in anatomy. See ANATOMY, Part II. Sect. ii.

The motion of the muscles of animals has been thought a matter of such curiosity and importance, that an annual lecture upon it was founded by Dr Croone, one of the original members of the Royal Society at London. In consequence of this, the investigation of the subject hath exercised the pens of a great number of very learned and ingenious men; notwithstanding which it still remains involved in almost as much obscurity as ever. Many curious observations, however, have been made; and as far as the laws of dead mechanism can be applied to a living machine, the investigators have been successful; but still there has been a ne plus ultra, a certain barrier by which their investigations have been limited, which no person has hitherto been able to pass, and which it is very improbable ever will be passed. To give an account of all the different theories which have appeared on this subject is impossible; but in the year 1788 a lecture on the subject was delivered by Dr Blane, F. R. S. of which, as it seems to contain the substance of all that can be laid upon the subject, we shall here give the following abridgement.

The Doctor considers as muscles not only those large masses of flesh which compose so great a part of the bulk of the body, but likewise all the minuter organs subservient to circulation, nutrition, and secretion; since not only the heart itself, but the whole vascular system and the intestines, owe their action to certain powers of irritability and contractility peculiar to muscular fibres.

The first and most obvious considerations with respect to the muscles is the regular organization of their fibres in a parallel direction. In this they are distinguished from every other matter of a fibrous structure, whether vegetable or mineral, by a certain degree of moisture, tenacity, and elasticity, entirely peculiar to themselves.

The fibres of the muscles visible to the naked eye are composed of others discoverable by glasses, and these others of fibres still smaller; neither hath any person been able to discover the ultimately fine fibres of a muscle, which are not composed of others. Some have indeed imagined that they have been able to do this, but their observations have been found fallacious; and it is now universally allowed that the fibres are divisible beyond what the best assisted sight can trace, and that they are to all appearance uniform. In this regular and fibrous organization they resemble the crystals of salts, many of which are found composed of fibres more and more fine, and which, like those of the muscles, can never be ultimately traced.

The doctor next touches a little upon the vis inertiae of matter; and, contrary to the generally received opinion of modern philosophers, considers matter as an active substance. What is called the vis inertiae, he thinks, "is not a resistance of change from rest to motion, or from motion to rest, but a resistance to acceleration or retardation, or to change of direction." The activity of matter is further proved by the attractions and repulsions which take place universally among its parts; and every instance of motion within the cognizance of our senses, may be referred, either in itself or its cause, to some mode of attraction or repulsion. These may both be considered as one principle, being both expressive of that state of activity originally inherent in matter; and because any two particles, having affinity with each other, must either attract or repel, according to their distance, their common temperature, and other circumstances; and it is so universal an agent in nature, that some modern philosophers have made it absorb, as it were, every other power and property in matter. It is evident, however, whether this hypothesis be just or not, that the cause of muscular motion cannot be referred to mechanism, which is itself only a secondary principle. Some have had recourse to a fluid conveyed into the fibres of muscles, by which they were swelled, and thereby shortened. One of the most plausible of these hypotheses supposes this fluid to be the blood; but this is plainly a petitio principii; for in order to set the blood in motion, muscular motion is necessary. Other fluids have been supposed to have this effect; but even the existence of these has not been proved, and indeed the most solid objections might be brought against all the theories that have hitherto been invented.

Our author having now established it as a maxim, that the primary properties of matter are attraction and repulsion, and that mechanism is only a secondary property, he next considers muscular motion as referable to an original law of animated matter, whereby its particles are endowed with an attractive power, for which no cause can be assigned any more than for gravitation, cohesion, or chemical affinity. If the shortening of a muscular fibre depends on this increased power of attraction between its particles, the effect will be to add to the power of cohesion in the fibre; and to determine this the Doctor made the following experiment: Having taken the flexor muscle of the thumb of a man newly dead while yet warm and flexible, ible, he appended a weight to it, continually augmenting it until the muscle broke; and this he found was done when 26 pounds had been added: yet a living man of the same apparent strength and age could with ease lift a weight of 38 pounds by the exertions of the same muscle. "It is farther in proof of this fact (adds he), that in the case of a violent strain from muscular contraction in the living body, it is the tendon that gives way; whereas we have seen that in the dead body the muscle is the weaker of the two. It is also well known, that in cases of our exertion the muscular fibres themselves do not give way, though the strongest tendons, such as the tendo Achillis, and even bones, such as the knee-pan, are broken by their living force, which in such instances must be many times greater than the strength of the dead fibres. There is a case related in the Philosophical Transactions by Mr Amyand, wherein the os humeri was broken by an exertion of the muscles. Every one has heard of fractures happening from very slight accidents. These occur most probably from a jerk of the muscles concurring with the external violence. The sensible increase of hardness in a muscle, when in a state of contraction, may also be considered as a proof of an increased attraction of its particles to each other at that time."

