SECRETION. 



445 



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 from the blood ; for the 

 tubuli of the absorbent system, like those of 

 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 glandulas or ganglia, with which 

 the absorbent vessels of man and the mam- 

 malia are copiously furnished. These bodies 

 are composed of lacteal or lymphatic 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. 



There is strong reason for regarding the 

 spleen, the thymus and thyroid glands, and 

 the supra-renal capsules, as parts of the same 

 assimilative apparatus, their office apparently 



being, to withdraw certain crude matters from 

 the blood, to submit these to an elaborating 

 action whereby they shall be rendered more 

 fit for the nutrition of the tissues, and then to 

 restore them to the circulating current. The 

 details of the structure of these organs will be 

 found under their respective names ; and it 

 will be sufficient to state here, that they all 

 show an essential correspondence with the 

 true and recognised glands in every respect 

 but this, that they have no efferent ducts. 

 Each of them may be described as consisting 

 essentially of a number of vesicles, which are 

 either closed and isolated, or open into a 

 common reservoir, which is itself closed ; the 

 vesicles in either case are lined with epithelial 

 cells.* Around these, as around the follicles 

 or tubuli of the true glands, blood-vessels are 

 copiously distributed ; and the elimination of 

 products from the blood appears to be 

 effected by their agency, precisely as if these 

 products were destined to be cast out of the 

 body. The mode in which they are taken 

 back into the circulation, after they have been 

 subjected to the elaborating process, is not 

 very clear ; both blood-vessels and absorbents 

 have been supposed to participate in the 

 operation ; and this idea may not be regarded 

 as improbable, when the large size and number 

 of the lymphatics distributed to these organs 

 is considered. 



Having thus taken a general survey of the 

 principal varieties of secretory structure, and 

 of the chief aspects under which the secreting 

 function presents itself, we shall pass on to a 

 more particular consideration of the mode in 

 which this operation is performed, and of the 

 instruments by which it is effected. For this 

 purpose it will be preferable to select a par- 

 ticular gland, and to examine the minutiae of 

 its structure in the most diverse forms and 

 conditions under which it presents itself; and 

 there is none which suits our purpose so well 

 as the liver, which is the gland of most 

 universal existence throughout the whole 

 animal series, and which presents almost 

 every leading variety that is found in the 

 whole series of glandular structures. And 

 we gladly avail ourselves of the opportunity 

 thus afforded, of bringing the account already 

 given of that gland (see LIVER) into con- 

 formity with the increased knowledge of its 

 structure that has been since acquired. 



There are few animals possessed of a dis- 

 tinct digestive cavity, in which some traces of 

 a biliary apparatus (recognisable by the colour 

 of the secretion) may not be distinguished. 

 Thus in the Hydra, some of the cells that 

 form the lining of the stomach contain a 

 brownish-yellow matter, strongly resembling 

 bile, which is probably poured into the cavity 

 on tha rupture of the cells. In the walls of 

 the stomach of the Actinia, Dr. Thomas Wil- 

 liams has described sulci formed by duplicatures 

 of the lining membrane, in which are lodged 

 a set of cells of glandular appearance, some 



* Anatomical and Pathological Observation 

 46. 



* See Prof. Ecker, in Annales des Sciences Na- 

 turelles, Zoologie, Aout, 1847. 



