of these principles depends the recovery of those apparently dead from asphyxia (suffocation, drowning, &c.). When the circulation becomes languid from indolence, from depressing passions, or the want of accustomed stimuli, we feel about the breast, a peculiar sensation, which physicians call anxiety, and which is relieved by a deep inspiration; by sighing, yawning, &c. Violent exertions of the respiratory organs, such as laughing, coughing, sneezing, talking unusually long or loudly, quicken the circulation, sometimes to an alarming degree, so as to occasion hemorrhage in such as are predisposed to that affection. Breathing in an atmosphere that is much rarified, as on the top of a high mountain, has often the effect of producing plethora and hemorrhage; though this, perhaps, is imputable rather to a want of the ordinary pressure on the surface of the body.

When the circulation through the lungs is impeded with tenacity or obstructed, a determination of blood takes place to other parts, especially to the head. The effects produced on the brain and other organs of sensation, by the breathing of impure air, are dreadful. When the same quantity of air is repeatedly respired, there is experienced, still, great anxiety about the breast, and this soon becomes intolerable; the face swells, becomes livid, or even black, and feels excessively hot; sparks of fire seem to dance before the eyes; the sight becomes depraved; giddiness, ringing in the ears, and confusion of thought succeed; and if fresh air be not soon supplied, the subject of the experiment loses both sensation and motion, and falls into a state resembling apoplexy†. Rite's When rarified air is breathed, the nervous system experiences a kind of excitement; agreeable sensations are produced, with a disposition to mirth and cheerfulness; but if the person continue for some time in such a situation, an unusual languor, heaviness, and disposition to sleep, come on‡. We need not here describe the pleasurable sensations excited by the respiration of nitrous oxide, as these have been already related under CHEMISTRY, No 366. The exhilarating effects which a pure and serene atmosphere produce on the general system, and the uneasy sensations experienced under a thick and clouded sky, are partly referable to this head. The nervous system also acts on the organs of respiration. In some affections of the brain, respiration is much quickened, while in others, especially the comatose affections, it is slow, laborious, and often attended with that peculiar noise called flertor. It is well known what effect anxiety, eagerness, hope, or desire, have on the respiration. According as one or other of these passions is predominant, the breathing becomes hurried, irregular, or suspended.

An evident relation takes place between respiration and motion. The breathing is quickened by exercise; and when there is a considerable debility of the muscular system, the slightest exertion produces hurried respiration, panting, &c. In those animals that possess the greatest powers of motion, respiration is most free, and the air most extensively diffused over the body. In birds, not only the lungs are very extensive, but the air is conveyed into the bones of the skull, and into the hollows of the larger cylindrical bones; and in insects which have the most rapid motions, the air penetrates to every part of the body. Motion, as well as sensation, becomes unusually free and vigorous in rarified air, and during the respiration of nitrous oxide; while in cafes of impeded or obstructed respiration, the action of the muscles is languid and feeble. Indeed, if we may implicitly rely on the experiments that have been made on the respiration of de-oxygenated gases, the muscular fibres are among the first organs that are injured. We are told that by the admission of black blood, or blood that has not undergone the necessary changes by healthy respiration, the muscles lose their power of contraction, and even their irritability.

The organs and function of respiration sympathise with those of digestion. When the former function is most free, the latter is generally most healthy; the respiration of pure or rarefied air, or of the nitrous oxide, is attended with an increase of appetite, and of the digestive powers, as was experienced by M. Sauflaire while wandering among the Alps, and by Davy while respiring the gas of Paradife. Again, when digestion is impaired, or when the stomach is overloaded, the breathing is rendered difficult, laborious, or irregular, and in many cases of affection of the stomach, cough is a very common symptom. These effects produced on the respiratory organs in consequence of impaired digestion, are ascribable chiefly to the pressure on the diaphragm by the distended stomach.

Many other relations might be pointed out between respiration and the other functions of the animal economy, but our limits do not permit us to enlarge further on the subject.

For an account of the morbid affections of respiration, such as sneezing, hiccup, coughing, anxiety, dyspnea, or difficulty of breathing, see the article MEDICINE.

**CHAP. IX. Of Nutrition and Assimilation.**

The function by which the nutritious particles received by a living being are assimilated to the nature of that being, or become part of its substance, is properly called nutrition. This is the completion of the processes which, in most animals, is the combined result of several other operations. Thus, in the superior animals, from man to the mollusca, the whole process of nutrition consists of digestion, absorption, circulation, and respiration; by the two last of which the nourishment received is changed into perfect blood, and fitted for the support and renewal of the several parts of the system. From the account of the constituent parts of the blood given under CHEMISTRY, No 2660, it will appear, that this fluid contains within itself the principles of which every part of the body is composed. Thus it contains fibrine, which is the chief principle of the muscular parts; phosphate of lime, which forms the basis of the bones; albumen and gelatine, the chief constituents of cartilages, ligaments and tendons, &c. These principles are conveyed by the arterial blood, during its circulation, to those parts of the system where they are required, for renewing waste, or supplying deficiencies, and thus they are assimilated to the nature of the body.

The power of assimilation, so remarkable in living bodies, is not the same in every assimilating organ; but each has the property of converting the materials it receives (provided they be susceptible of this conversion) into a peculiar substance. Thus, the stomach always converts the food into chyme; the intestines change it into chyle; but if chyle, or what is very similar to it, of fresh milk, be received into the stomach, this organ and all exerts on it the usual change, and does not pass it forward into the intestines unaltered, though we know by experiment, that fresh milk is capable of being taken up unchanged by the absorbents of the bowels. Again, blood is always perfected within the circulating vessels; and if chyle or fresh milk be injected into the arteries, it produces dangerous effects, while the fresh blood of another living animal may be transfused into these vessels without injury. In like manner, if a piece of fresh muscular flesh be cut from a living animal, and applied to the muscles of another living animal, also newly divided, the two parts unite, and are immediately assimilated; and even fresh bone may, in the same manner be ingrafted on the living bones of the same, or of a different species of animal; while substances that are foreign to the nature of the animal body, when introduced into the blood-vessels, prove fatal, and when inserted into a wounded muscular or bony part, prevent the wound from healing.

These circumstances show that assimilation is a chemical process, though modified and regulated by the action of the living principle. The chemical nature of assimilation is most distinctly proved by the well-known experiment of colouring the bones of an animal, by feeding it on madder. The particles of the madder, which we know to have a strong affinity for phosphate of lime, are carried unchanged from the stomach into the blood-vessels, and are thence conveyed, probably in combination with the phosphate of lime there contained, into the substance of the bones, where they are deposited, and remain for a considerable time.

We have considered nutrition as performed by the circulating vessels. It has been supposed that the nerves not present are the organs of nutrition; but this strange hypothesis formed by Monro, which proves that the limb of a frog may be preserved alive and nourished by the blood-vessels, after its communication with the brain has been cut off by dividing the nerves.

In insects and zoophytes, where there are no circulating vessels, nutrition must be a very simple operation. According to Cuvier, it is performed by imbibition; and the pores of the animal's body receiving immediately the nutritious fluids on which it feeds.

In plants and animals, the assimilating power has always certain limits preferable to it; its influence is very generally confined to the sort of food congenial to the species, and its strength is varied according to circumstances, as the age, the habits, and the state of health. Those which are young assimilate faster than those which are old; and one species, which may partly be owing to the nature of their food, will assimilate much faster than another. Certain worms that feed on animal and vegetable substances will, in twenty-four hours after their escape from the egg, become not only double their former size, but will weigh, according to Redi, from 155 to 210 times more than before. Most oils are of very difficult assimilation; and those which are volatile will often resist the long-continued and varied action of the living organs; will mingle with the parts, and, undecomposed, communicate their flavour.

Other circumstances respecting nutrition have been noticed. noticed in the first part of Anatomy, No 130; and the chemical doctrine of assimilation is more fully considered under Chemistry, No 2567—2571.

Chap. X. Of Secretion.

That function by which any organ, or set of organs, separates from the general mass of blood certain principles intended to perform some important office in the animal economy, is called secretion; and the substances so separated, are called secretions.

The secretory organs in the more perfect animals are very numerous, and some of them very complex. The most simple of them seem to be the cellular texture, and the mucous membranes. The next in simplicity are the conglomerate glands, and perhaps the spleen, while the more complex organs are the liver, the testicles, the atrabiliary capillaries, &c. An account of all these organs, as they occur in the human body, has been given in the first part of the article Anatomy; and the corresponding organs of the inferior animals, with others not found in man, are described by writers on comparative anatomy, especially Cuvier and Blumenbach.

Secretion appears to be of three kinds: 1. Transudation, in which the secreted matters merely ooze through the pores of the secreting organ. This takes place in the lowest classes of animals, as in zoophytes, insects, and some worms, but rarely in the human subject. 2. Exhalation, in which the secreted fluids are poured out into cavities by certain branches of the arteries with open mouths, called exhalants. This appears to take place in many organs of the most perfect animals, especially from the mucous membranes, the synovial glands, &c. 3. Secretion, properly so called, in which the blood passes through glandular bodies, where a part of it is decomposed, and carried out in another form by particular tubes called excretory ducts. This is the case with most of the secreting organs, as the salivary glands, the lacrimal glands, the liver, the pancreas, the testicles, and a few others.

The secreted fluids are chiefly the following: lymph, serum, tears, mucus, saliva, pancreatic juice, gallbladder juice, enteric juice, bile, semen, synovia, fat, marrow, cerumen or ear-wax, and in the female, milk. The other matters secreted, which may rather be termed solid than fluid, are albumen, gelatine, fibrine, and phosphate of lime. On the nature and properties of all these substances, see the article Chemistry, Chap. xix. sect. 3.

With respect to the secretions in general, we may remark, that they are considerably influenced by age, sex, various affections of the mind, and various bodily diseases. They are formed by organs which are sometimes capable of supplying the deficiencies of each other; they are subjected to the influence of the atmosphere, and to the temperaments of the body; they are sometimes mixed together, and by this combination their nature is changed.

We shall now briefly examine the action of three of the secreting organs, viz. the cellular membrane, the liver, and the spleen.

From the extensive distribution of the cellular membrane it is reasonable to conclude, that it is intended to perform several important offices in the animal economy. One of its most obvious uses is to form a general connecting medium between every part of the structure, while it at the same time separates and distinguishes every organ. From its elasticity, and the lubricating fluid which it holds within its cells, it facilitates motion, and thus assists the action of all the muscular parts and organs. That it is susceptible of great dilatation is proved by the phenomena of anarous dropsy; and the gradual evacuation of the water when anarous limbs are punctured, as well as the passage of extraneous bodies below the skin from one part to another, seem to show that it possesses considerable contractile powers. It is chiefly, however, as a secreting organ that we are here to consider the cellular membrane; and in this way its function is of the utmost importance. The fatty matter, that is so copious in most of the superior animals, is contained within particular cells or bags of the cellular membrane, and is found in greatest quantities below the skin, especially on the sternum part of the belly, and about the kidneys. In some animals, as the hog, the seal, the walrus and the cetaceous tribes, it forms a layer several inches in thickness, and in all the water animals above mentioned it is nearly fluid. To these animals it not only serves the purpose of a warm covering by the flowness with which it conducts heat; but, by diminishing their specific gravity, renders their motions on the surface of the water much more easy and expeditious. One of the most important uses of the fat seems to be to supply nourishment to the body, when the ordinary channel is obstructed, or the system rendered incapable, from torpor or disease, of receiving food. When fat persons are attacked by fever, or similar acute diseases, they become emaciated, sometimes to a great degree, as to appear a mere skeleton; and those animals who sleep during winter, though very fat when they retire to their dormitories, are extremely lank and lean when they quit these on the return of spring. In all these cases the fat alone is absorbed, and supplies the waste that takes place in the body, and would otherwise prove fatal.

On the actions of the cellular membrane, see Bichat, Anatomie Generale, tom. i.

Some physiologists have supposed that the bile secreted by the liver is not formed entirely from the blood of the liver, the vena portaria, but partly from the hepatic artery. Dr Saunders, who has examined the arguments in favour of this supposition, decides against it, and considers the usual opinion of the bile being solely secreted from the blood of the vena portaria, as quite satisfactory. It has also been supposed, that the whole of the bile is not secreted by the liver, but that the gall bladder has a share in this office, and is not merely a reservoir, like the urinary bladder. This supposition is highly improbable, although we think there can be little doubt that the bile undergoes, within the gall bladder, some peculiar changes, which render it better fitted for the functions it has to perform. We know that the gall bladder is very muscular, and that there is an appearance of follicles within it. It is therefore probable that some matter is secreted from its internal surface, which produces a necessary change in the bile.

The principal use of the bile seems to be to stimulate the intestines, and thus keep up their energy and peristaltic motion, though it is probable that, besides this office, it performs several others of importance in the animal economy, such as assisting in the decomposition of the food, and thus forming chyle; and acting as a general... general stimulus to the system. That it has this last effect is probable from the torpor, inactivity, and dejection that attend hypochondriacal and chlorotic affections, in which this secretion is defective. Too great a secretion, or rather excretion of bile, is attended with

* See Sezn-violent purging.

On the nature of the bile, and biliary concretions, see Chemistry, No 2664.

Uses of the spleen.

Few subjects in physiology have given rise to more discussion, and few have been considered with so little success, as the use of the spleen in the animal economy. That an organ so large, and so well supplied with blood, should be intended for some important function, is scarcely to be doubted; and yet the instances of animals that have lived, seemingly with little inconvenience, after the spleen had been cut out, seem to prove that this organ is not of such great importance as it appears to be. The conjectures respecting its uses are various, and some of them not a little ludicrous. Some have supposed that it acted by its weight and pressure on the stomach, and thus promoted digestion at one time, and counteracted hunger at another; some, that it was intended to dilute and attenuate the blood; others, to deprive that fluid of its superabundant oil; another party, that it contributed to form the red globules of the blood; and some of the older physiologists supposed that it secreted that fluid to which they gave the name of black bile. Dumas is of opinion that it is a sort of supplementary organ, both to the liver and the kidneys, separating from the blood part of its ferocity, and then delivering it over to the liver in a proper state for the formation of bile; and furnishing to the kidneys another portion of ferocity to form the watery part of the urine*. That at least a part of these opinions of Dumas has some foundation, will appear from the following summary of the late experiments of Mr Everard Home.

