The case is not different, in any essential respect, with regard to the organs by which the recrementitious secretions are formed. Thus the lachrymal, salivary, pancreatic, and mammary glands are in like manner composed of a continuation of the limitary membrane of the true skin, or of the mucous membrane lining the alimentary canal, involuted into tubes and follicles, the free surfaces of which are covered with epithelial cells. These cells, drawing into themselves certain constituents of the blood, are cast off when they have completed their full development ; and their contents, set free by the disintegration of the cell-walls, are carried off by the ducts, which collect them from different portions of the glandular structure, and deposit them in the situation where the purposes of the secreted product are to be answered.
If we attentively consider the character of what is commonly designated as the absorbent system, we shall see that this, too, may be re garded as a glandular apparatus ; possessing, as it does, the essential characters of a gland in regard to its structure, and being analogous to the true glands in itS mode of performing its function, and the difference of its purpose in the general economy being accordant with the difference of its anatomical relation. Putting out of view for the time the absorbent glands, or ganglia, we find the absorbent system to consist of two series of long tubuli, one set ex tended through almost the entire body, whilst the other is distributed upon the intestinal canal. These tubes appear to commence either in cmcal origins or in loops ; they coalesce with each other ; and at last dis charge themselves into a common receptacle, just as do the tubuli of the kidney. That their origins should be widely scattered, in stead of being bound together in one compact mass, is a fact of no physiological importance ; having reference only to the remoteness of the sources, whence are derived the materials on which the particular agency of this appara tus is exerted. These materials are of two kinds ; for they consist in part of the crude materials selected by the lacteal division of the system from the contents of the aliment ary tube, over whose walls the origins _of the lacteals are dispersed; and in part of substances taken up by the lymphatic or interstitial divi sion, and probably consisting chiefly of par ticles which are set free by the continual disintegration of the living structure, but which, not being yet decomposed, are capable of being again employed for the purposes of nutrition. The materials derived from these
sources appear to require a considerable pre paration or elaboration, before they are fit to be introduced into the current of the circula tion ; and this elaboration is effected by an agency of precisely the same nature with that which is concerned in the removal of various products of secretion ftom the blood ; for the tubuli of the absorbent system, like those the kidney or the testis, are lined by epithe lial cells, and their duty seems to be altogether analogous. The alterations which the ab sorbed matters undergo during their passage along this system of tubes, and the evidence that these alterations are in great part due to the elaborating action of cells, having been heretofore considered (see NUTRITION), need not be again dwelt on ; but a few words may be added respecting the structure and func tions of the glandulm or ganglia, with which the absorbent vessels of man and the mam walla are copiously. furnished. These bodies are composed of lacteal or lyrnphatic trunks, convoluted into knots, and distended into cavities of variable form and size, which are known as the " cells " of these glands. Amongst these cells there is a copious plexus of blood-vessels, but there is no direct com munication between their cavities. Accord ing to Prof. Goodsir*, the epithelium which lines the absorbent vessel undergoes a marked change where the vessel enters a gland, and becomes more like that of the proper glandu lar follicles in its character. Instead of being flat and scale-like, and forming a single layer in close apposition with the basement mem brane (as it does in the lacteal tubes before they enter the gland, and after they have emerged from it), we find it composed, within the gland, of numerous layers of spherical nucleated cells, of which the superficial ones are easily detached, and which appear to be identical with the cells that are found floating in the chyle and lymph, especially after their passage through these bodies. The absorbent glands may be regarded, therefore, as concen trating within themselves that agency, to which the whole system of tubuli is more or less subservient. Such an idea is strictly ac cordant with the facts of comparative anatomy; for in reptiles, in which there are no glands, the tubuli or vessels are enormously length ened by the convolutions which they present along their course, as if to furnish a suffi cient extent of epithelial surface.