The Doctor next considers whether or not a muscle, when in a state of contraction, undergoes any change of density. "Every homogeneous body (says he) possesses a certain degree of density, determined by the distance of its integrant particles. The most common means in nature by which the density of such bodies is altered are heat and cold; the one universally producing expansion, the other condensation. Whether mechanical force has the same effects, is a point in natural philosophy not so well ascertained; for though tension and collision produce in solid elastic bodies a change of figure, which they immediately resume when the force is withdrawn, it has not been inquired whether in such cases a change of density takes place while the body is in a state of elongation or compression. Two elastic balls in the act of collision undergo a momentary change of figure, so that there must be an approximation of particles in the direction in which they are flattened; and in the elongation of an elastic cord by tension there must be an increased distance of the particles in one direction: but while these changes take place in one dimension of the respective bodies, they may be compensated by contrary changes in the other dimensions, so that the levered bodies may preserve, upon the whole, the same solid contents. In order to ascertain this in the case of tension, which is the only case bearing analogy to muscular motion, I made the following experiment: I took a piece of the elastic gum or caoutchouc, three inches square, and about the eighth of an inch in thickness; I procured a piece of sheet tin three inches broad and about five inches long, cut into sharp teeth at each end. The gum was first weighed in air, and found to be 380.25 grains. It was then weighed in water along with the tin, to which it was loosely attached, and the weight of both was then 758.75 grains. The gum was then stretched upon the tin by means of the teeth at each end to a surface of about five inches square, the tin being bent so as to leave a free space between it and the gum, in order that when immersed in water no air-bubbles might be entangled. In this situation, the weight of both in water was found to be 746.75 grains. Here was a difference of 12 grains, which could be owing only to a diminution of specific gravity; and in order to be sure that there was no fallacy nor inaccuracy in the experiment, the gum was immediately disengaged from one end of the tin so as to allow it to shrink; and being again weighed in this state in the water, it was found to have recovered exactly its former weight."

From this very remarkable experiment, the Doctor argues to what may probably happen in the contraction of the muscles. "This point (he says) cannot be decided but by an experimental examination. It might be determined whether a muscle occupies most space when relaxed or when contracted, by finding its specific gravity in each of those states by means of the hydrostatic balance. But this would be found extremely difficult; for the state of contraction is very transitory, and the motion itself would produce such a disturbance as would render the result unsatisfactory. As there is this obstacle to an experiment on a living muscle, it occurred to me that it might be performed on the muscles of a fish which had undergone the operation of crimping, as it is called; for in consequence of dividing the muscles, by cutting them when alive, they undergo a contraction which continues after death; and upon comparing, by the hydrostatic balance, portions of muscle which had been crimped with those of the opposite side of the same fish which had on purpose been saved from this operation, it did not appear that there was any difference in the specific gravity. Two trials were made; one with the matter of muscles of a skate, the other with the sides of a large trout."

To determine whether the contraction or relaxation of a living muscle made any alteration in its density, our author took one half of a living eel, and put it into a glass flask, of which the mouth was afterwards fused by a blow pipe, and drawn out like the stem of a thermometer. The flask and tube being then filled with water, our author observed, with great attention, whether the convulsive agonies of the creature would make the fluid rise or fall; but it did neither. The tail part of the eel was made use of in this experiment, that there might be no deception from the other, which contained the organs of respiration and the air-bladder. In one of his trials, the tail portions of two eels were introduced into the flask; but though they were frequently both in convulsions at once, not the least motion of the fluid in the tube could be perceived. On this occasion also the Doctor made some experiments to decide the question, Whether the mere circumstance of life made any alteration in the gravity of bodies? His first trials were with animals of warm blood inclosed in oil-skin and close tin-vessels; but not being satisfied with the accuracy of these, from the difficulty of cutting off all communication with the external air, he inclosed live eels in flasks; and having sealed them hermetically, he found that the weight of them when alive and dead was the very same.

The result of all our author's experiments is, that "the contraction of a muscle produces no change in its density, and that animal life differs from inanimate matter in this respect, as well as in most of its other properties and laws. One purpose in nature for muscles always Muscle always preserving the same density may be, that as some of them act in confined cavities, inconveniences might arise from their occupying more space at one time than at another. In the extremities of crustaceous animals, for instance, which are filled with muscles, a change of density would be apt to burst them.

