Amongst a multitude of examples, which teach us how little we can infer the importance of anything in nature from its size, or other impressions which it may convey to mere sense, we might adduce the wonderful little tubes, certain relations of which were the objects of this paper. Those constant mutations in animal bodies which are every moment in progress, are, in great part, due to a very curious order of vessels, of such extreme minuteness and tenuity, that, being in the dead animal usually empty and transparent, they are very commonly invisible, and thus long eluded discovery. There is one situation, however, in which circumstances combine to expose them to observation. Transparent though they be, they are here usually rendered visible; first, by being loaded with a milk-like fluid; and secondly, by being placed between the folds of a membrane, itself beautifully transparent (the mesentery). This fluid they have just taken up from the digestive surfaces on which their mouths open, and they are now carrying it off to pour it into the blood-vessels, that it may be added to the general stock of the circulation.
In the situation above mentioned they were at length discovered, about the commencement of the 17th century. Every thing destined to support the body with new material, as well as the old, which is to be taken away, must first be sucked up by the myriads of inconceivably minute mouths of these vessels, which, from their office, are called the absorbents. These absorbents may therefore be regarded as the sentinels of the body. They are very sensitive and excitable; but, besides this, there are placed in the course of their journey, from the surfaces whence they bring their contents, and the blood-vessels to which they are carrying them, a number of douaniers, or custom-house officers (the glands, or kernels, as they are popularly called), whereby, as we have every reason to believe, the fluids they are importing are subjected to rigid examination; and, if found to be injurious, to some modification, tending to render them more fit for admission into the system.
If the contents are very irritating, these vigilant guards—these kernels—become very painfully affected, and sometimes inflammation is set up, sufficient even to destroy the part; as if, faithful to their trust, they perished themselves, rather than give entrance to anything injurious to the body.
We should never advance, however, in our story, if we were to tell all the interesting peculiarities of these curious vessels.
When first discovered, and the office assigned to them could no longer be disputed, the general distribution of them was still doubted. As it was usual to render them visible by filling them with quicksilver, so, with a kind of reasoning which has too often characterized mere anatomical research, when they could not be made visible, it became the fashion to doubt their existence. Amongst other structures, Bone was formerly one in regard to which people found a difficulty. How could such delicate vessels exist in such an apparently dense structure? But Mr. Abernethy, who, like Bacon, had always opposed mere eye-reasoning, used to observe, with equal simplicity and good sense, that, for his part, he could see no more difficulty in an absorbent taking up a particle of bone, than he could in comprehending how a vessel could lay it down, which nobody doubted. We now know that bone is not only supplied with all the vessels which characterize a living structure, but so liberally, that, in comparison with some other structures of the body, we regard it as a part of high organization.
Nevertheless, the extreme minuteness and transparency of these absorbent vessels naturally led persons to regard with considerable interest any magnified view of them, such as that afforded by larger animals. In the paper before us, which was published in the "Philosophical Transactions" for 1793, Mr. Abernethy gives the account of his examination of the absorbents in a whale; and his object was to help to determine a question long agitated, whether the glands or kernels were composed of cells, or whether they were merely multiplied convolutions of vessels. He selected the absorbents from the situation to which I have already referred. He threw into the arteries which carry blood to nourish the gland, a red solution containing wax, which of course became solid on cooling; and into the veins which return the blood from all parts, a similar solution, only coloured yellow. He filled the absorbents with quicksilver.
He found, in filling the absorbents, that wherever the quicksilver arrived at a gland, there was a hesitation—its course became retarded, and that this retardation was longest at those glands which were nearest the source whence the vessels had drawn their contents, viz. the alimentary canal: as if the surfaces over which the fluid had to pass were more multiplied where most necessary, or, recurring to our metaphor, as if the more strict douanier had been placed on the frontier. He says that he found that some of the absorbents went over the glands, whilst others penetrated these bodies. That he found that the melted wax which he had thrown into the vessels had formed round nodules of various sizes. He then extended his examination of these vessels to those of horses and other large animals; and the result of his investigation was, that it inclined him to the conclusion that the glands were not merely made up of convolutions of vessels, but were of a really cellular structure.
The paper is very modestly put forth, and he concludes it by observing that he offers it merely for the facts which it contains, and not as justifying any final conclusion; but "as all our knowledge of the absorbents," he continues, "seems to have been acquired by fragments, I am anxious to add my mite to our general stock of information on the subject."
It may not be uninteresting to some unprofessional readers to know that the glands here alluded to are the organs which are so seriously diseased in those lamentable conditions popularly expressed, I believe, by the term mesenteric disease, or disease of the mesentery.