LYMPH, a fine colourless fluid, separated in the
body from the mass of blood, and contained in peculiar vessels called lymphatics.
The late ingenious Mr Hewson published a dissertation on the lymph and lymphatic system in the human body, and that of other animals. The lymph he considers both as contained in its proper vessels, and exhaled into the different cavities of the body. In each of these states, the lymph coagulated when exposed to the air, and seemed more to resemble the buffy coat of the blood than its watery parts, to which it hath been compared by authors. This coagulum, Mr Hewson found to vary considerably in its degree of cohesion, being always firm in strong animals, but in weak ones of a much looser texture. And, in the course of his experiments, he observed, that the fluid collected from the different cavities of the body uniformly agreed with that found in the lymphatic vessels of the same animal, both in degree of tenacity, and time of coagulation. Thus far these fluids correspond with the coagulable part of blood; but they differ from it in the time necessary to form the coagulum, the blood always jellying much sooner than the lymph, and later in strong than in weak animals; the very reverse of what happens to that fluid.
From the following observations, Mr Hewson is of opinion, that the vessels exhaling fluids into the cavities of the body, whatever their structure may be, have a power of changing the nature of their contents.
1. A coagulum is often found covering the internal surface of inflamed cavities, without the smallest erosion. This can only be the natural exhalation, thrown out by inflamed vessels, with a strong disposition to coagulate.
2. As the lymph is discharged, variously changed in tenacity and other properties, by disease; and as pus is sometimes found in the same manner in cavities, without any ulceration; the author thinks it at least probable, that this purulent matter is only the lymph itself, still more altered by flowing through vessels in a higher or different degree of inflammation; and in this notion he is confirmed, by observing, with the microscope, globules in pus like those of the milk, a secreted liquor.
On a review of his experiments, he thinks they evidently point out the following general conclusions: that the lymph and exhalations are of a similar nature, but differ widely from the watery part of the blood; that they coagulate on exposure to air, and are probably a species of the lymph of the blood; that they differ in the time and firmness of coagulation, in various degrees, from the caecetic habit, where they scarcely jelly at all, to the inflammatory, where very short time is requisite to a strong cohesion; and that, in some cases, the inflamed vessels can even convert the fluid passing through them into real purulent matter.
Having thus given a correct idea of the properties of the lymph, he goes on to trace it from the circulating mass, and to investigate the manner of its separation; with regard to which, he observes, two different opinions have been entertained. The first and most general is, that the lymph is poured into cavities, either by exhalant arteries, or pores of an organised nature on the sides of vessels. The other has lately been started by Dr Hunter, who considers the lymph as merely a transudation of the thinner parts of the blood through
Lymph. through interstices or inorganized pores in the vessels circulating the red-blood; which, tho' they retain the serum, allow the more fluid parts to pass with ease. In support of this new doctrine, Dr Hunter adduces the following facts.
1. Water injected into the blood-vessels of a dead animal readily imitates this transfusion.
2. Blood itself transfused after death, the lymph to which it owed its consistence being coagulated.
3. Bile transfused through the gall-bladder; for, on opening an animal, all the adjacent parts are tinged.
With a view to disprove this notion, Mr Hewson maintains, that there is a tension in the vessels of a living body, forsaking them with life; and that, did such an exsudation take place, it would defeat one great intention of vessels, the conveyance of fluids, and involve us in a continual round of exhalation and absorption, very contrary to the common simplicity of nature.
A second objection of Mr Hewson is, that, if liquors thus filtrate into cavities, not only must the vessels, creeping on their sides, but the membranes lining these cavities, be porous. But, if these pores are admitted in every part, we cannot deny them to the whole extent of these membranes. And, were this the case, a partial dropsy could never occur; as a fluid would as easily escape, from within, onwards, as it formerly infiltrated itself from the vessels into any cavity.
Another argument against the opinion of Dr Hunter, is drawn by Mr Hewson from his former experiment, where he found the lymph, in different states of the body, so various in degree of viscosity. He is at a loss to conceive, how pores of an inorganized nature should pour out fluids so different in consistence, much less filtrate pus from the blood, where it never existed.