Another circumstance in which the contractions of muscles differ from simple elasticity is, that the former, however frequent and violent, does not produce any heat, as collision and tension are known to do. This may admit of some cavil with regard to animals of warm blood; for one of the theories with regard to animal heat is, that it arises from the perpetual vibration of muscular fibres, particularly those of the vascular system; but this will not hold with respect to animals of cold blood, in which the actions of life are equally vigorous. The principal phenomena, therefore, of muscular motion are, the shortening of the fibres, the lateral swell, the increase of cohesion and hardness, and the unchanged density and temperature. It would appear from the two last circumstances, that the intimate motions of the particles in relation to one another must be different from what take place in the several instances of contraction and expansion of dead bodies. In the expansion arising from the action of heat and the contraction from cold, the change of density shows, that in the one case the ultimate particles must recede from each other, and in the other they must approach. The same may be said of elasticity. But as there is no alteration in the density of a muscle in passing from relaxation to contraction, this change cannot consist in the approximation of the integrant parts of the fibres, but must depend on some other circumstances in the intimate dispositions of the particles. In attempting to conceive in what this consists, the following explanation may be offered. It is probable that the regular structure of solid bodies depends on the polarity and shape of their integrant parts. Now all bodies, except such as are spherical, must have a long and a short axis; and let us imagine the fibres of muscles to be composed of spheroidal particles, we may then conceive relaxation to consist in their being disposed with their long axis in the line of their fibres, and contraction to consist in their short axis being disposed more or less in that direction. This will not only account for the decuration and uniform density, but for the lateral swell, and also for the increased hardness and cohesion; for though the particles do not approach or recede, as in bodies simply elastic, yet their power of attraction will be increased by their centres being brought nearer, and by being applied to each other by more oblate surfaces. This hypothesis accords with what has been before proved concerning the unchangeable density, for what is lost in one dimension is gained in another; and the cause for there being no increase in temperature depends probably on the same circumstance by which the density is preserved unaltered."

Thus far the Doctor has proceeded upon a plan, which may become plausible by means of an hypothesis at least; but in the prosecution of his subject he is involved in the same difficulty which has proved too hard for every other person, and which he, indeed, does not attempt to solve. This is the action of stimuli, by which the muscles are exerted to contraction, and upon which all the phenomena of life depends, and which indeed is the thing that particularly ought to be explained; but of this our author is forced to confess his entire ignorance, and to content himself with enumerating the stimuli of which he cannot explain the action. Stimuli then, according to him, are divided into internal and external. An example of the former kind is the circulation of the blood, which is kept up by an exciting influence of the blood upon the heart and vessels which contain and impel it. The earliest perceptible instance of muscular motion is the beating of the heart, as it is seen in the first rudiments of the embryo in an egg, and called the pundum falens. There seems to be established by nature a certain habitu of action between the vessels and their fluids; for if a fluid even more mild than the blood, such as milk, be injected into the circulation, it will produce great disturbance; and if the blood, by being deprived of the influence of respirable air, becomes destitute of a certain property which it would naturally acquire in the act of respiration, it does not prove a stimulus to the heart.

In like manner, all the containing parts are accommodated to the nature of their respective contents.—The intestines are so calculated as to have proper motions excited in them by the aliment and the secretions which are mixed with it; and there are bodies which, though perfectly mild, such as alimentary substances of difficult digestion, yet excite more violent commotions in the stomach than other substances which are very acrimonious. The various effects of poisons in different parts of the body may also be mentioned as an illustration of the peculiar susceptibility of the several organs of the body. The poison of a viper, for instance, is perfectly innocent, not only in the receptacles of the animal which produces it, but may be taken into the stomach of any animal without the least bad effect, and only exerts its deleterious power when brought in contact with a wounded part. Some vegetable poisons, on the contrary, such as that of laurel water, prove deadly when taken into the mouth, or applied to any part of the alimentary canal, but are innocent when injected into the veins. It may be remarked also, that the receptacles of the several secreting fluids, such as the gall-bladder and bladder of urine, are so adapted to their natural contents, by a due measure of irritability, as to bear their accumulation to a certain degree, and then to expel them. We have here also a proof that irritability is not in proportion to sensibility; for both these receptacles are extremely sensible to pain and irritation from extraneous acrimony, though so moderately sensible to the acrimony of their natural contents. This disposition in the several organs to perform their natural functions, in consequence of the stimulus of the respective fluids they contain, has aptly enough been called the natural perception of their organs.