Prosecuting the inquiry respecting the state of the stomach during digestion, which we have formerly alluded to (see No 161), Mr Home found that during digestion, the fluids taken into the stomach are principally contained in the cardiac portion; and he inferred, from the uniform confidence of the chyme in the pyloric portion, that a great part of the fluids are carried out of the stomach without ever reaching the pylorus. As he conceived very naturally, that the lymphatics of the stomach were inadequate to this office, he conjectured that the fluids might pass off by the spleen. He proved, by a decisive experiment, that liquors might pass through the stomach without going through the pylorus, and he also found at the same time, that the spleen was turgid, unusually large, and its external surface very irregular; and when cut into, small cells were everywhere met with, containing a watery fluid, and occupying a considerable portion of its substance. Rudiments of these cells had been seen before by Malpighi, who considered them to be glands, and by Cuvier, who calls them cur-pules; but the cells in a distended state seem not to have been examined till Mr Home was led to look for them, in consequence of the above experiment. Mr Home varied this experiment, by giving animals a decoction of madder, and an infusion of rhubarb, and obtained similar results. Part of the fluid swallowed was again rejected by vomiting; but of that which remained it was always found that a part had escaped, without any possibility of passing by the pylorus, as this was secured by ligature. It did not probably escape by the absorbents, as these were not so much distended by fluid as to be visible; and it certainly did enter the spleen, as there was there found a quantity of liquor, which was proved, by an alkaline test, to contain rhubarb. A large quantity of urine was found in the bladder, also impregnated with rhubarb*.

From these experiments it appears, that the spleen is capable of carrying off from the stomach, a part of its fluid contents, thus affording a much nearer passage to the bladder than through the absorbents. If this investigation, on further trials, shall prove equally satisfactory, it will explain why the bladder is often distended with urine in a short time after drinking; and will do away the necessity of having recourse to the disputed hypothesis of the retrograde action of the absorbents.

In the inferior classes of animals there are a number of peculiar secretions, which, from their utility in medicine or the arts, or from their noxious or unpleasant effects, are deserving the attention of physiologists. We can here only mention a few of the more important. The nature and properties of most of them are explained under Chemistry.

Some of the mammalia secrete matters that have a very strong smell, as musk, civet, caflor, and in particular the fluid emitted from the hide of several of the weasel tribe, beside the civet cat. Ambergris is secreted by some species of whales. Birds, especially water fowls, secrete a large quantity of oily matter, which they use in dressing their feathers. Some reptiles, especially the toad, secrete an acrimonious fluid, which seems to serve them as a weapon of defence. Many serpents, as is well known, produce a most virulent poison, which they infuse into the wounds inflicted by their fangs. Some fishes secrete fluids of a similar tendency. Few animals, however, form secretions so various and so useful, as the insect tribes, from whom we procure cochineal, kerres, lac, silk, &c. Some of the mollusca, as the mulete, and the spinning flug, also secrete a matter similar to silk, by means of which they either secure themselves firmly in their situations, or facilitate their progressive motion. The ink of the cuttle fish, supposed with no small probability, to be the basis of Indian or Chinese ink, is also a remarkable animal secretion, which seems intended by nature to screen the animal, and assist it in eluding the vigilance of its pursuers.

There are also many important vegetable secretions, constituting what are called gums, resins, and gum-resins; secretions as gum-arabic, gum-dragant; guaiac, dragon's blood; affaefetida, gamboge, myrrh, aloes, and many others; for an account of which see Chemistry and Materia Medica.

CHAP. XI. Of Excretion.

The function of excretion differs but little in its nature from that of secretion already considered. As the organs of secretion separate from the blood those substances which are useful in the animal economy, so the excretory organs separate from the blood, or from the food taken into the stomach, those substances which are to be conveyed out of the body as excrementitious, viz. the solid excrement, the urine, and perspiration.

The organs of excretion, then, are chiefly the bowels, especially the larger intestines, the kidneys, with their appendages, appendages, the ureters and urinary bladder, and the skin. For an account of these in the human body, see Anatomy, Part I.; and for the varieties of these organs in such of the inferior animals as possess them, and for the means by which their absence is supplied in others, see the works of Cuvier and Blumenbach already quoted.

The physiology of intestinal excretion requires little explanation. The remains of the food, after the nutritious chyle has been extracted from it by the lacteals, are carried onward through the colon and rectum by the peristaltic motion of the intestinal canal, excited to action by the stimulus of their contents, and of the bile, and assisted by the pressure of the abdominal muscles, till they reach the extremity of the rectum, when becoming more stimulant, partly by their bulk, and partly by their increased acrimony, they rouse the muscular fibres of that intestine to greater action, so as, with the assistance of the abdominal muscles, to overcome the resistance of the sphincter, and are thus expelled.

Intestinal excretion is influenced by most of the preceding functions. 1. By the nervous power; as we find that in cases of paralysis, the excrements are not passed without artificial means, or they are voided involuntarily. 2. By motion. Thus we find that the action of the bowels is increased by exercise, and lessened by indolence and a sedentary life; though the quantity of excrement passed is greater in the latter case than in the former, shewing that the stimulus of the excrementitious matter is not sufficient without muscular action, to produce the regular performance of this function. 3. By digestion. It is well known that the stronger the digestive powers of the stomach, the more active are the bowels, and again, when these latter are overloaded with excrement, the functions of the stomach are disordered. 4. By secretion. The action of the intestines is increased, when that of the secretory organs which pour their contents into the alimentary canal, becomes unusually great, viz. in unusual secretions of bile or mucus, as in cholera and diarrhoea; while, when those secretions are defective, as in cases of jaundice, the intestines become unusually torpid.

The morbid affections of intestinal secretion have been considered under Medicine, No. 109—112, and 114, 115.

The organs destined for the excretion of urine afford the most complete apparatus for this function of any that we shall have occasion to notice, consisting of an assemblage of glands, collected within one membrane; an excretory duct; a reservoir for collecting the excreted fluid, and a canal for conveying it out of the body. Indeed we may consider the kidneys rather as secreting than excreting organs, as the urine there formed differs so much in its nature and properties from the circulating fluids. We know, by a decisive experiment, that the kidneys perform the whole of this office, and that the other organs are intended for the excretion of the urine; for when the ureters are tied or obstructed next the bladder, we find that the secretion of urine still goes on, and that the ureters above the obstruction soon become filled and prodigiously distended.

The nature and properties of urine, in its natural state, and as altered in certain diseases, have been considered under Chemistry, No. 2670.

As the urine contains two substances that are not found in the blood, viz. urea and uric acid, Dr Thomson concludes, on very probable grounds, that the office of the kidneys is not merely to separate from the blood, a quantity of water and salts, but that they exert on the fluid some peculiar action, decomposing some part of the blood, and forming some new substance or substances.

The mutual relations between this excretion and the preceding functions, are not many, or very important. During certain affections of the nervous system there is a sudden and copious excretion of limpid urine, and some mental emotions produce an involuntary flow of it. And in cases of palsy, an incontinence, or a total suppression of urine, is very common. This excretion is considerably influenced by motion, being less copious in those who use much exercise, or lead a laborious life.

The morbid affections referable to this excretion, are noticed under Medicine, No. 118—122.

The excretion by the skin, or perspiration, has excited the ingenuity of many physicians and physiologists, by the skin ever since the time of Sanctorius; and though it is not now considered as so essential to life and health as it was in the beginning of the 18th century, is certainly of great importance. The quantity of watery fluid occasionally thrown out by the skin in the form of sweat, proves that, by means of this organ, the blood is freed from a great deal of useless or perhaps injurious matter, which could not so conveniently, or so perfectly, be expelled by other outlets. The nature of the matter perspired, and the quantity of ordinary perspiration, have been investigated by many able experimentalists, the result of whose labours is given in the first volume of Johnson's Animal Chemistry; the fifth volume (third edition) of Dr Thomson's System of Chemistry; in a paper by Dr Kellie in the second number of the Edinburgh Medical and Surgical Journal, and in our article Chemistry.

The principal facts that have been ascertained with respect to the perspirable matter thrown out by the skin, relate either to its quantity, or its chemical composition.

I. The experiments on the quantity perspired, on Quantity of which we can the most rely, are those of Lavoisier, Seignouin, and Mr Abernethy. From these experiments we may deduce the following conclusions: 1. The greatest quantity of matter perspired in a minute, amounts to about 26 grains troy, the least to about 9 grains; giving an average of about 17 grains in the minute, or about 52 ounces in 24 hours. Dr Kellie estimates the quantity at about 30 ounces, which seems too small. 2. The quantity of perspiration is increased by drink, but not perceptibly by solid food. 3. Perspiration is the least in quantity immediately after a meal, and reaches its highest proportion during digestion.

II. The perspirable matter is chiefly composed of a large quantity of water, some carbon or carbolic acid, a small quantity of another acid, supposed to be the phosphoric, and a peculiar oily matter of an odorous quality, differing in different animals, and, as it should seem, in different individuals. The perspiration of quadrupeds is frequently found to contain phosphate of lime, and sometimes urea. See Chemistry, No. 2532.

As to the relations of perspiration with the preceding functions, we may remark that this excretion is increased by various passions of the mind, by exercise, by healthy and rapid digestion; that it is generally in proportion to the vigour and quickness of the circulation and respiration, and that it is capable of supplying the defect of the two former excretions. On the contrary, it... Of integumentation it is lessened by inactivity; by the impaired state of the digestive organs; by languid circulation and respiration, and by violent purging, or evacuation of urine.

**Chap. XII. Of Integumentation.**

All living bodies are furnished with integuments, which are intended to afford them a defense against those injuries to which their situation is commonly exposed. Of the integuments some are useful in preventing the dissipation of the fluids; some in resisting acid and corrosive substances; some of them are indigestible in the stomach of animals, and some appear to be incorruptible in the earth. By these properties, seeds and the ova of insects are preserved for a considerable time, waiting the changes of soil or of season that are favorable to their evolution. They are protected from the action of weak membranous stomachs, and thus the animals who may swallow them contribute to their propagation. It is thus the seeds of the melon tree are dispersed, and deposited on the bark of the oak or the ash. There is a gelatinous substance frequently ejected by birds, and commonly called tremella, nofoce, or star-fall, which Dr Barclay has proved to be nothing else than the oviducts of frogs, which, as the embryo in form of an egg, moves along their winding canal, secrete that transparent and viscid matter which constitutes the albuminous part of the ovum, and feeds and protects the embryo while in water.

The most important circumstances with respect to integumentation relate to the varieties of the integuments themselves, and to the changes or renovation of these in different animals.

I. Some integuments are useful, chiefly from their strength and hardness. The elytra or flaky coverings of the beetle tribes afford an excellent defense for their membranous wings, when folded up; the shell of the snail lodges the intestines, when the animal comes forth to search for food, and affords a safe retreat to the animal, when it is threatened with any danger from without. The shells of some animals can be opened and closed by a muscular power; and some of them, as in the tail of the lobster, are so disposed in plates or scales, as to be no hindrance to the animal's motion. Several insects which pass a part of their time in the water, always compose for themselves a shell, where it is needful.

Some integuments are covered with feathers, some with hair or thick down. Besides many other obvious uses of these coverings, they generally serve to repel insects; and being bad conductors of heat, tend to preserve an equal and necessary temperature. Some integuments are covered with prickles, which oppose the attacks of an enemy by the strength of their points, or by the venom which they infuse, as in the stings of nettles, and the down of some other plants, and some insects. Others again are moistened by a viscid secretion, which preserves the necessary softness of the parts, prevents evaporation, resists acrimony, enables some beings to destroy their enemies, and afflicts others in performing their progressive motions.

Both plants and animals, but particularly the former, are often protected by odoriferous effluvia from their integuments. These effluvia are the finer parts of their volatile oil, always inflammable, and so subtle, that the continual emission of it from wood or flowers does not sensibly diminish their weight. To this fragrance it is owing that the deadly nightshade (atropa belladonna), the henbane, hounds-tongue, and many others, are seen on almost every high road untouched by animals. The manchineel-tree of the West Indies emits so very dangerous vapors, that the natives poison their arrows with its juices, and those have died who have ventured to sleep under its shade. The lobelia ingens of America produces a suffocating opprobrium in the breath of those who respire in its vicinity. The return of a periodical disorder has been attributed to the exhalation of the rhus toxicodendron. Of all the vegetable effluvia, however, that afford defense to the plant from which they proceed, or annoyance to the animals that approach it, none are equal to those of the celebrated bobun-upar, or poison-tree of Java, whose exhalations have been said to extend to the distance of several miles, preventing all access of animals, or punishing the intruders with certain death. It is rather unfortunate for the botanical poets, that the effects of this poison have been greatly exaggerated, if indeed such a marvellous tree really exists.

Various colours of the integuments afford a species of defense. Caterpillars which feed on leaves are generally dull green; and earthworms the colour of the earth which they inhabit. Butterflies which frequent flowers are coloured like them. Small birds which frequent hedges have greenish backs like the leaves, and light-colored bellies like the sky, and are hence less visible to the hawk who passes under them or over them. Those birds which are much amongst flowers, as the goldfinch, are furnished with vivid colours. The lark, partridge, and hare, are of the colour of dry vegetables or earth on which they rest; and frogs vary their colour with the mud of the streams which they frequent, and those which live on trees are green. Fish which are generally suspended in the water, and swallows which are generally suspended in the air, have their backs the colour of the distant ground, and their bellies of the sky. The sphinx convoluta, or unicorn moth, resembles in colour the flower on which it rests; and among plants, the nectary and petals of the ophrys, and of some kinds of the delphinium, resemble both in form and colour the insects which plunder them, and thus sometimes escape from their enemies by having the appearance of being pre-occupied. From colour being thus employed as a defense, many animals vary their colours with the seasons and circumstances; and those which are of different colours in summer according to the places which they inhabit, in winter assume in common the colour of the snow.

II. The changes that take place on the integuments consist either of a partial, or complete, renewal of integuments. As the more superficial integuments are commonly inflexible to stimuli, and possess little or nothing of the vital principle; in all cases where they cannot be enlarged to admit of an additional increase of growth, or, where they are not furnished with organs for repairing those injuries which they may suffer from accident or disease, the body is endowed by nature with a power of throwing them off, and of producing others in their stead. Thus, serpents and toads slough their skins; the crustacea cast their shells; the larvae of insects change their cuticle; and several trees, especially the trans the cork tree, throw off their outer bark. Even man himself generally changes the cuticle, which peels off in the form of scales. Most animals once a year change their hair, wool, or feathers, and have these renewed by a fresh covering; a process well known by the name of moulting. Some animals who do not usually cast their external covering, have the power of repairing this when injured. This is the case with most of the testacea, especially snails.

**Chap. XIII. Of Transformation.**

The alterations which organized beings undergo from metamorphosis or transformation, are more striking than those which we have described in the preceding chapter. It has indeed been asserted, that these alterations consist in throwing off certain temporary coverings; but this expression is inaccurate, and arises from a want of precision of ideas. Transformation and change of integuments are really different; the truth is, transformation often takes place without any change of integuments, and there is often a change of integuments without any change of form. This new form is sometimes occasioned by a change of shape, consistency, and colour, as when the lobes of a seed are converted into seminal leaves. It is at other times occasioned by a change of proportion among the parts, and at others by the addition of new organs, as when the emmet receives wings, and the plume of the seed is fed by new roots striking into the ground; or, lastly, it is occasioned by a change in the form and organs, and in their mode of operation, as happens in a remarkable degree to some insects. Indeed, though all living bodies, both plants and animals, undergo some degree of transformation in the course of their existence, these changes are most remarkable in insects.

