But, besides this kind of absorption, which is brought about by the substances being, as it were, mechanically forced into the pores of the skin, and thus applied to the mouths of the lymphatics, it was an opinion very generally embraced by the older physiologists, and still retained by many of our contemporaries, that the lymphatics, which are distributed over the surface, possess the power of imbibing water, when simply applied to it by the immersion of the body, or even when it is exposed to aqueous vapour diffused through the atmos phere. This supposed power of cutaneous ab sorption was called in to account for various physiological or pathological facts, for which it appeared to aflbrd a plausible explanation, while, on the other hand, the easy mode in which it appeared to account for these facts was made use of as the great argument to prove its existence. The statical experiments of Sanctorius, which have, since his time, been so much multiplied and extended, were supposed to prove unequivocally that the body is capable of gaining weight independently of any substance received into the stomach, and to account for this addition, recourse was always had to the cutaneous absorption. Of late, indeed, it has been discovered, that a part of the effect ascribed by Sanctorius to the action of the skin is in reality due to the lungs, but still, after making the necessary deduction for the operation of the latter organ, there re mained a certain increase of weight, which it was supposed could only be accounted for by admitting the existence of the cutaneous ab sorption.* The doctrine of cutaneous absorption has, however, been altogether called in question by Seguin, who performed a series of experiments, which consisted in immersing a part of the body in a saline solution, for example, that of corrosive sublimate, the effects of which on the system at large would be easily recognized, if any part had been absorbed. The result was, that when the cuticle was entire, no effect that could be attributed to absorption took place, and the conclusion seemed not unna tural, that under ordinary circumstances it did not exist.-f- Currie was led to form the same conclusion by accurately weighing the body before and after immersion in the warm bath, under circumstances which were conceived to be favourable to the process,t and as the re sults of his experiments coincided with those of Seguin and others, the doctrine of cuta neous absorption, except under the particular circumstances mentioned above, was very generally abandoned. Experiments have been adduced to prove, that even under these par ticular circumstances, when substances are ap plied by friction to the surface, they do not enter into the mouths of the vessels, but being volatilized by the heat of the body, that the vapour thus produced is inhaled by the lungs;§ an opinion which one might be inclined to think was almost too extravagant to be seri ously maintained.
The subject of cutaneous absorption has been lately investigated by Dr. Edwards, with that skill and address which he has applied to so many departments of physiology. By a number of experiments, which were performed on cold-blooded animals, where it was more easy to observe the effects, he found that ab sorption was carried on, to a considerable extent, when the animal, or a part of it was immersed in water. The conclusion which the experiments seemed to warrant was, that transudation and absorption are, at all times, going forwards at the surface, but that the operations proceed at different rates, according to the circumstances in which the animal is placed, and that the body gains or loses weight, in proportion to the excess of one of them above the other. The analogy of the cold blooded animals he applies to those with warm blood, and he supposes that they are subject to the same double action, a conclusion which appears to be confirmed by some experiments that were performed on guinea-pigs immersed in moist air, when an increase of weight was found to have taken place, which, after taking every circumstance into consideration, seemed necessarily to depend on absorption.* With respect to the experiments of Seguin, Dr. Ed wards is not disposed to call their accuracy in question, but he points out various circum stances connected with them, which he con ceives would tend to increase the transudation, and to diminish, or even entirely to suspend the absorption.- The experiments of Dr. Ed wards, considered in all their relations, are generally conceived to decide the question respecting the existence of cutaneous absorp tion, under the ordinary circumstances, and in the natural conditions of the system.
§. 4. Of the specific uses of the different parts of the absorbent system, and of the rela tion which that system bears to the other vital functions.—Whatever opinion we may form on the controverted question respecting venous absorption, and in whatever manner we may explain the action of the lacteals and the lym phatics, there can be no doubt that their spe cific use is to absorb certain substances which are presented to their extremities.X There is, however, so well marked a distinction between the situation and the anatomical relations of these two kinds of vessels, as well as between the substances that are found to be contained in them, that we are naturally led to conclude, that they are destined for different uses, and serve different purposes in the animal economy. With regard to the lacteals, their use seems to be clearly marked by their connexion with the digestive organs, and by their contents, as constituting the channel by which the chyle is conveyed from the intestines to the thoracic duct, and ultimately to the bloodvessels. We cannot doubt that their primary function is to supply the body with the elements which com pose the blood, and thus become the imme diate agents in its nutrition. Although, from the experiments which have been related above, it will appear that, on certain occasions, the lacteals are not incapable of receiving extraneous bodies, yet we may conclude, that this is the case only under extraordinary cir cumstances, or in an unnatural state of the parts.
With respect to the lymphatics, their specific use is less obvious. As their contents are ul timately mixed with those of the lacteals, we may suppose that they contribute indirectly to the nutrition of the body; but this would appear not to be their primary, or even their principal destination. Still we can scarcely refuse our assent to the position, that absorp tion is the specific function of the lymphatics ; and this will be equally the case, although we may suppose that the veins cooperate with them in this action.
We are indebted to the genius of John Hun ter for a consistent or plausible theory of the use of the lymphatics, which, with certain mo difications, is generally admitted to be correct. Conceiving that the appropriate and specific action of the lacteals is to nourish the body, and to support the system by the addition of new matter, that of the lymphatics is to mould and fashion the body, to admit of the growth and extension of the whole, while each in dividual part retains its proper form and position. When we consider in what manner an organized part increases in its dimensions, we immediately perceive that it is not by mere accretion, nor by simple distention; it is, on the contrary, by an addition to every individual portion, while they retain the same relation to each other and to the whole. If we take the case of a muscle, we find that each particular fibre must be increased in length, so that the distance may be augmented between the ten dinous extremities, while probably the number of fibres that are contained in the membranous covering is also increased ; the whole organ consequently becomes larger in every one of its individual parts, while they each retain their former proportions and connexions.* We may apply the same train of reasoning to the bones, which offer a still more remark able example of this change of form, inas much as the firmness of their texture must render it less easy to conceive of any alteration in their dimensions and in the disposition of their component parts. Here it is still more obvious than in the case of the muscle, that the change cannot be effected either by accre tion or by distention, but that a completely new disposition of the integrant parts must have taken place. The only means, however, by which this can be accomplished is by the former particles of the body being gradually removed, and new ones deposited to supply their place ; the process being so gradual, that, although the deposition of the new particle is not precisely in the same situation with the former, yet that of each particle is so nearly so as to cause no obstruction or interruption to the action of the organ. Now it is evident that this removal of the old matter can be effected by no process but by absorption, and we may therefore conclude that the lymphatics, either alone or in conjunction with the veins, are the agents destined to perform this office.