Our author now considering that the internal organs are calculated to perform their functions in consequence of certain stimuli, concludes the application of chemical and mechanical stimuli is not a mode of experiment likely to produce useful knowledge; and hence, he thinks, we may suggest the most likely means of restoring lost irritability and action to the vital functions, when suspended by suffocation, strangulation, or immersion. In these cases, he says, that all other means are far inferior to that of inflating the lungs, with atmospheric air, and stroking and pressing the ribs in such a manner as to imitate natural respiration. The only other thing which he supposes to be any way useful, is the application of heat to such as have been immersed in cold water; but of cool air to those who have suffered from mephitic vapours.

The Doctor having then considered some other parts of the animal economy, enters into an investigation of the analogy between motion and sensation. "This analogy (says he) is the more exact, that the nerves seem to be the instruments of both; for not only the organs of sensation and voluntary motion, but those of involuntary motion, are supplied with nerves, and dependant upon them; for if the influence of the nerves leading to the heart or intellects is interrupted by cutting, ligature, or palsy, the function of these parts is thereby destroyed. Thus, as there is a peculiar sensibility belonging to the several senses, so is there a peculiar irritability belonging to the several organs of motion. The intention of nature, therefore, in distinguishing nerves to every muscular organ, was probably in order to constitute those peculiar perceptions on which the various vital and natural functions depend. But I give this only as a conjecture; and though the nervous influence may thus modify irritability, there is reason to think that it does not below it."

Our author controverts the principle which has been held by some very able physiologists, that all muscular irritability depends upon a sentient principle. "There have been several instances (says he) of the production of fetuses without the brain; and a principal fact in support of this opinion is, the existence of animals without brain or nerves. That there are such, was, I believe, first observed by Haller, and has been confirmed by Mr Hunter; who maintains farther, that the stomach is a centre or seat of life more essential to it than the brain. That the stomach should be an organ of so much consequence, seems natural enough from the importance of its function, which is that of assimilation; and life can be more immediately and completely extinguished by an injury to it, such as a blow, than by the same violence to any other part of the body. It is also well known, that the muscular fibres of animals endowed with a nervous system, will retain their irritability for some time after their separation from the brain and nerves.—It is evident likewise, from the phenomena of vegetation, that irritability may exist in nature without sensation, consciousness, or any suspicion of the existence of a nervous system. In favour of this opinion, it is further observable, that those animals which are destitute of brain and nerves are of the class of vermes, the most simple in nature, having only one function, viz. that of assimilation; and therefore not requiring that variety of action, and those perceptions which are peculiar to more complex animals. Lastly, the state of an egg before incubation, and the condition of those animals which become torpid from cold, and afterwards revive, afford facts which favour this opinion; as they show that there is a certain principle of self-preservation, independent not only of the operation of the nervous system, but even of the circulation; for in this quiescent state, those portions of animal matter are preserved for a great length of time from that corruption to which they would otherwise be liable, and their fluids are prevented from freezing in a degree of cold which would congeal them, were they destitute of every principle of life."

In the course of his reasoning, our author considers the nervous system not only as a mere appendage to life, but as tending to impede its operation, and shorten its existence. "Simple life (adds he) will not only survive sensation, but will survive it longer, if the animal is killed by destroying the nervous system, than if it had been destroyed by hemorrhagy, suffocation, or other violence. If a fish, immediately upon being taken out of the water, be stunned by a violent blow on the head, or by having the head crushed, the irritability and sweetness of the muscles will be preserved much longer than if it had been allowed to die with the organs of sense entire. This is so well known to fishermen, that they put it in practice in order to make them longer susceptible of the operation called crimping. A salmon is one of the fishes least tenacious of life, inasmuch that it will lose all signs of life in less than half an hour after it is taken out of the water, if suffered to die without any farther injury; but if, immediately after being caught, it receives a violent blow on the head, the muscles will show visible irritability for more than 12 hours afterwards."

To the same purpose, our author observes, that in warm-blooded animals an excessive exertion of voluntary motion immediately before death, prevents the muscles from being rigid when cold, and renders them more prone to putrefaction. Thus, if an ox is killed immediately after being overdriven, the carcass will not become stiff when it grows cold, nor is it capable of being preserved by means of salt. In confirmation of the same hypothesis also, our author observes, that in some disorders of the brain, such as hydrocephalus, and apoplectic palsy, in which the functions of the brain are suspended, the office of digestion is sometimes better performed than in health.