Many reptiles undergo very curious changes, but these are most remarkable in the frog tribe. The larva or tadpole, as it is called, of the frog, is an animal with a large head, a long tail, no limbs, and commonly possessed of gills, all obviously very different, both in form, proportion, and uses, from the parts of the perfect animal. The curious appearance of what has been considered as the tadpole of the *rana paradisoa*, has led some naturalists to describe it as an animal of a different genus, either a fish or a lizard; see *Erpetology*, p. 284.

Many insects appear to consist of two distinct animal bodies, one within the other; the exterior, a creature of an ugly form, residing in the water, or under the earth, breathing by gills, or sometimes by tracheae projecting from the tail, possessing a voracious and growing appetite, and having a system of sanguiferous vessels that circulates the blood towards the head. When all its parts decay and fall off, the creature inclosed succeeds in its stead: this often is an animal of a different form, generally lives in a different element, feeds on a different species of food, has different instruments of motion, different organs of sense, different organs of respiration and differently situated; and being endowed with the parts of generation, inclines to gratify the sensual propensity, and produces an embryo, which becomes like the first, and from which, in process of time, a creature is evolved similar to the former.

If the embryo or egg be deposited on a leaf, the leaf is frequently observed to bend, to wrap it in folds, intended for the purpose of protecting it from injuries and danger. If deposited in the body of an animal or plant, they accommodate themselves to its wants and necessities, and furnish a tumor which serves it for a nidus, and besides, like an uterus, supplies it with nourishment; and if deposited in the body of an insect, the creature provides for the future destination of its young charge with all the tender care of a parent, and then dies.

These circumstances, added to the great variety of forms which insects assume, render it sometimes difficult to know who is the parent. We cannot, for instance, pronounce with certainty who is the true parent of the gordius, known by the name of the *feta equina*, or hair cell. A set of experiments which Dr Barclay once began with a view to throw some light on the subject, were unfortunately interrupted by an accident. He learned only, from a number of observations, that certain black beetles, at the end of summer, have the strongest propensity to run into the water, where they soon die; and that one or two, and sometimes three or four of these cells gradually drop from the beetle by the anus. Whether other insects provide for the gordius in this manner, we have not yet been able to determine.

In all living bodies possessed of a *jenorium*, the changes of form, as well as the changes of habit and of age, are usually accompanied with new propensities, appetites, and passions.

Microscopic observations having demonstrated, that the forms of the plant and animal existed previously in the seed or embryo; transformation must be owing entirely to the evolution of the different parts by means of nutrition.

What nature intends by transformation, we cannot determine; but by means of it different elements are its peoples, the different features variously adorned, and animated nature wonderfully diversified without a multiplication of beings.

**Chap. XIV. Of Reproduction.**

In the present chapter we shall notice, first, the partial reproductions that take place in some classes of animals, and then take a general comparative view of the principal phenomena of generation, in the various classes of living beings.

Experiments have proved, that even in the superior classes of animals, many parts of the body, when de-mammalized, destroyed or removed, may be reproduced. A bone may be broken in such a manner, that part of it must be taken away, but in a few weeks the separated ends are brought together by the secretion of new bony matter, called callus. Little more than 24 hours have elapsed after a fracture, before nature begins her operations. The soft parts inflame; the periosteum becomes swollen; the vessels pour out coagulable lymph, and a pellucid, gelatinous substance appears about the broken extremities of the bone. Into this minute blood-vessels are gradually sent off from the arteries, and in no very long time phosphate of lime begins to be secreted, for rendering the whole firm and compact. In cases of *necrosis*, where the old bone entirely loses its vitality, new bony matter is secreted into the surrounding periosteum, which thickens and enlarges, and in time supplies the place of the old bone. When a muscular part is cut away, as in removing flesh that... Of Reproduction.

That has become gangrenous, if the healthy function of the surrounding parts can be restored, and the loss of flesh has not been too great, the wound gradually fills up, not indeed with fleshy fibres, but with granulations very much resembling the ordinary cellular substance. It is well known, that the hair, nails, and skin, are occasionally renewed; but it will appear extraordinary that blood-vessels, and even nerves, have been reproduced. In cases of aneurism, where the trunk of the divided artery is divided, so as to cut off the usual channel for the blood, the anastomosing branches become gradually enlarged, and even new branches appear to be formed for carrying on the circulation. What has been said above respecting the formation of callus, also proves the formation of new blood-vessels; and the observations of Mr John Hunter put this beyond a doubt. Till within these few years, it was thought impossible that a divided nerve should re-unite; but some late experiments of Mr Cruickshank have proved that this re-union may take place.

Under this head we may mention some curious experiments that have been lately made by Dr Jones, on the means by which nature suppresses the hemorrhage from divided arteries. These experiments were made on dogs, and the results of them lead us to conclude, that the following is nearly the process by which the hemorrhage is suppressed. First, the divided artery contracts, and is drawn within the neighbouring parts; blood is gradually effused into the sheath of the artery and the adjoining cellular substance, where it is entangled, and affords a basis for the formation of a coagulum or clot, which surrounds the extremity of the divided artery, and prevents the farther effusion of blood, till another clot is formed within the mouth of the artery, plugging it completely up. Soon after there oozes out between the external and internal clot, a quantity of coagulable lymph, which cements all the parts together, and thus in time, if the artery divided be not very large, and the force of the circulation very great, the cavity of the vessel and the divided extremity, is obliterated, and all further loss of blood effectually prevented.

It is, however, in the lower classes that we are to look for the most remarkable instances of this provision of nature, particularly among the reptiles, crustacea, mollusca, worms, and polypes.

In many reptiles, the legs and tail, when cut off, are soon renewed, and even the eyes have been re-produced. Some interesting experiments on this subject by Spallanzani have been related under Reproduction, p. 316, to which we refer the reader.

In the crustacea, the legs and claws are very often torn away, either by accident, or by some voracious animal; but they never fail to be renewed in a short time, provided the animal is in good health. This is most remarkable in the craw-fish (cancer atlanticus, Lin.). It has been observed that when the claw of this animal is broken, the most distant part is gradually cast off, and about a day or two after, a red membrane, not unlike a bit of red cloth closes up the aperture. This is at first plain; but in the course of four or five days it assumes a convexity, which gradually augments till it takes the appearance of a small cone, which exceeds not a line in height. It continues, however, to stretch out, and in ten days it is sometimes more than three lines, or about one-fourth of an inch high. It is not hollow, but filled with flesh, and this flesh is the basis or rudiment of a new claw. The membrane that covers the flesh performs the same office to the young claw as the membranes do to the fetus of the larger animals. It extends in proportion as the animal grows; and as it is pretty thick, we can perceive nothing but a lengthened cone. When 15 days are elapsed, this cone inclines towards the head of the animal. In a few days more its curvature increases, and it begins to assume the appearance of a dead claw. This claw, though at the end of a month or five weeks it has acquired the length of six or seven lines, which is more than half an inch, is still incapable of action. The membrane in which it is enclosed becoming gradually thinner, in proportion as it extends, gives an opportunity of observing the parts of the claw, and we now perceive that this conical substance is not a simple congeries of flesh. The moment is now arrived when the claw begins to be brought forth. The membrane at last bursts, and the new claw, though still soft, appears without incumbrance or investment. In a few days more it is covered with a shell; and though still delicate, and not the half of its former length, it is able to perform all the natural functions. It has likewise been discovered, that, whether the claw has been lopped off at the fourth articulation, or anywhere else, the animal in a short time recovers all that it had lost. The same reproduction takes place also in the horns; but, if the tail is cut off, the animal survives a few days only.

Many of the mollusca exhibit curious instances of reproduction, especially the actinia, the star-fish, and snails. The abbe Dicquemarie made several experiments to ascertain the reproductive power of the actinia rufa (purple sea anemony). He first cut off all its tentacles, which grew again in less than a month; and on repeating this a second and third time, he had equal success. He cut off the upper part of one, and a few days after, the base of the animal was found to have fallen from its place; but it soon entirely recovered its limbs. But if the base of these animals is injured by the incision, the wound commonly proves mortal. The arms of the star-fish are often torn off, but appear always to be reproduced. The power of snails in this respect is very great; for Spallanzani has ascertained, that even if their heads are cut off, they are regenerated in no very long time. There can scarcely be a more surprising instance of animal reproduction than this, as we shall readily allow, if we consider the complicated structure of the head of a snail; that it contains a brain divided into two parts; that the horns attached to it are furnished with muscles, and that on the tops of the larger horns there are eyes, composed of two coats and three humours; that the head contains a mouth, lips, teeth, and a palate; and yet all these parts, when cut away, have been reproduced in the course of a few weeks.

On the reproductive power of polypes we have been sufficiently minute under Helminthology, No. 84.

As this subject of partial reproduction is extremely curious, and as we cannot here enter upon any particular detail on the experiments and observations that have been made on the subject, we shall conclude this part of the present chapter by enumerating the principal works to which the reader may refer for a more satisfactory account of the subject. These are chiefly Trembley, We have already stated it as our opinion, that plants, as well as animals, reproduce their like by generation. We shall not now enter on a discussion of the controversial point of the sexual system; and as the parts that appear subservient to this function in plants, and their various modes of propagation, have been sufficiently explained under the articles Botany and Plant, we shall in this chapter confine our attention to the generation of animals.

The human organs of generation are described in the article Anatomy, sect. xv. those of the cetacea under Cetology, No. 161.; those of birds under Comparative Anatomy, 277.; those of fishes have been noticed under Ichthyology. For a more full comparative view of these organs, we refer to Cuvier and Blumenbach.

The nature of generation which is the greatest mystery in the economy of living bodies, is still involved in impenetrable obscurity. The only circumstance common to all generation, and consequently the only essential part of the process, is, that every living body is attached at first to a larger body of the same species with itself. It constitutes a part of this larger body, and derives nourishment, for a certain time, from its juices. The subsequent separation constitutes birth, and may be the simple result of the life of the larger body, and of the consequent development of the smaller, without the addition of any occasional action.

Thus the essence of generation consists in the appearance of a small organized body in or upon some part of a larger one; from which it is separated at a certain period, in order to assume an independent existence.

All the processes and organs, which co-operate in the business of generation in certain classes, are only accessory to this primary function.

When the function is thus reduced to the most simple state, it constitutes the gemmiparous, or generation by shoots. In this way the buds of trees are developed into branches, from which other trees may be formed. The polypes (hydra) see Helminthology, No. 84., and the sea anemones (actinia), multiply in this manner; some worms are propagated by a division of their body, and must, therefore, be arranged in the same division. This mode of generation requires no distinction of sex, no copulation, nor any particular organ.

Other modes of generation are accomplished in appropriate organs; the germs appear in a definite situation in the body, and the assistance of certain operations is required for their further development. These operations constitute fecundation, and suppose the existence of sexual parts; which may either be separate, or united in the same individual.

In most animals the embryo of the future young is fecundated within the peculiar organs of one individual, while another of the same species is provided with the means of giving activity to the embryo by a fecundating fluid. In some animals, however, both these offices are performed by the same individual, which is then said to be androgynous, or hermaphrodite.

The office of the male sex is that of furnishing the distinction fecundating or seminal fluid; but the manner in which of sexes that contributes to the development of the germ is not yet settled by physiologists. In several instances, particularly in the frog, the germ may be clearly recognized in the ovum before fecundation; its pre-existence may be inferred in other cases from the manner in which it is connected to the ovum when it first becomes visible; for it is agreed on all hands, that the ovum exists in the female before fecundation, since virgin hens lay eggs, &c.

The combination of the sexes and the mode of fecundation are subject to great variety. In some instances hermaphrodites, they are united in the same individual, and the animal impregnates itself. The acephalous mollusca and the echini exemplify this structure. In others, although the sexes are united in each individual, an act of copulation is required, in which they both fecundate, and are fecundated; this is the case with the gastropodous mollusca and several worms. In the remainder of the animal kingdom the sexes belong to different individuals.

The fecundating liquor is always applied upon or Mode of about the germs. In many cases the ova are extruded impregnated before they are touched by the semen, as in some bony fish and the cephalopodous mollusca. Here, therefore, impregnation is effected out of the body, as it is also in the frog and toad. But in the latter instances the male embraces the female, and discharges his semen in proportion as she voids the eggs. In most animals the seminal liquor is introduced into the body of the female, and the ova are fecundated before they are discharged. This is the case in the mammalia, birds, most reptiles, and some fishes; in the hermaphrodite gastropodous mollusca, in the crustacea, and insects. In all the last-mentioned orders ova may be discharged without previous copulation, as in the preceding. But they receive no further development; nor can they be fecundated when voided*. We shall not gratify the prurient imagination of the philosophe senefaliste, by any details of the mode in which these operations are carried d'antem. on in the various classes of animals, except in one in-Comp. tom. stance, which is so curious that we shall be excused v. p. 12. for describing it. We allude to the copulation of the snail.

These animals meet in pairs, and stationing themselves an inch or two apart, launch several little darts, of snails not quite half an inch long, at each other. These are of a horny substance, and sharply pointed at one end. The animals, during the breeding season, are provided with a little reservoir for them, situated within the neck, and opening on the right side. On the discharge of the first dart, the wounded snail immediately retaliates on its aggressor by ejecting at it a similar dart; the other again renewes the battle, and in turn is again wounded. Thus are the darts of Cupid, metaphorical with all the rest of the creation, completely realized in snails. After the combat they come together. Each of them lays its eggs in some sheltered and moist situation,

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* Cuvier

** Lecons

*** Comp. tom. The effect of a single copulation varies in its degree; it usually fecundates one generation only; but sometimes, as in poultry, several eggs are fecundated; still, however, they only form one generation. In a very few instances one act of copulation fecundates several generations, which can propagate their species without the aid of the male. In the plant-louse (aphis) this has been repeated eight times; and in some monoculi 12 or 15 times.

When the germ is detached from the ovary, its mode of existence may be more or less complete. In most animals it is connected, by means of vesicles, to an organized mass, and the absorption of which nourishes and develops it until the period of its birth. It derives nothing, therefore, from the body of the mother, from which it is separated by coverings varying in number and solidity. The germ, together with its mass of nourishment, and the surrounding membranes, constitutes an egg, or ovum; and the animals which produce their young in this state, are denominated oviparous.

In most of these the germ contained in the egg is not developed until that part has quitted the body of the mother, or has been laid; whether it be necessary that it should be afterwards fecundated, as in many fishes; or require only the application of artificial heat for its incubation, as in birds; or that the natural heat of the climate is sufficient, as in reptiles, insects, &c. These are strictly oviparous animals.

