With respect to the parts of the body, or to the animals of an inferior order, which were supposed not to be furnished with ab sorbent vessels, by prosecuting their examina tion with more care they gradually detected the existence of these vessels in many cases where they had not been previously known to exist ; and they were discovered in so many new situations that it became a fair inference that every part of the body, and every animal whose structure is generally analogous to that of the mammalia, is provided with an appropriate apparatus of absorption, although, from the texture or the peculiar nature of the vessels, it may be very difficult actually to demonstrate their existence. In this train of investigation the labours of William Hunter and Monro were ably seconded by various anatomists, both in this country and on the continent, among whom we may select the distinguished names of John Bunter, Hewson,t Cruikshank, and Mascagni.
The experiments of Hunter may deserve to be particularly noticed, because they consisted not merely in repeating and correcting those of preceding anatomists ; but, in addition to these, he entered upon a series of original researches, which are highly characteristic of that ingenuity and acuteness for which he w..Ls so eminently distinguished. The experiments essentially consisted in filling portions of the small intestines with a fluid, the sensible pro perties of which might be easily recognized, and retaining it. there so as to allow of its entering into the veins of the mesentery, were they capable of absorbing it ; the result, however, is stated to have been that in no instance could the fluid be detected in these veins. These experiments appeared to have been so carefully conducted, and so frequently repeated, as to have impressed the minds of anatomists and physiologists with a conviction that the lacteals were the only vessels which are concerned in the absorption of the chyle ; and although it was not possible to perform analogous experi ments on the lymphatics, yet it seemed a natural inference, that we might extend to them the conclusion which had been established with respect to the lacteals.* In proof of lymphatic absorption various facts were brought forward, which seemed clearly to show that when extraneous or noxious substances were introduced into the system, it was clone by the medium of the lymphatic vessels rather than of the veins; and it was thence argued that, as these vessels perform the func tion of absorption under certain circumstances, and that we are not acquainted with any other office which they serve in the system, we may conclude that they are the sole agents in the action of absorption. Although the argument,
as applied to the lymphatics, was far less direct and conclusive than to the lacteals, yet the analogy between the two organs appeared so strong, and so many concurring circum stances appeared to favour the doctrine, that it was very generally received, and may be considered as having been the established opinion at the conclusion of the last century.f This unanimity of opinion was, however,' of very short duration; for anatomists had scarcely ceased to contend for the honour of the dis covery of the exclusive action of the lacteals and the lymphatics in the function of absorp tion, when the doctrine itself was impugned by physiologists of the first eminence, who supported their views by a powerful train of arguments, enforced by numerous experiments, and by various pathological considerations. Of these authors, one of the first, both in point of time and of ability, is M. Magendie, whose opinions on this subject, connected as as they are with some of those of his most distinguished countrymen, have been brought forward in a form which entitles them to the fullest and most respectful attention.
Of the two sets of observations by which Hunter and Monro attempted to establish their hypothesis respecting lymphatic absorption, those derived from the analogy of the lacteals may still be considered as maintaining their ground ; while the conclusion which they de duced from their experiments has been called in question, partly because it was thought not to be the legitimate inference from the experi ments, and partly in consequence of the ex periments themselves having been conceived to be imperfect or incorrect. It is principally upon the latter ground that the force of the objections has been rested ; and it has been, first, by repeating the experiments of Hunter, and afterwards by varying them in different ways, that their insufficency has been attempted to be proved. It has been stated above that the main support of the doctrine of the ex clusive action of the lacteals and the lymphatics was derived from those experiments of Hunter, in which it appeared that, when the circum stances were the most favourable for the re ception of substances into the veins of the mesentery, they could not be proved to have entered these vessels; and hence it was con cluded that the veins did not, under any cir cumstances, possess the power of absorption.