in one glandular mass, we have an illustration in the accessory glands of the genital apparatus, in several animals, which discharge their secretion into the urethra by numerous outlets (Fig. 112); or in the Mammary glands of Mammalia in general, the ultimate follicles of which are clustered upon ducts that coalesce to a considerable extent, though continuing to form several distinct trunks even to their termination. Such glands may be subdivided, therefore, into glandulæ or

lobules, that remain entirely distinct from each other (Fig. 113).-In the highest form of Gland, however, all the ducts unite; so as to form a single canal, which conveys away the products of the secreting action of the entire mass. This is the condition in which we find the Liver to exist, in most of the higher animals; also the Pancreas, the Parotid Gland, and many others. In some of these cases, we may still separate the gland into numerous distinct lobules, which are clustered upon the excretory duct and its branches, like grapes upon a stalk; in others, however, the branches of the excretory duct do not confine themselves to ramifying, but inosculate, so as to form a network, which passes through the whole substance of the gland, and which connects together its different parts, so as to render the division into lobules less distinct. This seems to be the case in regard to the Liver of the higher Vertebrata (§ 723).

718. Whatever degree of complexity, however, prevails in the general arrangement of the elements of the Glands in higher animals, these

Ultimate follicles of Mammary gland with their secreting cells, a, a, b, b, the nuclei.

Two follicles from the liver of Carcinus manas (Common Crab), with their contained secreting cells. elements are themselves everywhere the same; consisting of follicles that enclose the real secreting cells (Figs. 114 and 115). Now from

the history of the development of Glands in general, it appears that the follicles may be considered as parent-cells and that the secreting cells in the interior constitute a second generation, developed from the nuclei germinal spots on the walls of the first. Of such parent-cells, we have characteristic examples in the Peyerian glandulae of the intestinal canal (§ 450), and also in the Thyroid gland (§ 513) and Supra-Renal capsules (§ 510); and the most elaborate glands, in their earliest stage of development, present a similar condition. These closed cells become follicles, by opening at one extremity, either upon a neighboring free surface, or into a canal which is prolonged from it. Thus the first rudiment of the Liver is formed by a thickening of the cellular mass in the walls of the alimentary canal, at the spot in which the hepatic duct is subsequently to discharge itself. This thickening increases, so as to form a projection upon the exterior of the canal; and soon afterwards the lining membrane dips down into it, so that a kind of cæcum is formed, surrounded by a mass of cells, as shown in Fig. 108. The increase of the mass seems to take place by a continual new buddingforth of cells from its peripheral portion, which takes place to a considerable extent before the cæcum in the interior begins to ramify. Gradually, however, the cells of the exterior become metamorphosed into fibrous tissue for the investment of the organ; those of the interior break down into ducts which form continuations of the principal canal; whilst those which occupy the intervening space, and which form the bulk of the gland, seem to be developed into follicles, and to give origin to the proper secreting cells. As this is going on, the hepatic mass is gradually removed to a distance from the wall of the alimentary canal; and the cæcum is narrowed and lengthened, so as to become a mere connecting pedicle, forming, in fact, the main trunk of the hepatic duct.The development of the Pancreas, Salivary glands, &c., seems to follow. the same plan.

719. It has been pointed out by Prof. Goodsir, that the continued development and decay of the glandular structure,-in other words, the elaboration of its secretion,-may take place in two different modes. In one class of Glands, the parent-cell, having begun to develope new cells in its interior, gives way at one point, and bursts into the excretory duct, so as to become an open follicle, instead of a closed cell: its contained or secondary cells, in the progress of their own growth, draw into themselves the matter to be eliminated from the blood, and, having attained their full term of life, burst or liquefy, so as to discharge their contents into the cavity of the follicle, whence they pass by its open orifice into the excretory duct: and a continual new production of secondary cells takes place from the germinal spot, or nucleus, at the extremity of the follicle, which is here a permanent structure. In this form of gland, we may frequently observe the secreting cells existing in various stages of development within a single follicle; their size increasing, and the character of their contents becoming more distinct in proportion to their distance from the germinal spot (which is at the blind termination of the follicle), and their consequent proximity to the outlet (Fig. 114). In some varieties of such glands, however, as in the greatly-prolonged follicles or tubuli uriniferi of the kidney, the produc

tion of new cells does not take place from a single germinal spot at the extremity of the follicle, but from a number of points scattered through its entire length.-In the second type of Glandular structures, the parent-cell does not remain as a permanent follicle; but, having come to maturity and formed a connexion with the excretory duct, it discharges its entire contents into the latter, and then shrivels up and disappears, to be replaced by newly-developed follicles. In each parentcell of a gland formed upon this type, we shall find all its secondary or secreting cells at nearly the same grade of development; but the different parent-cells, of which the parenchyma of the gland is composed, are in very different stages of growth at any one period: some having discharged their contents and being in progress of disappearance, whilst others are just arriving at maturity and connecting themselves with the excretory duct; others exhibiting an earlier degree of development of the secondary cells; others presenting the latter in their incipient condition; whilst others are themselves just starting into existence, and as yet exhibit no traces of a secondary generation.-The former seems to be the usual type of the ordinary glands; the latter is chiefly, if not entirely, to be met with in the Spermatic glands.