The ovum, after being fecundated and detached from the ovarium, remains in some animals within the body of the mother, until the contained germ be developed and hatched. These are false viviparous animals, or ovo-viviparous. The viper, and some fishes, afford instances of this process. Mammalia alone are truly viviparous animals. Their germ possesses no provision of nourishment, but grows by what it derives from the juices of the mother. For this purpose it is attached to the internal surface of the uterus, and sometimes, by accident, to other parts, by a kind of root, or infinite ramification of vesicles, called placenta. It is not, therefore, completely separated from the mother by its coverings. It does not come into the world until it can enjoy an independent organic existence. The mammae cannot, therefore, be said to possess an ovum in the sense which we have assigned to that term.

From this view of the subject, generation may be said to consist of four functions, differing in their importance, and in the number of animals to which they belong.

1. The production of the germ, which is a constant circumstance; 2. Fecundation, which belongs only to the sexual generation; 3. Copulation, which is confined to those sexual generations, in which fecundation is accomplished within the body; 4. Uterogestation, which belongs exclusively to viviparous generation.

There is a general rule observable among all quadrupeds, that those which are large and formidable produce but few at a time; while such as are mean and of contemptible are extremely prolific. The lion, or tiger, have seldom above two cubs at a litter; while the cat, that is of a similar nature, is usually seen to have five or six. In this manner, the low tribes become extremely fecundly numerous; and, but for this surprising fecundity, from their natural weakness, they would quickly be extinguished. The breed of mice, for instance, would have long since been blotted from the earth, were the mouse as slow in production as the elephant. But it has been wisely provided, that such animals as can make but little resistance, should have at least a means of repairing the destruction which they must often suffer, by their quick reproduction; that they should increase even among enemies, and multiply under the hand of the destroyer. On the other hand, it has as wisely been ordered by Providence, that the larger kinds should produce but slowly; otherwise, as they require proportional supplies from nature, they would quickly consume their own store; and, of consequence, many of them would soon perish through want; so that life would thus be given without the necessary means of subsistence. In a word, Providence has most wisely balanced the strength of the great against the weakness of the little. Since it was necessary that some should be great and others mean, since it was expedient that some should live upon others, it has afflicted the weakness of one by granting it fruitfulness, and diminished the number of the other by infecundity.

In consequence of this provision, the larger creatures, which bring forth few at a time, seldom begin to generate till they have nearly acquired their full growth. On the contrary, those which bring many reproduce before they have arrived at half their natural size. Thus the horse and the bull are at their best before they begin to breed; the hog and the rabbit scarce leave the teat before they become parents in turn. Almost all animals likewise continue the time of their pregnancy in proportion to their size.

For an account of the principal phenomena attending the reproduction of the human species, viz. the requisites for conception and its signs; the effects of impregnation; the gradual evolution of the fetus, and the successive changes that take place in the uterine system during uterogestation, see the article MIDWIFERY, chap. i. and ii. The phenomena of reproduction in other viviparous animals are so analogous to those in the human species, that we need not enter on an examination of them. We shall here only give a view of the successive changes that take place in the egg of birds during incubation, taken from the observations of the celebrated Blumenbach.

The following observations refer to the egg of the common hen, as affording the most familiar example of changes of the chick during incubation.

A small shining spot, of an elongated form, with rounded extremities, but narrowest in the middle, is perceived at the end of the first day, not in nor upon the

(1) In the following table are noted the time of gestation, incubation, and the number of young produced in several species of quadrupeds and birds, as far as these have been ascertained; and the second column shews the period to which the life of each is usually extended. The cicatricula (m), but very near that part on the yolk-bag. This may be said to appear before-hand, as the abode of the chick which is to follow.

No trace of the latter can be discovered before the beginning of the second day; and then it has an incurvated form, resembling a gelatinous filament with large extremities, very closely surrounded by the amnion, which at first can scarcely be distinguished from it.

### Quadrupeds

| Species | Period of Life | Time of Gestation | Number of Young | |------------------|----------------|-------------------|-----------------| | Apes | | | About 2 | | Bats | | | From 2 to 5 | | Sloth | | | One | | Rhinoceros | 70 or 80 years.| 21 months | One | | Elephant | Above 100 years.| 9 months | One | | Arctic walrus | | | One | | Seal | | | About 2 | | Bitch | 12 or 16 years.| 9 weeks | From 4 to 10 | | Wolf | | 3½ months | 5 or 6 | | Fox | About 14 years.| 6 weeks | From 3 to 6 | | Jackall | | | From 6 to 8 | | Lioness | Above 70 years.| | 4 or 5 | | Tigress | | | Ditto | | Cat | From 10 to 18 years.| 8 weeks | From 4 to 6 | | Ferret | | 6 weeks | 6 or 7 | | Otter | | 9 weeks | 4 or 5 | | Virginian opossum.| | | Ditto | | Kangaroo | | | One | | Mole | | | 4 or 5 | | Porcupine | | | Two | | Guinea pig | 6 or 7 years | 3 weeks | From 5 to 12 | | Common rat | | 5 or 6 weeks | From 12 to 18 | | Moufe | From 2 to 3 years.| 6 weeks | From 6 to 10 | | Common squirrel | | | From 4 to 5 | | Hare | | A month | 3 or 4 | | Rabbit | | Ditto | About 8 | | Camel | 40 or 50 years.| 12 months | One | | Rein deer | 15 or 16 years.| 8 months | Two | | Stag | Near 50 years | 5 months | One | | Goat | 15 years | 5 months | About 2 or 3 | | Ewe | | Ditto | From 1 to 3 | | Cow | | 9 months | 1 to 3 | | Mare | 30 or 40 years.| 11 months | 1 or 2 | | Sow | Nearly 20 years.| 4 ditto | From 10 to 20 |

### Birds

| Species | Period of Life | Time of Incubation | Number of Young | |------------------|----------------|--------------------|-----------------| | Eagle | Above 100 years.| 30 days | 2 or 3 | | Raven | Near 100 ditto | 20 ditto | 5 or 6 | | Cuckoo | | | 1 or 2 | | Humming bird | | 12 days | Two | | Blackbird | | 14 ditto | 4 or 5 | | Canary bird | 10 or 20 years | Ditto | Ditto | | Wren | | A month | From 10 to 18 | | Pigeon | | Ditto | Two | | Turkey | | Ditto | 18 or 20 | | Hen | About 13 years | 3 weeks | About 20 | | Ostrich | | 6 weeks | One or two each| | Swan | Above 100 years.| Ditto | 6 or 8 | | Goose | Near 100 years | A month | 9 to 12 | | Duck | | Ditto | 12 to 14 |

(m) The structure of an egg has already been described under the article Egg; but for the better understanding of Blumenbach's observations, it may be necessary to enumerate the several parts, with the names given to them by that author. The membrane lining the shell is called membrane albuminis, and includes the two whites of the egg, the inner of which surrounds the yolk, which is contained within a peculiar, very delicate membrane, called the yolk-bag. From two opposite sides of this bag proceeds a white knotty body, terminating in the white of the egg, by a flocculent extremity. These bodies are called the chalaza or grandines; the cicatricula, tread, or traddle, is surrounded by one or more whitish concentric circles called halones, or circuli, the use of which is not known. About this time the halones enlarge their circles; but they soon after disappear entirely, as well as the cicatricula.

The first appearance of red blood is discerned on the surface of the yolk-bag, towards the end of the second day. A series of points is observed, which form grooves; and these closing, constitute vessels, the trunks of which become connected to the chick. The vascular surface itself is called figura venosa, or area vesiculosa; and the vessel by which its margin is defined, vena terminalis. The trunk of all the veins joins the vena portae; while the arteries, which ramify on the yolk-bag, arise from the mesenteric artery of the chick.

On the commencement of the third day, the newly formed heart is discerned by means of its triple pulsation, and constitutes a threefold punctum saliens. Some parts of the incubated chicken are destined to undergo successive alterations in their form; and this holds good of the heart in particular. In its first formation it resembles a tortuous canal, and consists of three dilations lying close together, and arranged in a triangle. One of these, which is properly the right, is then the common auricle; the other is the only ventricle, but afterwards the left; and the third is the dilated part of the aorta.

About the same time, the spine, which was originally extended in a straight line, becomes incurvated; and the direction of the vertebrae is very plain. The eyes may be distinguished by their black pigment, and comparatively immense size; and they are afterwards remarkable in consequence of a peculiar slit in the lower part of the iris.

From the fourth day, when the chicken has attained the length of four lines, and its most important abdominal viscera, as the stomach, intestine, and liver, are visible, (the gall-bladder, however, does not appear till the fifth day), a vascular membrane begins to form about the navel; and increases in the following days with such rapidity, that it covers nearly the whole inner surface of the shell, within the membrana albuginea, during the latter half of incubation. This seems to supply the place of the lungs, and to carry on the respiratory process instead of those organs. The lungs themselves begin indeed to be formed on the fifth day; but, as in the fetus of the mammalia, they must be quite incapable of performing their functions while the chick is contained in the amion.

Voluntary motion is first observed on the fifth day, when the chick is about seven lines in length.

Ossification commences on the ninth day, when the osseous juice is first secreted, and hardened into bony points. These form the rudiments of the bony ring of the sclerotica, which resembles at that time a circular row of the most delicate pearls.

At the same period, the marks of the elegant yellow vessels on the yolk-bag, begin to be visible.

On the fourteenth day the feathers appear; and the animal is able to open its mouth for air, if taken out of the egg.

On the nineteenth day it is able to utter sounds; and on the twenty-first to break through its prison, and commence a new life.

Blumenbach concludes his observations with a few remarks on those very singular membranes, the yolk-bag and the chorion, which are so essential to the life and preservation of the animal.

The chorion, that most simple yet most perfect temporary substitute for the lungs, if examined in the last quarter of incubation in an egg very cautiously opened, presents, without any artificial injection, one of the most splendid spectacles that occurs in the whole organic creation. It exhibits a surface covered with numberless ramifications of arterial and venous vessels. The latter are of a bright scarlet colour, as they are carrying oxygenated blood to the chick; the arteries, on the contrary, are of a deep or livid red, and bring the carbonated blood from the body of the animal. Their trunks are connected with the iliac vessels; and, on account of the thinness of their coats, they afford the best microscopic object for demonstrating the circulation in a warm-blooded animal.

The other membrane is also connected to the body of the chick; but by a two-fold union, and in a very different manner from the former. It is joined to the small intestine, by means of the ductus vitello-intestinales, and also by the blood-vessels, with the mesenteric artery and vena portae.

In the course of the incubation the yolk becomes constantly thinner and paler, by the admixture of the inner white. At the same time innumerable fringe-like vessels with flocculent extremities, of a most fingerlike structure, form on the inner surface of the yolk-bag, opposite to the yellow ramified marks, and hang into the yolk. There can be no doubt that they have the office of absorbing the yolk, and conveying it into the veins of the yolk-bag, where it is assimilated to the blood, and applied to the nutrition of the chick. Thus in the chick which has just quitted the egg, there is only a remainder of the yolk and its bag to be discovered in the abdomen. These are completely removed in the following weeks, so that the only remaining trace is a kind of cicatrix on the surface of the intestine.

Many of the causes which contribute to the formation of a living body have hitherto eluded human research; may in all probability never be discovered; and perhaps are beyond human comprehension.

Some philosophers, discovering the extreme divisibility of matter, and learning from the microscope that generation transformation is but the development of certain parts that previously existed, have thence imagined that generation is somewhat analogous; that all organized bodies received their form at the beginning; that the first of every genus and species contained by involution the numberless millions of succeeding generations; and that the union of the two sexes gives only a stimulus, and brings into view forms that had existed since the world began.

By this hypothesis they have attempted to explain a thing that is unknown by what must remain for ever incomprehensible to the human mind in its present state. They absurdly appeal from observation to conjecture; and suppose that bodies which are originally brought into view, which are daily augmented, frequently repaired, and sometimes renewed by organic action, do nevertheless in their first formation require an effort superior to what omnipotence is able to perform by secondary agents. Had the supporters of this hypothesis considered that many herbaceous plants produce new flowers when... when the first fet are untimely cut off; that lobsters and many other animals renew their limbs, and that certain polypes can raise a structure so perfectly resembling a vegetable form as to puzzle the naturalist whether or not he should clasps them under plants, they would surely not have preferred such bounds to omnipotent wisdom and almighty power, or declared with such confidence what the Author of nature, to speak with the vulgar, must necessarily perform by his own hands, or what he may intrust to secondary causes regulated by his laws.

These philosophers will find it difficult to account in a very satisfactory manner for monstrous productions, and for those changes of structure and of form which for a while continue hereditary from the influence of habit. They object to others, that all the parts of a living body are mutually dependent on one another, and that they must necessarily have been coeval, or have existed at once. But though every attempt that has yet been made to ascertain which of the vital organs are prior and which posterior in a living body has proved unfruitful, it has not been demonstrated that either themselves or their functions are coeval. It may, on the contrary, be plainly demonstrated from observation, that the lungs and the stomach do not begin to perform their functions so early as the heart and the vascular system; that even the heart and its system perform their functions with some considerable changes, immediately after birth; that the vegetable tribes are without nerves; and that the brain and nerves in the animal kingdom perform more and more of their functions, according as the system approaches towards maturity. It has even been shewn, that bones will unite; that the limbs of an animal continue to be nourished without the nerves; that there is a principle of life in the blood; that the heart will act under other stimuli beside that of nervous influence; and found logic does by no means require us to suppose, that the first action of the foetal heart, or the punctum fulgens, is owing to the influence of stimuli from the brain, or that the brain must have existed when the heart first moved. Although the minuteness and transparency of the parts may prevent us from seeing the first gradual formation of the embryo, yet every observation corroborates the opinion, that it is formed by secondary causes, and through the medium of organic powers.

Most physiologists have believed, the certain inorganic particles are contained in the system of one sex or of the other, and that by the union of the sexes these particles have become organised. It has, however, been asked whether or not is the embryo formed by the joint operation of the two sexes? or is it formed entirely by one, and brought into action by a stimulus from the other? The former of these questions supposes that each of the sexes has a seminal fluid; that some mixture takes place in the uterus, and produces an embryo, in the same manner that a neutral salt assumes a certain and determinate form. The notion implies some general and confused ideas of chemical combination; but does not bepeak a very clear understanding, profound reflection, or much acquaintance with the nature and properties of living bodies.

For a long time past the most rational physiologists have generally thought that the embryo is formed gradually and slowly in one or other of the two sexes, not by chemical combination, but a system of organs, directed by laws and prompted by stimuli, with many of which we are yet unacquainted. From the great Hippocrates to Fabricius and Harvey, the credit of furnishing the foetal embryo was almost universally given to the females of oviparous animals. Among the viviparous, the appearances were such, that the female was left to contest it with the male. At last the eclat of Embryo Leeuwenhoek's discoveries seemed to put an end to all doubts entertained upon the subject. He very plainly from the saw through his microscope that very great profusion of male particles that move to and fro with amazing rapidity in the male semen. Upon this he embraced the doctrine of Hamme, who had seen them before, and supposed from their motions that these particles were not only animalcules, but the principles or rudiments of that animal in whom they were formed, and that they were deposited in the uterus of the female only to be nourished and augmented in size.