The argument of Dr Hunter, founded on the transfusion of blood, proceeds, he thinks, on an erroneous principle; for the blood of a living animal is actually thinned by the presence of the coagulable lymph: and, if this transfusion takes place in a dead, and not in a living animal, it ought rather to be attributed to a change in the vessels, than their contents. And this is almost proven by the attentive examination of one of the strongest of the doctor's own facts, viz. the bilious tinge on parts contiguous to the gall-bladder, which only takes place after an animal has been some time dead; but is never found in one recently killed.
From all these, the author concludes, that, as the lymph is very different from water, it cannot be filtrated by inorganical passages; and that the common opinion is founded on reason and experiment, the lymph being not only transmitted thro' exhalant vessels, but by them changed in its properties, and adapted to the office of lubricating parts.
The manner in which the lymph is discharged into the cavities of the body being thus ascertained with a great degree of probability, Mr Hewson proceeds to examine how it is again absorbed, whether by the common veins, or by the lymphatic system. Besides the authority of all the ancients, the advocates for the first opinion think they are possessed of several conclusive arguments in its favour.
They allege, that, in dead bodies, injections easily
flow from the veins into several cavities. This Mr Hewson thinks, for many reasons, to have little force. The veins are very delicate, and may be ruptured by a very small distending power. And this seems in general to have been the case; for most of the injections used in these experiments were far too gross to reach the extremities of the veins. And when those of a more penetrating nature are employed, they imitate that transfusion from vascular laxity, which takes place after death.
A second argument adduced in favour of the old opinion is, that chyle has been seen in the mesenteric veins. But this is a very inconclusive one, as the whole serum has been found of a white colour, and, in these vessels, might easily be mistaken for chyle. Another has been furnished by the structure of the penis: but this analogy is now given up; it being generally allowed, that the blood is not absorbed, but forced from its cells into the venous system.
Ligatures and compression on veins have afforded a fourth argument; but two circumstances unite to render this less satisfactory: 1st, The lymphatics, being contiguous to the veins, may be affected by the same cause. And, 2d, the compression of a vein will throw a greater quantity of fluids on the exhalant arteries, by obstructing their flow through the red ones.
It has been farther alleged, that there is no continuation of vessels nor any lymphatics in the placenta, though absorption evidently takes place. To this Mr Hewson replies, that there may be lymphatics, though not discovered, and continued vessels from the mother to the fetus, though never injected.
The sixth argument is supplied by an experiment of Kaau Boerhaave. He injected a fluid into the intestines, which soon appeared in the mesenteric veins. But Mr Hewson is much inclined to believe that there was some deception in this experiment, as it has been often repeated, without similar success. Did the fact even happen, it could never be by absorption, that ceasing soon after death, while this happened at the end of several hours; and, as pressure was applied to the stomach, the author thinks a venous rupture might give rise to the appearances observed by Kaau Boerhaave.
The seventh and last argument in favour of absorption by the veins was, that many animals were destitute of any other vessels capable of performing that office, as birds, fishes, and amphibia; but the discovery of lymphatics in every one of these, has totally overthrown this argument.
To all these, the ingenious Mr Meckel has, of late, added some others, derived from injections of mercury into the lymphatic glands, vesicle feminales, bladder, and lacteal ducts of the breast, which he found to pass readily, and without extravasation, into the adjacent veins. But Mr Hewson observes, that extravasation is detected with difficulty; and as the lymphatic vessels often lie contiguous to veins, it is not impossible that, in some cases, the mercury might burst from the one of them into the other, as often happened to Mr Hewson in the mesentery of the turtle. From all these, the author considers the old opinion as at least open to many and weighty objections.
Unsatisfied, however, with this negative proof, he goes on to show, that absorption in reality is carried on by the lymphatic system, from the analogy of all these
Lymphatics these vessels with the lacteals, which form a part of that system, and are without doubt absorbents. Besides this, the course by which poisons reach the circulating fluids when externally applied, evidently points out that they enter the lymphatic vessels alone, and are by them conveyed towards the heart. To these Mr Hewson adds his observation, that the same fluid is always found in these vessels, and the cavities whence they arise. Thus there will remain little doubt of the absorbent power of lymphatics, a function of such importance in the economy, that, for it alone, nature has provided a distinct set of vessels: they had indeed, by some, been suspected to be only reflected arterial branches; but this has been long confuted by the experiments of Dr Monro and Dr Hunter.