2. Of the Liver and the Bile.

720. The Liver is more rarely absent than any other Gland; being discoverable, under some form or other, in all but the very lowest members of the Animal kingdom. Its simple condition in the higher Polypes has been already noticed (§ 716); and it is met with, under an almost equally simple form, in the Starfish. As we ascend the scale, however, we find it assuming a much greater importance, and presenting a great increase in size. This is particularly the case in the Molluscous classes; and also in the Crustacea, a class which, in mode of respiration and in general habits, bears a great resemblance to the Mollusca. In nearly all such animals the Liver makes up a large proportion of the mass of the body. It usually consists of a series of large follicles,

which branch out into smaller ones (Figs. 116 and 117), and of which several open into one excretory duct; but these ducts remain separate, and discharge their contents into the intestine by several distinct ori

fices.-In Insects and other air-breathing Articulata, however, the Liver is much less developed; and its type remains much simpler. We usually find it consisting of a small number of cæcal tubuli, which open separately into the intestinal canal, just below the stomach. These tubuli are often so long, as to pass several times from one extremity of the visceral cavity to the other, being doubled upon themselves; in other instances, we find that the principal tube or canal is beset with rows of short follicles, somewhat in the manner of Fig. 111. But they never cluster together so as to form a solid glandular mass. The low development of the liver in these animals, bears an evident relation with the high development of their respiratory apparatus; whilst, the respiration being comparatively feeble in the aquatic Mollusca and Crustacea, the development of the liver in those classes is enormous.

721. There is much difficulty in ascertaining the mode in which the elementary constituents of the Liver are arranged, in the fully-developed condition of that organ in the higher Vertebrata. At an early period of its development, as already remarked, it may be easily shown to consist, in the Fowl, of a series of distinct cæca, clustered round a projection from the intestinal canal, and opening separately into it (Fig. 108); and it is a peculiarly interesting fact, that this very condition should exist as the permanent form of the Liver, in that curious little fish, the Amphioxus or Lancelot, which retains the embryonic type in so many parts of its conformation. In the Tadpole, again, the distinct cæca are very evident (Fig. 118); but here we see that the projection of the intestinal canal, instead of being a simple wide cæcum, has become extended in length and contracted in diameter, at the same time dividing and subdividing, so as to form an arborescent excretory duct, whose ramifications extend through the entire glandular mass. In this manner, then, is formed the complex system of hepatic ducts, which we find in the liver of the higher Vertebrata, branching out from the main trunk. But the mode in which the ultimate ramifications of these are arranged, and their relations with the secreting cells which make up the parenchyma of the gland, have not yet been fully elucidated. The following are the principal facts, that have been ascertained on the subject.

722. The entire Liver is made up of a vast number of minute lobules, of irregular form, but about the average size of a millet-seed. Each of these lobules contains the component elements of which the entire organ is made up; namely, branches of the hepatic artery and vein, branches of the portal vein, branches of the hepatic ducts, and secreting cells. The lobules are connected together in part by areolar tissue, but in great part by the anastomosis of the blood-vessels and hepatic ducts, which supply the adjoining lobules; indeed there is frequently no definite division of the glandular substance into lobules, other than that which is marked out by the arrangement of these canals (Figs. 119 and 121). The branches of the Hepatic Artery are principally distributed upon the walls of the hepatic ducts, and upon the trunks and branches. of the portal and hepatic veins, supplying them with their vasa vasorum ; also upon Glisson's capsule and its prolongations into the substance of the liver, which prolongations form the greatest part of the connecting

structure that holds together the several elements. There is strong reason to believe, that the blood which the liver receives from the

Liver of Tadpole; showing distinct and free cæcal terminations of the biliary ducts.

hepatic artery is not destined to supply the materials for the biliary secretion, until it has become venous by travelling through the network, in which it is subservient to the nutrition of the tissues it permeates, as it is in other parts of the systemic capillary system.-The supply of blood, from which the materials of the biliary secretion are chiefly drawn, is afforded by the Vena Porta, which collects it as a Vein from the chylopoietic viscera, and which then subdivides as an Artery to distribute it to the different parts of the Liver. Its branches proceed to

Horizontal section of three superficial lobules of the Liver, showing the two principal systems of bloodvessels; 1, 1, intra-lobular veins, proceeding from the Hepatic veins; 2, 2, inter-lobular plexus, formed by branches of the Portal veins.

the capsules of the lobules, covering the whole external surface of the latter with their ramifications, and sending capillary twigs inwards, which converge towards the centre of each lobule (Fig. 119, 2, 2). As