What raised objections against this theory was, that numerous animalcules were discoverable by the microscope in other fluids, and that a vast profusion of young embryos appeared in cases where never more than one or two arrive at maturity. It was an objection to it, that some females had been impregnated, where the hymen remained unbroken, so that the impregnating fluid could have reached only the mouth of the uterus. Again, in frogs, fishes, and many other animals, the ova are not impregnated till after extrusion; and lastly, Haller had observed the chick completely formed in eggs that had not been fecundated.

It is now, we believe, pretty generally known, that the embryo does not commence its existence in the cavity of the uterus. De Graaf observed it on its passage down the fallopian tube; he saw the place where it first began in the ovarium of the female, and cases have occurred where it has missed the fallopian tube, where it has fallen into the abdomen, in which the placenta has been formed, and the fetus has grown among the bowels.

From these facts it has been concluded, notwithstanding some feeble objections, that the female ovaria contain the embryos in the form of eggs; that these eggs exist in the male semen, which is sometimes thrown into the cavity of the uterus, sometimes applied only to its mouth, and sometimes sprinkled on the eggs after they are extruded. For more on this subject see the article Midwifery.

A view of the relations between the present function Relations, and those which we have before examined, is a subject of so delicate a nature, that we must not enlarge on it. The sympathy between the reproductive and the nervous systems is well known. The effect which desire has occasionally produced on the brain is very great, madness being not unfrequently the consequence, either of too much indulgence, or of total continence. We are of opinion that the influence of the fancy or imagination on the uterine system has been overrated, though the accounts given of monsters or deformed births, in consequence of terror experienced by the mother, appear to be too well authenticated to warrant our total disbelief in this influence. Too much indulgence in this appetite produces a debilitated state, both of Of Sleep the mental and bodily functions, and deprives the system of that natural stimulus which seems essential to the activity and vigour of the body in the male.

**Chap. XV. Of Sleep and Torpor.**

We have already considered the active means by which the waste of the body is repaired; but all these would have little effect, and could not indeed be carried on for any considerable time, without some general relaxation of the system. This relaxation is brought about by sleep, in which the active functions find a repose from the labours which they have undergone during the day; and in this way the system is recruited more completely than by any other means.

Sleep may be considered as an affection of mind, and therefore more properly a subject of metaphysical, than of physiological speculation. It is, however, generally treated of in systems of physiology; and it will be necessary to take notice of some circumstances respecting it. We shall chiefly consider the state of the body and mind during sleep, and some of the principal theories that have been contrived to account for it.

Natural sleep returns at certain intervals, which are, however, different in different animals. Most animals, and especially man, sleep only during the night, but most of the predatory species, as beasts and birds of prey, choose this time for their predatory excursions, and repose during the day. Sleep comes on with an unusual languor and listlessness; an aversion to motion; the mind becomes unfit for its usual exertions; and the desire of rest pervades the whole system. In particular, the extensor muscles lose their power of preserving the body in an erect posture; the eyelids involuntarily fall; the head bows forward; the joints bend, and the body sinks. During sleep, all the voluntary motions are in general suspended; but the involuntary actions of the heart and lungs proceed, though not so vigorously as in the waking state; the circulation and respiration being slower than usual. Most of the senses are also in a state of repose, especially those of feeling, smell, and probably of taste. Hearing is, in some animals, very acute during sleep, and they are thus enabled to escape any danger that threatens them. Some animals, as the hare, also sleep with their eyes open; and in most the impression of light, when the eyelids are raised, is very evident. The functions of digestion, absorption, and secretion, seem to proceed with greater ease and activity during sleep; and assimilation and nutrition are much promoted by this state of repose. Some of the faculties of the mind, especially the imagination, are, however, in full vigour, as appears from the dreams that take place during sleep. The duration of sleep is exceedingly various. Among the human species, young children, and very old persons, pass the greatest half of their time in sleep, while middle-aged and active people seldom sleep so much as one third of the 24 hours.

Though the returns of sleeping and waking depend much on custom, they may, however, be changed by various circumstances; and though the commencement of one of these periods happen to be altered, that of the other may remain as before. If a person is accustomed to go to sleep exactly at nine in the evening, and to rise again at six in the morning, though the time of sleep may be occasionally protracted till twelve, he will yet awaken at his usual hour of six: or if his sleep be continued by darkness, quietness, similar causes, till the day be farther advanced, the desire of sleep will return in the evening at nine.

Most of the causes that produce or prevent sleep, have been mentioned in the article Medicine, No. 94. Asleep to the immediate cause, the opinions of physiologists are much at variance, and the theory of sleep is as little understood as that of any function of the animal economy.

According to Haller, sleep arises, either from a simple absence, deficiency and immobility of the spirits, or of Haller, from compression of the nerves, and always from the motion of the spirits through the brain being impeded. That sleep is, somehow or other, connected with a compression of the brain, appears very probable, from the heaviness and coma that take place in cases where such a compression has evidently been produced; but how this compression acts has never yet been satisfactorily explained; and the obstruction that Haller supposes in the motion of the animal spirits, or nervous fluid, is gratuitous.

One of the fashionable doctrines of the present day respecting the immediate cause of sleep is, that this state is produced by an exhaustion of irritability or excitability. According to Brown, sleep succeeds a diminution of excitement, during which the excitability is either only so far diminished that it can be accumulated again, or so abundant, that the excess can be wasted, and in each case the excitement restored.

Similar to this is the doctrine of Zoonomia, that sleep depends on an exhaustion of senatorial power. Dr Dar. § 337, thus characterizes perfect sleep: 1. The power of volition is totally suspended. 2. The trains of ideas caused by sensation proceed with greater facility and vivacity; but become inconsistent with the usual order of nature. The muscular motions caused by sensation continue; as those concerned in our evacuations during infancy, and afterwards in digestion, and in priapismus. 3. The irritable muscular motions continue, as those concerned in the circulation, in secretion, and respiration. But the irritable senial motions, or ideas, are not excited; as the immediate organs of sense are not stimulated into action by external objects, which are excluded by the external organs of sense; which are not in sleep adapted to their reception by the power of volition, as in our waking hours. 4. The affecive motions continue, but their first link is not excited into action by volition, or by external stimuli. In all respects, except those above mentioned, the three last senorial powers are somewhat increased in energy during the suspension of volition, owing to the consequent accumulation of the spirit of animation.

Thus, the immediate cause of sleep consists in the suspension of volition produced by the exhaustion of senorial power; and hence, whatever diminishes the general quantity of senorial power, acts as a remote cause of sleep.

Beside the insufficiency of these hypotheses to account for many circumstances that take place during sleep, we may remark, that this state is often produced when no exhaustion of irritability, excitability, or senorial power, can be supposed to have taken place. Thus, the propensity to sleep often becomes irresistible from the effects of monotonous speaking, from stillness, darkness, or Plants have been said to sleep. At the approach of night, many of them are observed to change their appearances very considerably, and sometimes even to such a degree as scarcely to be known for what they are. These changes happen principally to the leaves and the flowers. During the night, many leaves, according to the nature and genus of the plant, are seen to rise up, to hang down, or to fold themselves in various ways, for the protection of the flowers, the buds, the fruits, the young stems; and many flowers, to escape a superabundance of moisture, to hang down their mouths towards the earth, or to wrap themselves up in their calices. These phenomena are owing to stimuli acting from without; we may add, that most of the motions are performed at the joints where the leaves and petals articulate with the stem. A period of rest is as necessary to plants as sleep is to animals. The irritable principle cannot act long under the influence of the same stimulus, except at intervals, and the rapid growth observable in plants during the night, is a strong proof that the organs employed in assimilation had been disturbed in discharging their functions during the day, when exposed to the action of heat and light and of other stimuli.

In our general outline, we had proposed introducing here an account of the phenomena of dreams, but we find that this subject has been so fully discussed under the article Dream, that any additional remarks would be unnecessary. To this article therefore we refer the reader.

Rules for the management of the body with respect to sleep, scarcely come within our present province; but as we pass so much of our time in this state, during which we are sometimes occupied in a very agreeable manner, while at others we are subject to most uneasy sensations, it is a matter of considerable consequence to take those measures which may secure to us the former, and enable us to avoid the latter. We have seen few rules better adapted to those purposes than those of Dr Franklin; but as more important matters press for attention within the circumscribed limits to which we are restricted, we must refer our readers to the original paper, which is published in the late 8vo edition of Franklin's Works, vol. iii. p. 437.

In a few cases, not only the imagination has a full range during sleep, but the voluntary motions of the body, and even the exercise of some of the external senses, are carried on with apparently as much perfection as when the person is awake. This state is called somnambulism, or sleep-walking, and is commonly considered as a variety of dreaming. Many surprising accounts have been given of sleep-walkers. They have been known to rise, dress themselves, go out of doors, and sometimes out of a window, from which they have climbed upon the roof of a house, dig in a garden, draw water from a well, saddle a horse and ride several miles; maintain a rational and interesting conversation, and even go through a laborious and difficult literary task; and after having performed these exploits, they have returned to their bed without being conscious of what they had been doing. This want of consciousness appears from their remembering nothing when they awake, of what passed during their sleep. It is disputed whether somnambulists incur as much danger in the actions which they perform, as those who are awake, in similar circumstances. We are inclined to think that the danger is much less in the former case, as sleep-walkers seem entirely free from the terror which commonly attends the attempting of any hazardous enterprise when awake; such as mounting to the roof of a house, climbing a steeple, &c. If suddenly awakened, however, while engaged in any of their hazardous actions, the danger is very great.

Dr Darwin considers somnambulism, not as a state of reverie, sleep or dreaming, but as a variety of reverie, carried to a morbid extent, so as to become a sort of epileptic or cataleptic paroxysm. In the state of reverie, according to Dr Darwin, the irritative motions occasioned by internal stimuli continue, those from the stimuli of external objects are either not produced at all, or are never succeeded by sensation or attention, unless they are at the same time excited by volition; the sensitive motions continue, and are kept consistent by the power of volition; the voluntary and associate motions continue undisturbed. He considers reverie as an effort of the mind to relieve some painful sensation, whence it is allied to convulsion and insanity.

The torpor that takes place in many animals during Torpor of winter, appears to be so nearly allied to sleep, that we animals shall consider it in this chapter.

A great variety of animals of almost every class, retire during the cold of winter, to the recesses of caverns, holes in old walls, hollow trees, or below the earth, where they remain in apparently a lifeless state till the return of spring rouses them from their trance. We shall here enumerate the different animals that have been known to undergo this state of hibernation.

Bats, especially the vesperilio murinus, auritus, and Hibernia noctula (see Mammalia, No. 39.); bears, especially the brown and the polar bear, and the badger; the hedgehog, (erinaceus Europaeus); several species of the mouse and rat tribe, but more especially the hamster (mus cricetus), the marmots, especially the arctomys marmota, (see Mammalia, No. 24.); the dormouse (myoxus mycardinus). Sheep appear capable of living for a considerable time in a torpid state, as they have been known to remain alive for several weeks, buried under the snow.

It does not appear that birds in general are capable of hibernation undergoing this state of existence; but the instances of birds swallows that have been found in this state in old walls and hollow trees, and even, as some affirm, below water, and have recovered life and activity on being exposed to gradual warmth, are too well authenticated to admit a doubt, that these at least sometimes hibernate.

Most reptiles and serpents pass the winter in a state of hibernation; but this is more particularly the case with the land tortoise (testudo graeca), see Reptology, p. 271.; frogs, and those lizards which inhabit cold climates.

It is not certainly known whether many species of fish of fishes become torpid in winter; but there is no doubt that several of them are susceptible of this state; and we are told that in North America, especially about Hudson's Bay, meopitir, p. 16. Of Sleep and Torpor.

Bay, fishes are not unfrequently found included within a body of ice, and when exposed to gentle heat, have recovered life and motion.

Almost all insects remain, during the winter, in a torpid state. This happens principally to the chrysalids, and such grubs as cannot, in that season, procure their food.

It will appear extraordinary that we should place man among the hibernating animals; and yet there seems little doubt, that even he is capable of having his life suspended for a considerable time, when exposed to those causes which bring about the torpidity of those animals that we have already mentioned. We are told of a woman, who, in February 1789, remained eight days buried in the snow, and still recovered; and the case of the three women who remained for 37 days in a stable at Bergamoletto, that had been overwhelmed by an avalanche, or snow heap, with no other sustenance than the milk of a half-starved the goat, is well known. These instances, added to others of persons who have passed several weeks in a state of almost uninterrupted sleep, tend to prove that man himself may, under certain circumstances, continue in a torpid state.

During this state of torpidity, the animals scarcely appear to live; sensation seems altogether lost; their irritability is so much diminished, that they may be cut, torn, or even broken to pieces, without expressing any mark of feeling, or giving any sign of motion; digestion seems entirely suspended; the secretions and excretions are discontinued. Some of the functions, however, are carried on. Respiration and circulation, though very languid, and sometimes scarcely perceptible, appear to go on in a degree sufficient to preserve the existence of the animal; and the action of the absorbents seems to be very little diminished, as appears from the gradual absorption of the fat. If the animal is taken from its place of confinement, and exposed to a gentle heat, it gradually recovers all its faculties; but if carried back to its cell, it relapses into the state of torpidity.

The long suspension of animation, of which several animals are susceptible, appears still more extraordinary than the torpidity above described. The common hair worm (gordius aquaticus) may, when dried, be preserved for an indefinite length of time, and when put into water, gradually recovers its usual activity of motion. See Helminthology, No. 32. One of the most remarkable cases of this suspended animation is that related of the garden snail, of which the following curious account has been given in the Philosophical Transactions for 1774. Mr Stuckey Simon, a merchant in Dublin, whose father, a fellow of the Royal Society, and a lover of natural history, left to him a small collection of fossils and other curiosities, had amongst them the shells of some snails. About 15 years after his father's death (in whose possession they were many years), he by chance gave to his son, a child about 10 years old, some of these shells to play with. The boy put them into a flower-pot, which he filled with water, and the next day into a basin. Having occasion to use this, Mr Simon observed that the animals had come out of their shells. He examined the child, who assured him that they were the same he had given him, and said he had also a few more, which he brought. Mr Simon put one of them into water, and in an hour and a half after observed that it had put out its horns and body, which it moved but slowly, probably from weakness. Of Sleep Major Vallancy and Dr Span were afterwards present, and found one of the snails crawl out, the others being dead, most probably from their having remained some days in the water. Dr Quin and Dr Ruttie also examined the living snail several times, and were greatly pleased to see him come out of his solitary habitation after so many years confinement. Dr Macbride, and a party of gentlemen at his house, were also witnesses of this surprising phenomenon. Dr Macbride has thus mentioned the circumstance: "After the shell had lain about ten minutes in a glass of water that had the cold barely taken off, the snail began to appear; and in five minutes more we perceived half the body pushed out from the cavity of the shell. We then removed it into a basin, that the snail might have more scope than it had in the glass; and here, in a very short time, we saw it get above the surface of the water, and crawl up towards the edge of the basin. While it was thus moving about, with its horns erect, a fly chanced to be hovering near, and, perceiving the snail, darted down upon it. The little animal instantly withdrew itself into the shell, but as quickly came forth again, when it found the enemy was gone off. We allowed it to wander about the basin for upwards of an hour, when we returned it into a wide-mouthed phial, wherein Mr Simon had lately been used to keep it. He was so obliging as to present me with this remarkable shell; and I observed, at twelve o'clock, as I was going to bed, that the snail was still in motion, but next morning I found it in a torpid state, sticking to the side of the glass."