From all this our author concludes, along with Mr Hunter, that the exercise of sensation is injurious to life, and that a sort of fatigue is induced by this as well as by voluntary motion; "so that all that intercourse carried on through the nerves, whether towards the brain in the case of sensation, or from the brain in acts of volition, tends to wear out the animal powers. And, as intense and long-continued thought, though not terminating in any outward action, tends also to produce an inability for farther exertions, it would appear that the brain or sensorium is more particularly the organ which is subject to that species of sufferance called fatigue. From these facts we perceive the necessity of sleep, which consists in a temporary suspension of sensation, volition, and thought, and is a resource of nature, whereby the powers of life recover themselves after satiety and fatigue, which are provided as guards to warn us when nature is in danger of being strained, either by repulsion or over-exertion; and it is evident that such barriers were absolutely necessary, in order to set bounds to operations which are only occasionally requisite, and which would otherwise depend on the caprice of the will. The exercise of sensation and voluntary motion in a moderate degree is conformable to the intention of nature, and therefore salutary; and it is only when they are excessive cessive that they tend to wear out the powers of life, and more especially if these are not duly recruited by sleep. It follows, from the same principle, that when life is threatened by certain diseases, of which the chief symptom is irritation, any means by which sensation, whether natural or morbid, and muscular motion, whether voluntary or involuntary, convulsive or spasmodic, can be soothed or suspended, will prove salutary, by allowing the powers of life to rally as it were, and to recover themselves. In this consists the operation of narcotic medicines, such as opium;—which, in complaints both of a general and local nature, proves useful, not merely as a palliative by the removal of temporary pain or spasms, or by procuring sleep, but as a principal instrument of recovery, by allowing the powers of life to exert their natural action, in consequence of the removal of irritation."

In treating this subject, the Doctor considers the effects of opium as affecting simple or sensitive life; and to determine this, he made the following experiments: Having made a solution of opium in water, he put into one portion of it some sound living eels, and others with their heads bruised; and in a number of trials it was found that the sound eels generally died much sooner than the bruised ones. This, however was the case only when the solution was of a certain degree of strength, such as half a grain of opium at least to an ounce of water; for when only about half this strength, the sound eels lived much longer, the time being then protracted to that in which the bruised eels would have died merely in consequence of their injury; but it must be observed, that even the wounded eels died considerably sooner than when put into plain water.

From all this, our author concludes, that "the great masses of muscle in the trunk and extremities of the body are the instruments of the mind in acting upon external bodies; and we may therefore rank in the list of stimuli the nervous power by which the will and the passions excite external motions. This is a function sufficiently important for the nerves, without admitting them as the principle on which irritability depends."

Having disclaimed all inquiry into the connection between muscular motion and volition, the Doctor proceeds to consider the effects of the different passions upon the muscles. Though these are distinct from the motions directly produced by the will, yet he considers them among those arising from consciousness; "for there are emotions of the heart which have visible and powerful effects upon the mind and vascular system, which are organs entirely out of the reach of the will. Not to mention the well-known effects of grief, fear, and joy, which affect the whole circulation, there are certain passions and sentiments which produce partial and local effects. These are established by nature, either to answer some important purpose in nature, as in the case of the congelation of the fluids in the parts of generation in consequence of the venereal appetite, or to serve as natural expressions, as in the case of blushing or weeping. One of the most striking effects of the passions upon muscular action, is the influence they have upon the strength or mechanical force of the voluntary muscles. Fear produces debility almost amounting to palsy. Courage and order of mind, on the contrary, add to the natural strength. When the mind is agitated by some interesting object, and calls upon the body for an extraordinary exertion to effect its end, the muscles are thereby enabled, as it were by magic, to perform acts of strength of which they would be entirely incapable in cold-blood. In circumstances of danger, for instance, where life or honour are at stake, exertions are made for overcoming mechanical resistance which seem incredible, and would be impossible, were not the mind in a sort of phrenzy; and it is truly admirable in the economy of nature, that an idea in the mind should thus in a moment augment the powers of motion, and inspire additional resources of strength adequate to the occasional calls of life. The great increase of strength in maniacs is also referable to the passions of the mind. These considerations would almost lead us to doubt whether or not the accounts we have of the great feats of strength ascribed to individuals in the heroic ages be fabulous or not. It is also worthy of remark, that in great and lasting exertions of strength to which men are impelled by active and generous affections, fatigue is not induced in the same proportion by many degrees as by the same quantity of muscular action in the cool and deliberate actions of common life."

