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 vvill 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 rnay 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 minutim 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 th3 rupture of the cells. In the walls of the stomach of the Adinia, Dr. Thomas Wil liams has described sulci formed by du plicatures of the lining membrane, in which are lodged a set of cells of glandular appearance, some of them containing scarlet-red, and others bright yellow granules ; the latter are regarded by Dr. W., and probably with justice, as the diffused rudiments of a liver.* In the Bower bankia densa, and in other Bryozoa, very distinct spots may be seen in the parietes of the stomach, which seem to be composed of clusters of biliary cells contained within fol licles ; and during digestion, the contents of the stomach are seen to be tinged with a rich yellow-brown hue, derived from the matter discharged from these follicles.-1- In the Astoias the digestive cavity is surrounded by a more complicated glandular apparatus, but it seems difficult to determine the precise portion of this which discharges the function of a liver. The central stomach is furnished with a pair of glandular appendages, each composed of a cluster of follicles, which open into its fundus ; and these, from their dull yellow colour, have been thought to be a liver. Dr. Williams states, however (loc. cit.), that their ultimate structure does not sanction that idea, the terminal vesicles abounding in a white elastic tissue, in the meshes of which are entangled a number of small, compact, and granular cells, which are by no means hepatic in their aspect. He is disposed to agree with Dr. Grant, who hints that this organ may be a rudimentary pan creas; we should, ourselves, regard it as more probably a salivary- gland, its secretion being apparently mingled with the food immediately upon the ingestion of the latter. In the walls of this central stomach, proper gastric follicles have been detected by Dr. Williams; and he regards in the light of an hepatic organ the dilated culs-de-sac, filled with large glan dular cells, which are disposed in great num bers along the ramifying cwcal prolongations of the central stomach that are extended into the rays.
In the lower groups of the Articulated series, we meet with a diffused form of the biliary apparatus, not unlike that which has been just described in the lower Radiata. Thus in the Earthworm, the large annulated alimentary canal is completely encased in a flocculent external coating, which, when exa mined with the niicroscope, is found to consist of a mass of minute flask-shaped follicles, held by tubular peduncles, several of which coalesce to form the excretory canals for the discharge of the secretion into the digestive cavity. These follicles are composed of a
membrane of extreme tenuity, and their inte rior is filled with cells containing granular matter and oil globules, which are the consti tuents of the hepatic secretion. In the Leech and some other Annelida, the alimentary canal is furnished with large sacculated appendages ; and in the walls of these, as well as of the central canal, the biliary cells are closely dis posed. These cells, according to Dr. Wil liams, are not included within follicles, as in the earthworm ; the absence of cwcal multi plications of the stomach in the latter being compensated bv ennrpntratinn nf narts in the biliary systetn. In the Myrapoda, there is a decided advance from this diffused form of hepatic structure, towards that more con centrated and isolated condition, in which we find the liver of Insects. The general distri bution of the biliary organs in this class has already been described. (See INSECTS, VOL II. p. 974.) They consist of a number of dis tinct filiform tubes, usually of a yellowish brown colour, placed in close apposition to the sides of the alimentary canal, and opening into it near the pyloric extremity of the stomach, usually by separate orifices, but sometimes after the junction of two or more with each other, to form short common trunks. Their number varies considerably ; the fewest, namely four, existing in the Dip tera, six being found in the Lepidoptera, and many more in the Orthoptera and Hymenop tera. When few in number, they are very long, sometimes three or four times the length of the alimentary canal, and are tortuous and convoluted ; when numerous, they are pro portionally short, and are more delicate in structure. In many larvm, they are furnished with lateral cmca, but these almost always disappear as the insect approaches the imago state. The following is the description re cently given of the minute structure of the biliary tubuli, by a well qualified observer :— " When more intimately examined, these tubes are found to consist of a delicate tube of clear, transparent, amorphous basement membrane, the inner surface of which is co vered with secreting cells. From the thin ness of the tube, the cells often project, so as to give it a granulated appearance when viewed by the naked eye, as in the flesh-fly, .111usca carnaria (fig.311. a, b) ; and generally towards the free extremities the sides of the tubes are so irregular, that they appear as if merely folded upon the secreting cells to keep them together. The secreting cells are round, oval, or nearly cylindrical from elongation. Their average measurement is about .09 millim. The contents are white, yellowish, or brownish, and consist of a finely granular niatter, numerous fine oil globules, a granular nucleus, and a transparent nucleolus. The cells in the extremity of the tubes are not more than half the size of those a little further on (or nearer the termination), and contain less granular matter and no oil globules, so that they are more distinct, and the nucleus more apparent. Upon advancing a very little, the cells are found to be of an increased size, and full of granular matter, so as considerably to obscure the nucleus from view. A little further, we find the addition of fine oil globules, readily distinguishable by their thick, black outline when viewed in a certain focus. Sometimes the cells become so filled with oil, as to be distended with it, rendering the granular matter and nucleus so transparent as totally to destroy all appearance of the for mer, and the latter only is to be perceived in faint outline. Such a state I have frequently observed in Dernzestes, Ateuchus, &c. The nucleus (fig. 311. c) is generally central, glo bular, and pretty uniform in size in the same species, averaging in measurement about .025 millim. The nucleolus is always transparent, and measures about .006 millim. The cen tral passage of the tubes, or separation of the cells in the middle line, is usually found filled with fine granules, and a great amount of oil globules. The biliary tubes of insects are bathed in blood, or the nutritive fluid, and the respiratory trachem are distributed to them with extreme minuteness, but are separated from the secreting cells by the intervention of the basement membrane."* According to Dr. T. Williams (op. cit.), sorne at least of the large cells which give the sacculated ap pearance to a biliary tubulus are really parent cells, filled with a second generation of hepa tic cells ; they are, therefore, analogous to fol licles, save that they have no proper outlet, for we shall hereafter see that the follicle in its earliest condition is probably nothing else than a parent cell. From the above descrip tion, it would appear that the hepatic cells originate towards the upper or meal end of the tubulus, that they are gradually being pushed onwards towards the outlet by the growth of new generations behind them ; and that, as they thus advance, they acquire an increase in size by their own inherent powers of development, at the same time drawing into themselves the peculiar matters which they are destined to eliminate from the circu lating fluids. The cells, having attained their full growth, and completed their term of life, give up their contents by the rupture or deli quescence of their walls, and these pass down the central cavity of the tube, to be discharged into the alimentary canal.