The still more extraordinary instances that have been related, on what many have considered authentic testimony, of toads having been found inclosed in the trunk of a large tree, or within a solid block of stone, appear almost incredible; and yet if we consider that M. Hérissant preserved toads in a state of suspended animation for 18 months, in boxes covered with a thick coating of mortar (see Entomology, p. 286.), that the snails mentioned in the above quotation, must have lain for at least 20 years; and that flies have been recovered after being immersed for many months in Madeira wine, it is difficult to say how long this suspended animation may not be continued.

Similar phenomena take place in the vegetable creation. Most of those plants which survive one year, often shed their leaves on the approach of winter; and, during this season, the motion of the sap ceases, and they have all the appearance of dead shrubs. The herbaceous tribes even die down to the roots, which, being mostly of the bulbous kind, afford shelter to the surviving germ; and are hence called, by botanists, the hypenacula of plants. On return of spring, the plant shoots anew from its winter's retreat, and flourishes with its former strength and beauty.

Some plants are even capable of having their vitality, or rather the exercise of all their functions, suspended, as in the gordius and the snail, for an indefinite length of time. Mistletoe have been kept in a dried state in a hortus siccus for many years, and have shown no sign of life, till they were moistened and exposed to air, light, and a moderate heat, when they have recovered all their powers, have erected their stems, shot forth new branches, and flourished as at first. It is almost impossible, in the present state of our physiological knowledge, to give any rational theory of these phenomena. The torpor of animals has been attributed to exhausted excitability, or exhausted sensorial power; to the effects of habit, and to the effect produced on the brain by suspended or diminished respiration. The last of these, though not quite satisfactory, appears to us the most probable hypothesis. It has been ably defended and illustrated by Dr George Kellie, in a paper in which he relates a remarkable case of torpor from cold.

"The powers of voluntary motion and of sensation (says Dr Kellie), are known to depend immediately upon the conditions of the brain and nerves; if, therefore, we could discover in what manner these organs are affected by any of the preceding events, we should advance considerably towards the solution of the questions above stated. (Namely, What is the order of succession between the diminished irritability of the heart, in consequence of the abstraction of caloric, and the complete torpor of the voluntary muscles and of the organs of sense, and how are the intervening effects connected?) Were the inactivity of these organs the direct effect of their diminished temperature; did the torpor in no case happen, till the heat of the brain and nerves was reduced beneath the natural standard, there could be hardly ground for any further inquiry. But, as it is not so, some other change, less direct, must have occurred, in consequence of the connection of the brain with, and its dependence upon, some other of the functions antecedently and more immediately affected; and this function I apprehend to be respiration, between which and the energies of the nervous system a very intimate connection is maintained, through the changes produced on the blood during the pulmonary circulation. This dependence of the brain upon the properties of the blood, maintained by respiration, is evinced by a great variety of observations. Whatever impedes the respiratory changes of the circulating fluid debilitates or destroys the powers of muscular motion, as the respiration of noxious gases, of reduced or rarefied atmosphere; while greater exertions of muscular powers call for, and give occasion to more frequent respiration, more rapid consumption of air, and greater changes of the blood; and the breathing of more effective gases, as of the nitrous oxide, increases the motive and sensitive powers of animals. That these effects depend immediately upon the properties of the blood, as modified by respiration, acting on the brain, has, I think, been proved by the experiments of Bichat, who, in a masterly manner, has traced the mutual connection and dependencies of the vital functions in his admirable Recherches Physiologiques sur la Vie et la Mort. The transfusion or injection of venous blood into the carotids induced asphyxia or death, the instant it reached the brain; an effect which did not follow the similar transfusion of arterial blood from the carotid of another. By these experiments, and by several other observations, he has shewn, that the asphyxia which so instantly follows impeded or suspended respiration is occasioned by the impression of dark, venous, unchanged blood upon the brain, and not, as has commonly been supposed, from this blood being incapable of stimulating the left side of the heart, which, on the contrary, continues to contract and to circulate the blood for some time after the voluntary functions are suspended; an observation confirmed also by Coleman and others.

"Such then appears to be the connection between the functions of respiration and those of the brain. Now, in animals rendered torpid from cold, there are many observations which lead us to believe, that the immobility of the nervous system depends much, and very directly too, on the state of respiration.

"In the perfect torpor of the hibernating amphibia, respiration is completely suspended, and the consequent changes produced on the blood by that function totally prevented. This, which appeared from a variety of observations on the winter quarters of such animals found imbedded in mud, incased in ice, or closed up in opercula of their own construction, for the occasion of excluding the air, has been amply confirmed by the pointed experiments of Spallanzani, lately published by Senebier."

"In every case of torpor from cold, where the respiration falls short of this complete suspension, it is at least more or less impaired. How much the torpid state depends on this condition of the respiratory functions, farther appears from observing, that hibernating animals, even those not of the amphibious order, warned by the approach of winter, instinctively or indifferently seek situations unfavourable to perfect respiration, where this function may be either inadequately or not at all performed, as by premature and involuntary interment under ground, in old walls, in mud, at the bottom of lakes, &c. The instinct of these animals, too, has been finely imitated by experiment, illustrating at once the object of this instinct, and confirming the opinion here advanced of its tendency. Thus the dormant hamster was found to regain and preserve its activity, when freely exposed to a pure atmosphere, the temperature, at the same time, not exceeding that at which it had formerly become torpid, or at which it returned to that state when again secluded under ground. These observations seem conclusive on this point, and, with those already brought forward confirming the general connection established between the properties of the blood, as modified by respiration, and the functions of the brain, render it, I think, highly probable, that the torpor of the voluntary powers, in the cases now under consideration, is the consequence of a limited and imperfect respiration, antecedently induced by diminished temperature.

"Observation, indeed, is more deficient on this point with regard to the higher orders of animals, and to men, who only occasionally become torpid from cold. Yet more than analogy, which is here very strong, leads me to believe that, even in these, the functions of respiration are much and necessarily affected. The examples of cattle and of men remaining long torpid, deeply buried under snow, are pretty direct and convincing proofs of this.

"If our induction from all these observations be admitted, we have the rudiments of a theory adequate to the explanation of the phenomena, in so far, at least, as the torpor of the voluntary powers is concerned.

"From the suspended or imperfect respiration, those changes, by which the blood is fitted for maintaining the activity of the sensorial system, are interrupted; this imperfect blood circulating slowly through the brain directly impedes its functions, and so debilitates the excitability of the motive and sensitive organs, that they become torpid. This enunciation may seem hypothetical; but let the proofs of the intimate connection between the respirable and sensorial functions be weighed; Of Sleep consider also the interrupted respiration of hibernating and torpid animals, their instincts with regard to this, and the greater facility with which torpor is induced in a confined situation, which they naturally seek; and compare all these with the observations and experiments of Bichat on the effects of the immediate impregnation of venous blood upon the brain, and you will perceive a connected system, not entirely fanciful, a theory not without foundation and strength, and which appears to me at least to merit some attention.*

For further particulars respecting the torpidity or hibernation of animals, we refer the reader to Spallanzani's Tracts on Animals and Vegetables; White's Natural History of Selborne; Barton's Fragments of the Natural History of Pennsylvania; Pennant's Arctic Zoology; La Cepède on Oviparous Quadrupeds, as translated by Kerr; Townson's Tracts on Natural History and Physiology, and the Inaugural Dissertation of Dr. Reeve de Animalibus Hicem Sepitis, published at Edinburgh in 1803.

**Chap. XVI. Of Death.**

In the article MAN, (No. 33, to the end) we have traced the progress of human life, from the cradle to the grave; and have briefly considered the phenomena and the consequences of natural death. In that article, and Longevity, we have also stated the natural duration of human life, and the circumstances that tend to prolong our existence beyond the ordinary period. We shall not here enter again on any of these topics, except to give a more ample account of the gradual approaches of natural death, and shall then enumerate the causes which usually produce violent or accidental death, and mention the opinions of some of the best writers on the nature of death.

Natural death is, in the present state of civilized society, by no means a common occurrence. When it does take place, its approach is slow and gradual. His whole life terminates in consequence of advanced age, (to use the language of a celebrated French physiologist,) dies in detail. His external functions successively cease to exert their action; all his senses are successively lost, or the ordinary causes of sensation pass over them without leaving their usual impressions. The sight becomes obscure, and at length the humours of the eye no longer transmit the rays of light; the ear receives only confused sounds, and frequently before death, is altogether insensible; the sense of touch, in consequence of the hardness and callousness of the cuticle, and the obliteration of many of the subcutaneous vessels, grows dull and uncertain; and all the parts depending on the skin show marks of weakness; the hair and the beard grow white, and a greater or less degree of baldness takes place; odours are no longer perceived, or they are perceived but faintly. The taste usually survives the rest of the senses; but that too, at last, grows equally obscure. The functions of the brain partake of the imbecility of the external senses. The imagination in particular becomes dull and often depraved; the memory no longer retains those occurrences which are every day taking place, though it recalls with increased relish and delight those of past times; the judgment becomes weak and wavering.

From the universal agency exerted by the nervous system on all the animal functions, we must expect that when the former is impaired, the latter will be proportionally enfeebled. The faculties of locomotion and of speech are commonly the first of these that fail; the body totters at every step, the voice grows weak, and the tongue falterers. The motion of the limbs is difficult and painful, and hence is but seldom willingly exerted. Not so with the vocal organs, though the impediments to utterance are evident and painful to his hearers, the old man himself seems scarcely to attend to them, but talks with proverbial garulity, and especially delights in recounting the scenes and actions of his youth.

While the external functions, and those of the brain, are thus gradually impaired; the internal, or what are commonly called the vital and natural functions, as digestion, absorption, circulation, respiration, and secretion, proceed with but little derangement. The circulation and respiration are indeed slower than before, and the appetite is in general less keen and returns less frequently; but the digestive powers of the gastric fluids remain in full vigour, and even after death has taken place, are exerted on the coats of the stomach; absorption is also very active, and nutrition, at least in many parts of the body, is sufficiently evident. At length, however, all these functions lose their powers; digestion languishes; the secretions no longer take place; the circulation, especially in the minute vessels, becomes obscure, and being deprived of the tonic powers by which it was carried on, gradually ceases altogether; the heart no longer propels the blood from its ventricles; and the circulation through the lungs being thus arrested, these organs cease to take air, make their last expiration, and thus the natural life of man is terminated.

Accidental death takes place in one of the following ways; either suddenly, in consequence of some great disturbance produced in the animal economy, as when a man is cut off by a sudden stroke of apoplexy, violent hemorrhage, dyspnea, &c., or by slow and gradual steps, in consequence of some less violent but still fatal disease. In the former case, it is sudden or violent death; in the latter lingering death.

Violent death may take place first, either in the brain, the lungs, or the heart; but when the action of one of these organs ceases, that of the others soon terminates. The entire cessation of life seems, however, to be more sudden in the two latter cases, and most of all in the last; when the heart is wounded or ruptured, the animal dies instantly; when the lungs are rendered inactive in consequence of suffocation, the animal may live for several minutes, or for an hour or two; but when the brain is overwhelmed, he may survive for hours or even days. Thus it sometimes happens, in cases of apoplexy, that the patient lies motionless, speechless, and quite insensible to external stimuli, while the circulation and respiration continue, impeded indeed, but not destroyed, for a considerable time, though life, as appears from the event, be in a state of irrecoverable declension. We shall presently show how these circumstances have been explained.

The usual signs of approaching death are, a very signs of quick and small pulse, scarcely distinguishable, and commonly intermitting; coldness, and generally clammy death. Sweats about the extremities; a "lack lustre" eye, sunk in features, want of expression about the countenance, and a prominence of the bones of the face, with a corresponding hollowness in the cheeks, orbits, and especially... death, especially at the temples. These last appearances constitute the marks of what has been called facies Hippocratica. They are all signs of a loss of activity and power in the circulating and nervous systems. Under these bodily circumstances, the powers of the mind seem to decline, generally with an equal pace with those of the body; and when the medium through which the activity of the soul is manifested can no longer act, we cannot expect to find any further traces even of its existence. Yet at the period of its separation, we are told of brilliant mental exertions of powers of intellect, not equalled in the best portion of existence. It has not been our fortune to see such intellectual animation. At the moment of death, anxiety for those we have loved will sometimes occasion apparently disproportionate exertions; and as they were unexpected, they have been exaggerated. But in no instance could we ever detect the activity of mind independent of the body. To this temporary prison the soul is confined, till, by the destruction of the machine, its animating principle is emancipated, soaring probably in higher, and, we trust, in more blissful, regions.

A few cases have occurred, in which persons, who were thought dead, have recovered from what was really a state of suspended animation; and there is reason to believe, that some unhappy beings have been buried while in this seemingly lifeless state. It becomes, therefore, a matter of the highest importance to ascertain, with certainty, whether or not death has actually taken place. The ordinary signs of death, as enumerated by one of the latest writers on this subject, are as follows:

1. The suspension of respiration. 2. The rigidity of the limbs. 3. The loss of sensation and motion. 4. The want of pulsation in the heart and arteries. 5. The spontaneous discharge of feces. 6. The collapse, opacity, and want of luster in the eyes. 7. The coldness of the body. 8. The paleness or lividity of the countenance. 9. The relaxation of the lower jaw. 10. The regurgitation of liquids to the mouth. 11. The insensibility of the pituitary membrane of the nose. 12. The collapse, stiffness, and wrinkling of the lips. 13. The hollowness of the temples, and thinness and contraction of the nose.