Having thus discussed the subject of internal stimuli, our author next proceeds to take notice of the second class, viz. such as are external. These are either immediate or remote, viz. such as are excited by mechanical means, or by acrimony directly and artificially applied to a muscular fibre; or such as occur in the instances of sympathy, and in the case of those instincts which nature has instituted for the purpose of self-preservation in brutes, and in the early part of human life. "There are certain habitudes (says he) between outward stimuli and the moving powers whereby natural propensities are constituted, equally necessary to the support of life as the internal functions. Thus, in a new born animal, the first contact of the external air excites the act of respiration, and the contact of the nipple excites the act of sucking; both of which actions are absolutely necessary to the maintenance of life, and require the nice co-operation of a great number of muscles prior to all experience. Actions of this kind are called instinctive; but though different from those of voluntary motion, they nevertheless run into one another; so that what was at first merely instinctive, may afterwards become a matter of deliberate choice. The same muscles are the instruments of both; and they differ from the muscles obeying the internal stimuli, such as the heart, in being liable to fatigue, and thereby concurring with the exercise of sensation and of thought, in rendering sleep necessary. There are no muscles except those of respiration, of which the constant action is necessary to life, and which are void of consciousness in their ordinary exercise, but which are yet in some measure under the control of the will. The principal end answered by this power of the will over the muscles of respiration in man, is to form and regulate the voice. But, though instinctive motions are in some cases convertible into those which are voluntary, they ought by no means to be confounded together; for even those animals which are destitute of brain and nerves, are capable of actions evidently of the instinctive kind." Muscle.

A leech, for instance, being brought into contact with a living animal, is impelled by an instinct of its nature to fall upon it, and suck its blood. There is something very similar to this even in vegetables, as in the case of tendrils and creeping plants being stimulated by the contact of other bodies to cling round them in a particular direction.

Besides these observations on the inferior animals, our author brings some experiments to show, that instinctive actions, even in animals furnished with a brain and nerves, do not depend on sensation. Having divided the spinal marrow of a live kitten a few days old, he irritated the hind-paws by touching them with a hot wire. By this the muscles of the posterior extremities were thrown into contractions, so as to produce the motion of shrinking from the injury; and the same effects were observed in another kitten of which the head was entirely separated from the body. In repeating this experiment he found, that when the spinal marrow was cut through between the lumbar vertebrae and os sacrum, the posterior extremities lost their irritability, but the tail retained it. Even the head retained its irritability after it was cut off; as appeared by touching the ears with a hot wire, or by pricking them; "and (says our author) as the extremities are also irritable, it will not be said that consciousness and sensation exist in two separated portions of the body."

The effects of habit are then considered; and the conclusion from the doctor's reasoning upon this subject is, that "there is a co-ordination, or pre-established harmony, as it were, between the faculties of animals and the laws of external matter, which is the foundation of all the instinctive habits of animals, as well as the rational conduct of man."

To the law of habit have been referred the effects of certain contagions, such as that of the small-pox, which do not produce their effect more than once in life. With respect to this he observes, "that upon whatever principle this property of the animal economy depends, it is an undoubted fact, that these morbid poisons, after exciting a certain degree of disturbance, and a certain series of diseased actions, no longer make any impression on the powers of life, otherwise there could be no such thing as recovery: for at the time in which a person begins to recover from the small-pox, the poison actually present in the circulating system is multiplied infinitely beyond what it was when it excited the disease. The constitution has therefore at that time, with respect to this acrimony, acquired an insensibility, or rather want of irritability; and this it preserves ever afterwards. This, however, holds only with regard to those morbid poisons which excite febrile affections, and seems to be a necessary provision of nature to guard against such noxious principles as are generated within the body itself."

Having lastly considered the effects of irritation upon the human body, the Doctor goes on to consider a very remarkable property of living muscles, viz. that of their being in a constant state of tension, more or less, independent of any temporary stimulus. This is evident from what happens when any muscle is cut; for then there is an immediate retraction of the separated parts; and that this is their natural state is farther proved by the spontaneous motion which takes place in consequence of the relaxation of an antagonist muscle, as when the mouth is drawn to one side in consequence of hemiplegia. Some degree of tension indeed is necessary for the performance of the natural motions of the muscles, whether voluntary or involuntary; and the vigour with which the several actions are performed depends on the degree of this tone.