Most of these signs singly have been shown to be fallacious; and none of them, except the last, are to be depended on with implicit confidence. Dr Davis recommends the following mode of procedure. "As soon as the evident signs of life cease, let us place the body in a warm or dry bed, give a proper temperature to the air of the apartment, and employ every means for restoring it to life. If we judge, from the nature of the disease which preceded the death, that these means are useless, we may content ourselves with keeping the body, until its decomposition become manifest; but let us never abandon an unfortunate person, who, perhaps by perseverance in the proper means, may be restored to life: should he recover, he will be a living monument of unexpected resurrection, and of the unceasing efforts of humanity. If a person die of malignant fever, scurvy, internal inflammation, or any other disease which corrupts the fluids, soon after death the belly becomes black and swelled; black or livid spots appear on the limbs and back, the eyes become hollow and soft, and discharge a puriform fluid; the eyelids grow yellow; the mouth opens; because the lower jaw is relaxed; the skin gets soft, the muscles flaccid; and, lastly, the whole body exhales a putrid odor. All these phenomena united, constitute an infallible proof of real death."

The changes which the animal body undergoes in consequence of death, and during putrefaction, have been amply detailed and explained under Chemistry, chap. xix. sect. 2.

In treating of the general phenomena of life in the comparative chapter of this article, we made a few observations five pertinacity of organized beings. There is scarcely a more curious part of the physiology of death than the consideration of the degree of vitality that appears in various tribes of life. Some are killed by a slight blow on the nose or the neck; this is the case with the seal, the rat, the hare, and the rabbit. Others again retain life with great pertinacity. Among the mammalia, the cat is proverbial for being difficult to kill; the sloth has been known to live for above 40 days clinging to a pole, and entirely without food; and Dr Sparman affirms us, that the ratel, or honey weasel (Civettura melitava), is so hardy that it is almost impossible to kill it; the colonists and Hottentots both assert, says he, that it is almost impossible to kill this creature, without giving it a great number of violent blows on the nose; and it is remarkable that such a number of hounds as are able collectively to tear in pieces a lion of moderate size, are sometimes obliged to leave the ratel only apparently dead. Some fishes live for a long time after being removed from the water, and even after being gutted and cut in pieces, as the carp, the flounder, and the eel. It is among the reptiles, mollusca, and zoophytes, however, that we find the most remarkable instances of pertinacity of life. Referring the reader to the article Erpetology for these instances in reptiles, and to Helminthology for those in zoophytes, we shall here only mention two among mollusca. The sea marigold (Actinia calendula) is destroyed with such difficulty, that after drilling the holes of the rock from which they appear, with an iron instrument, they have been known to rise again in the same places, and become as numerous as before in the course of a few weeks. Snails whose remarkable suspended animation we have already recorded, may be crushed beneath the foot, and will yet survive, and repair the breaches in their shelly covering; nay, they are capable of passing the ordeal of boiling water, as we learn from the relation of a lady who, wanting some snail shells for a piece of grotto work, attempted to kill the animals by repeatedly pouring over them boiling water; but to her horror and astonishment, she observed them next day crawling about the edges of the vessel.

(N) The work of M. Brulier, sur l'Incertitude des Signes de la Mort, from which these remarks of Dr Davis appear chiefly to be taken, created so much alarm in France, that every body dreaded being buried alive. To combat these terrors, M. Louis, in 1752, published his Lettres sur la Certitude des Signes de la Mort; in which he has very happily, and we think successfully, refuted the arguments of Brulier, and has thereby relieved the minds of his readers from one of the most dreadful apprehensions that can appal us on this side the grave. Of Death, in which she had scalded them. It is in vain for us to attempt any explanation of these extraordinary phenomena. We must refer them to some principle in the animal economy which is at present unknown.

The remote causes of death have been, by Dr Ontyd, arranged under 12 general heads, to which he gives the name of classes. These we shall enumerate, with their principal subdivisions.

I. Death arising from the mechanism of the body. II. Death from the passions of the mind. 1. Exciting passions; 2. Depressing passions. III. Death from superabundance or deficiency of heat. 1. From superabundant heat; 2. From deficient heat. IV. Death from electricity. V. Death from noxious gases. 1. From hyperoxygenized gases; 2. From deoxygenized gases; 3. From peculiarly stimulating gases. VI. Death from poisons. 1. Animal poisons; 2. Vegetable poisons; 3. Mineral poisons. VII. Death from universal disease. 1. Fevers; 2. Febrile diseases (exanthemata).

These seven classes are supposed to produce death by the immediate extinction of the vital principle; the five following are supposed to effect this by suppressing the action of some vital organ, or by disordering the chain of the vital powers by destroying the action of some of the intermediate links.

VIII. Death from inflammations. 1. Inflammations of the head; 2. Of the breast; 3. Of the belly. IX. Death from fluxes. 1. Aloine fluxes; 2. Hemorrhages. X. Death from cachexies. 1. Ulcers; 2. Atrophies; 3. Debilities and privations. XI. Death from diseases of the nervous system. 1. Atony; 2. Spasm. XII. Death from diseases of the secretory organs. 1. From altered action; 2. From altered structure.

The manner in which these causes operate in terminating life, is thus stated by the same author.

The causes of the first class act by inducing too great a rigidity of the solids, and by rendering them insensible to stimuli; the necessary effects of the continued action of the powers of life. In death from causes of the second class, the person dies in consequence of apoplexy, syncope, or suffocation, the brain, the heart, or the lungs being overwhelmed by accumulated blood. The causes of the third class act in a similar manner with those of the second; that of the fourth by suddenly extinguishing the vital principle; those of the fifth always act by inducing suffocation. The causes of the sixth class act in four ways: 1. By abolishing the vital principle by the violence of their stimulus; 2. By destroying the action of the brain, the heart, or the lungs; 3. By producing mortification of the intestinal canal; 4. By secretly and insensibly destroying life. Those of the seventh class act in six ways: 1. and 2. As in the last; 3. By local inflammation; 4. By mortification of some vital organ; 5. By a change in the organic structure of the intestinal canal inducing a colligative diarrhea; 6. By colligative sweats wasting the body.

The causes of the eighth class act in four ways: 1. By inducing violent convulsions; 2. As in the two last; 3. By suppressing the action of some vital function from the violence of the inflammation; 4. By mortification. The ninth class may act in five modes: 1. By spasm; 2. By fatal syncope; 3. By impeded action of some vital organ; 4. By mortification or sphacelus; 5. By wasting the strength in fruitless exertions. The tenth class may act in no less than nine ways: 1. By the consumption of some vital organ, or destroying the tone of the whole body; 2. By the violence of the noxious stimulus; 3. By suffocation; 4. By apoplexy; 5. By syncope; 6. By hemorrhage; 7. By colligative diarrhea; 8. By mortification of some organ; 9. By malignant fever from absorbed ichorous matter. The causes of the eleventh class act only in two ways: 1. By violent spasm; 2. By apoplexy. Those of the twelfth class produce death in three modes: 1. By the flow effects of the noxious stimulus; 2. By the continually stimulating noxious power alone, or by this and the continual wasting of the blood, to form some peculiar secretion; 3. By impeding or destroying the function of a vital organ.

Many of these modes of operation are very ill defined, and they may all be reduced to about eight or ten, or perhaps even fewer.

Death has been defined the separation of the soul from the body; the extinction of the vital principle; the extinction of the faculty of answering a stimulus, &c., &c.; Johnson. Perhaps we cannot describe it better than by calling it Ontyd, the irrecoverable cessation of all the bodily functions. By this character we distinguish it from suspended animation and lethargy, in which some of the functions continue; while we acknowledge the survival of the immaterial part of our frame.

It has been the general opinion among philosophers, both of ancient and modern times, that death produces only a change of the elements or principles of the organized body; and does not affect the annihilation of any part. Modern chemistry has fully confirmed this opinion, and has shown that by putrefaction the body is dissolved into a few earthly, saline, and gaseous products, all capable of entering into new combinations, and thus constituting a part of future bodies. See Chemistry, No. 2572, and Man, No. 44.

Of all the writers on the nature and phenomena of death, with whom we are acquainted, none has treated the subject with such accuracy and philosophic method, as Bichat. With a summary of some of the leading principles of this able physiologist we shall close the present chapter, and thus terminate our physiological enquiries.

We have already mentioned Bichat's division of life into animal and organic; see No. 49. Proceeding on Bichat's principle of this division, he conceives that the two lives terminate in different ways, and that one often terminates while the other remains active. In the natural death that happens from old age, the animal life gradually ceases in the order we have described, No. 367, while the organic life remains. The same happens in those cases of violent death where life first ceases in the brain, this organ being the centre of animal life. In other cases of violent or accidental death, the organic life first ceases in its central organs, the heart or the lungs; but in these cases, the animal life also is speedily suppressed. | I. SENSATION | VII. SECRETION | VIII. REPRODUCTION | |--------------|---------------|-------------------| | **1. ORGANS.** | Salivary glands, Liver, Pancreas, Kidneys, Testes, Mucous glands, Membranes, &c., Miliary glands, Brain? | Penis, Testicles, Vesicule Seminalis, Prostate gland, Spermatic vessels in Man; Vagina, Vagina, Uterus, and the Mammae, in Woman. | | Lymphatic and fluids, Nervous | Tears, Mucus, Saliva, Gastric juice, Pancreatic juice, Bile, Lymph, Synovia, Fat, Marrow, Cerumen, Semen, Urine, Milk, Nervous fluid? | Semen, Mucous fluid, Prostatic fluid, Liquor amnii, and Milk. | | **3. PHENOMENA.** | Sensation, Action of the bodies on Man, Perception of the external objects. | Separation of fluids useful in the economy, and Expulsion of noxious or useless parts. | Copulation, Conception, Parturition, Lactation. | | Sensibility and Vision. | Various. | Generative power. | | **5. RELATIVE PREDOMINANCE.** | Most predominant in females; female sex, lic temperament, dairies, hysteric nervous affections, warm climates. | In middle age; various as to sex and temperament; in warm climates. | In youth; in those of a sanguine temperament, and lively imagination. | | **6. PRINCIPAL MORBID AFFECTIONS.** | Vertigo, Coma, Angina pectoris, Infancy.—Painful Want of feeling, Abdominal pains; Tinnitus aurium; Intolerant Dyspnea; Cataracts. | Increased secretion, Diminished secretion, Depraved secretion, Jaundice, Calculus, &c. | Priapismus, Satyriasis, Nymphomania, Menorrhagia, Amenorrhea, Impotence, Sterility. | ### TABLE OF THE MOST IMPORTANT CIRCUMSTANCES RESPECTING THE ORGANIC FUNCTIONS OF THE HUMAN BODY.

| I. SENSATION. | II. MOTION. | III. DIGESTION. | IV. ABSORPTION. | V. CIRCULATION. | VI. RESPIRATION. | VII. SECRETION. | VIII. REPRODUCTION. | |---------------|-------------|-----------------|-----------------|-----------------|-----------------|-----------------|-------------------| | **1. ORGANS.** | Muscles, Tendons, Bones, Cartilages, Ligaments, and Mucous Bags. | Salivary Glands, Mouth, Teeth, Gullet, Stomach and Intestines, Liver, Pancreas, and Spleen. | Lacteals, Lymphatics, Thoracic Duct, Mesenteric Glands, Lymphatic Glands, Skin. | Heart, Arteries, Veins, and Exhalants. | Nephrits, Lungs, Windpipe, Lungs, and Diaphragm. | Salivary Glands, Liver, Pancreas, Kidney, Tejus, Mucous Glands, Membranes, &c., Military Glands, Brain? | Penis, Testicles, Vesiculae Seminales, Prostate Gland, Spermatheca Vesicalis in Man; Vulva, vagina, Uterus, and the Mammary in Woman. | | **2. FLUIDS.** | Lymphatic and Gelatinous Fluids, Nervous Fluid? | Gelatinous Fluid, Synovia, Marrow, Lymph, and Blood. | Saliva, Gastric Juice, Pancreatic Juice, Bile, Mucus. | Chyle, Lymph, Serous Fluid. | Blood, Lymph, and Various Exhaled and Secreted Fluids. | Blood and Mucus. | Tears, Mucus, Saliva, Gastric Juice, Pancreatic Juice, Bile, Lymph, Synovia, Fat, Marrow, Cerumen, Semen, Urine, Milk, Nervous Fluid. | | **3. PHENOMENA.** | Sensation, Action of External Bodies on Man, and Perception of These Actions. | Contraction, Dilatation, Locomotion, Progression, Action of Man on External Objects. | Maturation, Deglutition, Digestion in the Stomach and Intestines. Mutual Action of Alimentary Substances and the Digestive Organs. | Imbibition, Action of the Lymphatic Vesicles and Glands on the Fluids, Separation of Noxious or Useless Matters, and Selection of Useful Substances. | Contraction, Dilatation, Pulmation, Exhalation, Nutrition—Mutual Action between the Blood and Circulating Fluids. | Purification of the Fluids, Renewal of Action, Animal Heat, Mutual Action between the Air and the Animal Fluids and Fluids. | Separation of Fluids Useful in the Economy, and Expulsion of Noxious or Useless Parts. | | **4. POWERS.** | Sensibility and Vital Resistance. | Irritability, Contradictility, and Vital Resistance. | Diffusion, Affirmation, and Vital Resistance. | Irritability, Contradictility, Affirmation, and Vital Resistance. | Elasticity, Irricability, Contradictility, Dilatability, and Vital Resistance. | As in Circulation. | Various. Generative Power. | | **5. RELATIVE PREDOMINANCE.** | Most Predominant in Infancy; Female Sex; Melancholic Temperament; Hypochondriac Affection, and Other Nervous Affections, and in Warm Climates. | In Manhood; in the Male Sex; in the Sanguine Temperament, and in Mountainous Countries. | In Infancy; in the Female Sex; in the Sanguine Temperament; in Warm Climates and Cold Weather. | In Childhood; in the Female Sex; in the Sanguine Temperament; in Warm Countries; and in Feverish and Inflammatory Affections. | In Childhood; in the Female Sex; in the Sanguine Temperament; in Warm Countries; and in Feverish and Inflammatory Affections. | Much as in Circulation. | In Middle Age; Various as to Sex and Temperament; in Warm Climates. | | **6. PRINCIPAL MORBID AFFECTIONS.** | Vertigo, Coma, Delirium, Insanity—Pain, Itching, Want of Feeling; Ague; Tetanus; Tinnitus Aurium, Deafness; Intolerance of Light, Dyspnea; Catarrh, Amenorrhea. | Spasm, Convulsion, Twitching, Paralysis. | Bulimia, Phlegm, Nasus, Flatus, Eructation, Rumination, Vomiting, Heartburn, Pyrosis, Amenorrhea. | Glandular Obstruction, Atony of Lymphatics. | General Fever, Palpitation, Plethora, Aniamtion, Debility, Syncope. | Yawning, Sighing, Sobbing, Hiccups, Sneezing, Coughing, Anxiety, Dyspnea, Savor, Asphyxia, Dumbness. | Increased Defecation, Diminished Defecation, Depraved Defecation, Jaundice, Calculus, &c. |

*Priapism, Satyriasis, Nymphomania, Menorrhagia, Amenorrhea, Impotence, Sterility.* It is to violent or accidental death that Bichat principally confines his disquisitions, and in order to determine with precision the phenomena that take place in the three species, he examines at great length the relations that subsist among the three functions of circulation, respiration and sensation, as they are affected by the death of the heart, the lungs, or the brain. He first considers those cases of sudden death that commence with the death of the heart; then those originating in the lungs; and lastly those originating in the brain. He shows how, one of these functions ceasing, the others successively stop; he points out the mechanism by which the death of all the parts follows that of the organ first affected; and he determines, according to his own principles, the nature of the several diseases by which the life of the heart, the lungs, or the brain, is extinguished.