This tone of the muscles is everywhere maintained by a certain counteracting mechanical power: the great muscles are kept on the stretch by the bones, the heart and vessels by the mass of fluids, and the intestines by the aliment taken in, and their other contents. Differences of various kinds may arise from the different degrees of this tension, and the vascular system is more apt to be affected by different degrees of tension than any other part of the body; and our author considers what is called a nervous habit as one of the effects of want of tension. He likewise attributes to the different degrees of tension, more than to anything else, the great difference of constitutions observable among mankind. He observes also, that the tension of the muscles is greatly affected by sympathy. "This (says he) is particularly observed in the blood-vessels and intestines; for a relaxation in these will produce a like affection in every other part of the animal system. With regard to the intestines, it may be mentioned among other proofs, that it is common for persons in a state of great weakness to be affected by syncope, and even instantaneous death, in the act of evacuating the bowels. It seems to be from a like cause that a temporary lowness is produced by an abscess being opened.

The Doctor concludes his subject with considering the muscles as mechanical powers. "As they constitute the strength of animals, it may be proper to consider the relation of their strength to their bulk, and the relation of the bulk and strength of the body to the density and cohesion of its own materials; and to the bulk, density, and cohesion of the external inanimate bodies with which it is conversant.

"It has been demonstrated by Galileo, that in similar unequal bodies, of a cylindrical or prismatic shape, such as the limbs of animals nearly are, the ratio of their efforts to break by their own weight is in the quadruplicate ratio of their lengths; but that the resistance they make to the same force is only in the triplicate ratio of their lengths. It follows from this, that in order to endow the limbs of animals with the same relative force, it is not only necessary that the bones should possess an increased proportion of thickness, in order to give an adequate increase of what may be called the dead strength; but a similar increase of living strength is necessary, by a suitable addition of muscular power, in order to keep pace with the increased size of the bones. Now we observe, in fact, that in the large-sized animals, such as the bull and the elephant, the thickness both of their bones and muscles becomes greater in proportion to the length of their limbs than in the smaller animals, and they are therefore of a less elegant form. But nature has not carried this so far as to compensate for the disadvantage arising from the increase of size; for the greater animals have not the same proportional strength in relation to their bulk, that the smaller animals have.

It has been computed that a flea can draw from Muscle, to 80 times its own weight, whereas a horse cannot with ease draw more than three times his own weight. This disproportion between size and strength is very observable in different individuals of the human species; for tall men are not muscular, even in the simple proportion of their stature."

Our author now proceeds to assign some reasons why the stature of mankind in general is not larger than we see it. Some observations upon this subject are made under the article Giant, where it is attempted to show, that by increasing the proportional strength of the materials, the size of the human body might have been augmented in any proportion. To this, however, the Doctor replies, that "had the bones been harder, they would not have been calculated for the common duration of life, the effect of which being to increase their hardness and dryness, they must be endowed originally with a certain degree of softness and succulence; and, with regard to muscles, a degree of hardness much greater than they possess would have been incompatible with their contractility." But this reasoning does not seem to be conclusive. The bones of a lion are said to be much harder than those of any other animal; yet we do not find that these creatures are liable to any kind of disease in consequence of this superior hardness. Neither is any inconvenient degree of hardness in the muscles a necessary consequence of their increased strength; for silk, though equally soft and flexible, may much more so than hemp or flax, is nevertheless much stronger; and we cannot by any means doubt, that if men had formerly been of a larger stature than they are at present, the materials of their bones and muscles might have been proportionably stronger, without the least injury or impediment to any of the operations of life.

When we consider the manner in which the muscles act upon the bones into which they are inserted, we may be apt to think that nature has been very prodigal of mechanical power; for, considering the bones as levers, the muscles act upon them at a very great disadvantage, being always inserted much nearer the fulcrum than the weight to be raised. Thus the two muscles of the arm, named biceps and brachii internus, in order to support in the hand a weight of one pound with the fore-arm at right angles to the humerus, must exert a power equal to ten pounds. Another circumstance also which tends to waste the power, is the obliquity with which they are inserted into their bones; so that the greater part of the force is expended in pressing one bone against another at the articulation, and only a small part of it in making the flexures and extensions. These disadvantages, however, are compensated by a number of conveniences which could not have been obtained on any other plan. We must distinguish between those actions which consist in pressure and those which depend on percussion; for as the momentum of this last depends on velocity, it is evident that there must be a great advantage from the insertion of the tendon being near the centre of motion, as greater velocity with less expense of contraction will thus be communicated to the extremity. The muscles, for instance, which are attached to the olecranon, in performing those actions with the hand which require rubbing, act with a disadvantage exactly proportional to the inequality of the distance from their insertion to the joint of the elbow, and that from the same joint to the hand. This is an act of pressure. But in the case of percussion, as in the action of using a hammer, there is an evident advantage resulting from the velocity communicated to the extremity; for, in order to have produced the same velocity, with the insertion at a greater distance from the centre of motion, a much greater degree of contraction would have been necessary; and our author shows that fatigue principally depends on a contraction of the muscles. "If any one (says he) will take the trouble of comparing the fatigue of the biceps muscle, in bearing a weight in the hand with the elbow joint bent to a right angle, with that of bearing the same weight for the same length of time with the joint at an acute angle, he will be sensible how much the degree of fatigue depends on the extent of contraction; and, by attending to the relative situation of muscular fibres, it will appear, that Nature, in distributing the fibres of muscles obliquely, has had it in view not only to increase their number, but to save contraction."