We consider this as the most interesting part of his valuable work, and it well deserves the attentive perusal of every medical man. We regret that we cannot do more than extract from it the view given by the author of the successive phenomena produced by the influence which the death of each of the vital organs exerts on the general death of the body.

Whenever the heart ceases to act, says Bichat, general death comes on in the following manner. The action of the brain ceases for want of excitation; and from the same defect, the sensation, locomotion, and speech, which immediately depend on the general sensibility, are interrupted. Besides, for want of the excitation of part of the blood, the organs of these functions would cease to act, even though the brain were supposed capable of exerting on them its usual influence. The whole of the animal life, then, is suddenly arrested. The man, from the moment that his heart dies, ceases to exist with respect to surrounding objects.

The interruption of organic life, which has commenced through the circulation, operates at the same time through the respiration. The mechanical actions of the lungs no longer proceed when the brain ceases to act, since on this organ depends the action of the diaphragm and intercostal muscles. The chemical changes can no longer take place, when the heart can neither receive nor convey the materials necessary for their development. In short, general death continues to proceed in a gradual manner, by the interruption of secretion, exhalation, and nutrition. These are the effects produced when death is the consequence of a wound of the heart or large blood-vessels, a rupture of the heart, or similar accidents.

The series of phenomena that take place in death, as commencing in the lungs, is different according as the mechanical or the chemical action of these organs is first arrested. I. In the former case, as when death is produced by an extensive wound or laceration of the diaphragm, by the fracture of a great many ribs at the same time, &c., they proceed as follows: 1. Cessation of the mechanical action; 2. Cessation of the chemical phenomena, for want of the air which supported them; 3. Cessation of the brain's action for want of the red blood by which it was excited; 4. Interruption of animal life, of sensation, locomotion, and speech, from the loss of the exciting powers of the brain and the red blood on the organs of those functions; 5. Stoppage of the general circulation; 6. Stoppage of the circulation in the capillaries, of secretion, absorption, exhalation, for want of the excitation exerted on their organs by the red blood; 7. Cessation of digestion, for want of secretion, and of excitation of the digestive organs.

II. When the chemical action of the lungs is interrupted, as when an animal is confined in a vacuum; in cases of strangulation, suffocation, drowning, &c., the phenomena of death proceed in the following order: 1. Interruption of the chemical phenomena; 2. Consequent suspension of action in the brain; 3. Cessation of sensation, voluntary motion, voice, and the mechanical functions of respiration; 4. Stoppage of the heart's action, and of the general circulation; 5. Termination of the capillary circulation, of secretion, exhalation, absorption, and, by consequence, of digestion; 6. Cessation of animal heat, which, being the result of all the functions, must cease when all these are terminated.

The phenomena of general death commencing in the progress of brain come on in the following series: 1. Cessation of death commencing in the brain's action; 2. Sudden interruption of sensation and voluntary motion; 3. Simultaneous paralysis of the diaphragm, and intercostal muscles; 4. Interruption of the mechanical phenomena of respiration, and, by consequence of voice; 5. Cessation of the chemical phenomena; 6. Passage of the black blood into the system of red blood; 7. Impeded circulation, from the action of the black blood on the heart and arteries, and from the immobility of all the parts, especially the organs of the chest; 8. Death of the heart, and stoppage of the general circulation; 9. Simultaneous interruption of organic life, especially in the parts that are usually penetrated by red blood; 10. Abolition of animal heat.

We have now gone through the series of physiological enquiries, into which we proposed to enter in this article. In forming an estimate of the merit due to our labours, we request that our readers will consider the article as in a great measure supplemental to many that have preceded it in the course of the present work. It has been our principal object to fill up blanks and supply deficiencies, especially with respect to Comparative Physiology; and to form, with those preceding articles which have a reference to the animal economy, particularly Anatomy, Medicine, Midwifery, Chemistry, Man, one connected, if not uniform whole. The difficulty of the task we had undertaken will probably be admitted as some apology for the imperfect execution of it; while the variety and interesting nature of the subjects which we have treated, with the numerous references to the most respectable sources of information, will, we trust, render this article acceptable both to the general and the scientific reader. PHYSIOLOGY.

EXPLANATION OF PLATE CCCCXVIII.

Fig. 1. Exhibits a view of the exit from the head, and distribution in the chest, of the great sympathetic nerve, intended to illustrate the mutual relations between the head and the principal organs of the chest and belly.

A. The right parotid gland laid bare. B. The submaxillary gland. C, D, E. The digastric muscle, partly covered by the submaxillary gland. F. Part of the thyroid gland. G, G. The esophagus or gullet. H, H. The wind-pipe or trachea. II. III. IV. V. VI. VII. The bodies of the six lower vertebrae of the neck; VIII. IX. the two first vertebrae of the back. I, K, L. The heart, with part of the pericardium attached. P, P. The arch of the aorta, drawn aside. Q. The common trunk of the right subclavian (a) and right carotid (v) arteries. R. The vena cava from the superior parts; Q. That from below. S, T. The right lobe of the lungs. U, V. Part of the left lobe. W, X, Z. Muscular parts of the diaphragm. a. The first cervical or great ganglion, from which proceed, b. The trunk of the great sympathetic nerve, and c. The eighth pair of nerves, or par vagum. d. The lower cervical ganglion, opposite the fifth cervical vertebra. e. The upper thoracic ganglion, opposite the first vertebra of the back. f. The third dorsal ganglion, between the second and third rib.

g. The accessory nerve of Willis. h, i, k, l. Trunks of some of the cervical nerves. m. The cardiac plexus formed by branches from the sympathetic nerve. n, n. The par vagum running down to the diaphragm, through which it passes, unites with the intercostal, forms various ganglia, and gives branches to most of the abdominal viscera. o, o. The phrenic nerves distributed to the diaphragm.

Fig. 2. A section of the cuticle of the lilium chalcodonum, to show the lymphatic vessels, much magnified. Fig. 3. A similar magnified view in the onion. Fig. 4. Ditto in the pink. Fig. 5. Represents the atlantal extremity of the slow lemur (lemur tardigradus), to show the curious division of the subclavian artery. a. The subclavian artery, lying upon the subscapularis muscle. b. The division of the artery into equal-sized cylinders. c. The ulnar artery proceeding to divide in the usual manner.

Fig. 6. Represents the sacral extremities of the same animal, showing a similar division of the inguinal artery. a. The diaphragm. b. The descending aorta. c, c. The iliac arteries. d. The trunk of the inguinal artery, situated among the cylinders. e. The femoral artery under similar circumstances.

The annexed Table sufficiently explains itself.

INDEX.

A.

Absorbent system, discovery of, No. 41 Absorption, 26, 184 organs of, 185 by lacteals, &c. 186 by the veins, 187 by the skin, 188 of the lowest classes of animals, 190 of the ants, 191 theory of, 192 relations of, 203 Accidental colours, principal phenomena of, 100 Air, quantity of, received and emitted during respiration, 223 ascertained changes on, 226 volume of, sensibly diminished, 227 changes on, by the respiration of inferior animals, 232 by vegetation, 233 Alcmæon's opinions respecting man, 20 Anatomy, relation of, to physiology, 6 Anaxagoras's physiological opinions, 30 Animation, suspended, of some animals, 303

Appetite for food, No. 150 Aristotle's physiological opinions, 34 Arrangements in physiology, remarks on, 24 Arteries, action of, 202 distribution of, in the limbs of slow-moving animals, 214 Affiliation a chemical process, power of, limited, 262 Azote, doubts respecting its loss by respiration, 264 Barclay's ideas of vitality, principles of muscular motion, 73 Bats supposed to possess a fifth sense, 104 Bichat's physiological arrangement, 21, 49 division of life, 49 observations on death, 375 Bile, uses of, 273 Blood, how acted on by the vessels, 217 changes on by respiration, 235 Boerhaave's system, 42 Boerlock's explanation of the modern theory of respiration, 238

B.

Carbonic acid gas generated during respiration, 22 Cellular membrane, action of, 27 Chemists, physiology of, 5 Chemistry, relation of, to physiology, 17 Chyle, properties of, first formed in the pyloric portion of the stomach, 16 Chylification explained, 17 Clymification explained, 17 Circulation, ch. vii. 26, No. 5 discovery of, organs of, 19 proofs of, 13 in the human adult, 10 in the human fetus, 20 how carried on, 20 of the inferior animals, 20 of the mollusca, 20 in the vermes, 20 in crustacea, 20 in plants, 20

C.

Boerlock's defence of ditto, objections to Mr. Ellis's opinions, 24

Flying explained, No 143 Food first dissolved in the cardiac portion of the stomach, 164 comparative solubility of, 171 passage of, through the intestines, 180

Functions of living beings, 57

G. Galen's physiological opinions, 36 Gallop explained, 141 Galvanism an exciting cause of irritability, 118 Gastric juice, action of, 170 Generation, 311 essence of, 313 gemmiparous, 314 oviparous, 321 viviparous, 323 theories of, 330 relations of, 343

Girnanner's hypothesis respecting irritability, 126 Goodwin's opinions respecting life, 67 Gough's explanation of ventriloquism, 252

H. Haller, opinions and discoveries of, 46 opinion of the cause of irritability, 123 Harvey's merits discussed, 40 Hearing, sense of, 93 organs of, 94 comparative physiology of, 96 Heart, action of, 201 Heat not the sole cause of digestion, 173 animal, 246 Herophilus, opinions of, 35 Hibernation of mammalia, 356 of birds, 357 of reptiles, 358 of fishes, 359 of insects, 360 of man, 361 of plants, 364 phenomena of, 362 Hippocrates, physiology of, 32 Hoffman's physiological opinions, 44 Home's discoveries respecting the action of the stomach, 161 experiments on the use of the spleen, 276 Hufeland's idea of life, 68 Humboldt's idea of life, 69 hypothesis respecting irritability, 127 Hunter's, John, opinions on the life of the blood, 66

I. Impregnation, mode of, 318 Infiltration, explained, 15 Insects, want of circulation in, how supplied, 209 Integument, ch. xii. No 26 uses of, as defence, 290

Integuments useful by their hardness, No 291 by their external covering, 292 by their effluvia, 293 by their colour, 294 change of, 295

Irritability, ch. iii. No 26 general phenomena of, 111 definition of, 112 different acceptation of the term, 113 stimuli exciting, 114 causes of, opinions respecting, 121 chemical doctrines of, 125 laws of, 128

Kidneys, action of, 284

Leaping, nature and mechanism of, 138 Life, general idea of, 26, 51 effects of, 53 cause of, 64 duration of, 63 comparative pertinacity of, 371 Light, effect of, on the system, 102 absence of, supposed to favour obesity, 103 Liver, action of, 272 Locomotion, uses of, 148 Lymphatics, retrograde action of, 189

M. Magnification explained, 155 Mechanists, opinions of, 43

Motion, ch. iv. No 26, 57 organs of, 129 principles of, 130 progressive, of animals, 131 relations of, with sensation, 149 vegetable, 147

Muscles, motion of, assists circulation, 204

N. Nervous system, 76 fluid, theory of, 110 energy and exciting cause of irritability, 116 Nicholson's account of a ventriloquist, 251

Nutrition, ch. ix. No 26 nature of, 260 not performed by nerves, 263 of insects and zoophytes, 264

O. Observation a mean of improving physiology, 14 Odours, nature of, little understood, 91 Organized beings compared with inorganic matter, 52 Ornithorinchus, the connecting link between quadrupeds and birds, 168 Oxygen of the air diminished during respiration, 228

Respiration, relations of, with sensation, No 257 motion, 258 digestion, 259

Respirations, ordinary number of, in a minute, 224

Reverie, 354 Richerand's explanation of ventriloquism, 252

Rumination explained, 158 in man, cases of, Note G, page 490.

Running explained, 139 Rubby's ideas of vitality, 72

Secretion, ch. x. No 26 organs of, 267 kinds of, 268 matters furnished by, 269 modifications of, 270

Secretions, peculiar animal, 278 vegetable, 280

Sensation, ch. ii. No 26, 62 organs of, 76 laws of, 105 comparative physiology of, 107 theory of, 108 relations of, 149, 181, 212

Sensibility, necessity of, to organized beings, 74 of the animal body, 82

Sensitivity, 75 Sight, sense of, 97 organs of, 98 Sleep, necessity of, 344 phenomena of, 346 theory of, 347 of plants, 351 Sleep-walking, 353 Smelling, sense of, 89 organs of, 90 theory of, 91 comparative physiology of, 92

Snails, copulation of, 319 Somnambulism, 353 Sound, varieties of, 95 Speech, mechanism of, 250 Spleen, uses of, 274 carries off fluid from the stomach, 277 Stahl's system, 45 Standing on two feet explained, 133 on four feet, 134 Stomach, organic action of, 160 human, the link between carnivorous and phytivorous stomachs, divided into a cardiac and pyloric portion, curvature of, accounted for, 167

Swimming explained, 144 Systemic organs of circulation, 196

Tapping, sense of, organs of, variously affected, perfection of, modifications of, uses of,

Temperature, animal, equable, preservation of, 24 Torpor of animals, theory of, 36 Touch, sense of, use of, organs of, nature of,

Transformation, ch. xiii. No 27 of reptiles, 29 of insects, 29b accompanied by change of propensities, 30c confists in the evolution of parts, 30i

Trituration not the sole cause of digestion, 17 Trotting explained, 14

Van Helmont's physiological opinions, 3 Veins, valvular structure of, affinis circulation, 20 Ventriloquism, account of, how explained, 25 Vibration, a theory of sensation, 10 Vit infita, of Haller, 12 Vision, immediate feat of, probably the retina, 9 phenomena of, distinct requisites for, 10 Vital principle, opinions respecting, 65-7 existence of, denied by some, supposed to be divisible, 7

Vitality, degrees of, Voice peculiar to those animals that have lungs, human, amazing variety of, mechanism of, of brutes, of birds,

Volition, an exciting cause of irritability, 1

Walking on two feet explained, 1 four feet, 1 Whytt's arguments in support of nervous influence, as exciting irritability, 1

Zoological arrangement of physiology, PIA

PHYTOLACCA, Pokeweed, or American Nightshade, a genus of plants belonging to the decandra class. See Botany Index.