In considering the actions of the various muscles in producing the different actions of the body, we find scarce one produced that can be called direct. In some instances we find two muscles, or two sets of muscles, co-operating, so that the motion effected by them shall be in the diagonal of their direction. This is the case of the oblique muscles of the abdomen in some of their actions, and of the intercostal muscles in all theirs. Sometimes different portions of the same muscle combine in like manner to produce a similar effect; and in all the long muscles, however simple their origin and insertion may be, there is an internal obliquity of their fibres with regard to one another; for these do not run from end to end, but there are parts of the tendon running into the belly of the muscle, so as to divide it into penniform and rhomboidal portions. This distribution of the fibres takes off from the length; but as it takes place in those cases where the origin and insertion are at a considerable distance, this can be afforded; and this, as well as the waste of power, in consequence of oblique action, is more than compensated by the increased strength from the fibres being multiplied; for, in consequence of this structure, there is an extent of tendon afforded sufficient for the insertion of a greater number of fleshy fibres.

The Doctor illustrates this principle in the mechanism of muscular action from the example of fish; a species of animals which exert greater muscular powers than any others. "The muscles of most fish (says he) consist of regular series of oblique short fibres, forming those strata which every one must have observed in their muscular substance. Their motions are more simple and limited than those of land-animals, but much more vigorous; for a fish in the sea has to make its way through a medium about 1000 times more dense than air, and with more rapidity than those which inhabit the land. Nature, therefore, instead of giving them muscles whose fibres would run straight from one end of their body to the other, has multiplied their numbers, by distributing them into short and oblique portions. I have seen the sword of a sword-fish sticking in a plank, which it had penetrated from side to side; and when it is considered that Muscle. that the animal was then moving through so dense a medium, and in the same direction with the ship, we must form a high conception of its muscular power."

Lastly, our author gives a mathematical demonstration, that by the obliquity of the muscles a very considerable quantity of contraction is saved, and consequently a proportional degree of fatigue prevented.

"Let the line AB (says he) in the annexed diagram, represent a moveable bone, and the line CD a fixed bone parallel to it. Let FE, perpendicular to these lines, represent a muscle acting in its own direction, and the lines GE, HE, represent two muscles acting obliquely, and producing by a diagonal action the same effect as the other. If the bone AB be brought to the situation ab by the action of the muscle FE, the muscle will then be in the situation FK. If the bone is brought into the same situation by the action of the muscles GE, HE, these muscles will then be in the situation GK, HK.

"The proposition to be demonstrated is, that the line GK bears a greater proportion to the line GE, than the line FK does to line FE; for FK is to FE as GL is to GE (Eue. Elem. B. vi Prop. 2.); and the angle ELK being less than a right angle, the angle GLK, which is adjacent to it, must be greater than a right angle; and the angle GKL being in the same triangle with GLK, must be less than a right angle. The line GK, therefore, which subtends the greater angle, is greater than the line GL, subtending the lesser, and therefore bears a greater proportion to GE. But the line GL is to GE as FK is to FE; and therefore GK bears a greater proportion to GE than FK does to FE; that is, the fibres of the muscles acting obliquely, suffer a less proportional decimation than those of the muscle acting directly.

"It is farther obvious, that the more oblique the action becomes, the greater saving there will be of contraction; for in moving the line ab towards CD, the line FK diminishes in a swifter ratio than the line GK; and when the former has vanished, the latter is in the situation GF."

Besides these advantages in point of diminishing fatigue, there are others relating to the shape of the members. Thus, if the insertions of the muscles had been at a great distance from the joints, they must upon every occasion have passed like bowstrings from one bow to the other, and the limbs must have been exceedingly clumsy and unwieldy; all the motions must also have been extremely slow; and notwithstanding the superior strength which people would then have enjoyed, it is very plain that they would scarce have been fit for any of the offices of life which they now perform.

zoology. See Mytulus.

Muscovy. See Russia.

Muscovy-Glass, or Glimmer. See Mica.