On the surface of the liver are seen with the naked eye small rounded markings about the size of a pin's head, which give the organ a mottled appearance. This is much more striking in some animals (giraffe, bear, pig) than others, but is easily recognizable in the livers of all mammalia. These little areas are the surfaces of the lobules of the liver. They are surrounded by a dark-red boundary, and their centre is marked by a dark spot, between these is a paler, yellowish zone. The dark parts correspond to the blood vessels, and have a constant relation to the lobules.

The entire liver is made up of these little lobules, and each one of them has the same construction and blood supply, and therefore forms in itself a little liver perfect in all its structural arrangements, so that the description of one such unit will suffice to give an idea of the structure of the gland. For other details, anatomical works must be referred to.

The branches of the large portal vein and those of the small hepatic artery pursue the same course through the gland, and are enclosed in a sheath of connective tissue (capsule of Glisson), which also forms the bed of the hepatic duct and its numerous tributaries. If these branching vessels be followed to their final ramifications, they are found to pass around and between the neighboring lobules. The branches of the portal vein in this situation receive the name of the interlobular veins. They anastomose freely with the terminal veinlets in the vicinity, so as to form a network round each lobule. From this a number of capillary vessels pass into the lobule, and, lying between the gland cells, form a network with long meshes radiating from the centre. These are the lobular blood capillaries. The vessels of this radiating capillary network become larger as they unite and converge to the centre of the lobule, where they open into a central vein which lies in immediate apposition with the gland cells. This vein is called the intralobular vein, and is the radicle. of the efferent or hepatic vein, which carries the blood of the liver to the inferior vena cava.

Section of Lobule of Liver of Rabbit in which the blood and bile capillaries have been injected.

Fig. 72. Section of Lobule of Liver of Rabbit in which the blood and bile capillaries have been injected. {Cadiat).

a. Intralobular vein. b. Interlobular veins, c. Biliary canals beginning in fine capillaries.

The ultimate ramifications of the hepatic artery can be traced to various destinations. Some pass into the walls of the accompanying vein and duct and the connective tissue which surrounds these vessels. Many of the arterial capillaries unite with offshoots from the interlobular venous plexus, and thus reinforce the lobular capillaries. Other branches form a lobular capillary plexus, which joins the capillaries of the vena porta, together with that from the walls of the vein and duct.

Cells of the Liver. One large mass shows the shape they assume by mutual pressure..

Fig. 73. Cells of the Liver. One large mass shows the shape they assume by mutual pressure. (a) The same free, when they become spheroid. (6) More magnified, (c) During active digestion containing refracting globules like fat.

The blood flowing to the liver in the great vena porta and the hepatic artery is thus conducted by those vessels to the boundaries between the lobules (interlobular veins), and thence streams through the converging lobular blood capillaries to the intralobular vein, and is collected from the latter by the sublobular tributaries of the hepatic vein', by which it is conducted back to the general circulation, and enters the heart by the inferior vena cava.

Between the meshes of the lobular capillaries the gland cells are tightly packed. These are large, soft, polyhedral cells, with one, two, or even more nuclei, and no trace of a limiting membrane. Owing to the shape of the capillary meshes, the cells are placed in rows radiating from the centre of the lobule toward the periphery.

Section of injected liver showing the position of portal branches.

Fig. 74. Section of injected liver showing the position of portal branches (interlobular veins, vp) and radicals of hepatic veins (intralobular veins, hv) connected by lobular capillaries.

Below is a portion of the same highly magnified. (a)Liver cell with (n) nucleus; (b) Blood capillaries cut across, passing along angles of cells; (c)Bile capillaries between flattened sides of cells. (Huxley).

The blood capillaries are said to pass along the angles and edges of these cell blocks so as not to come into close relation to the bile capillaries (Fig. 71). The finely granular protoplasm of the liver cells is capable of undergoing some slight change in form while alive. In the protoplasm are situated varieties of granules, the commonest being bright, refracting fat globules, which vary in amount with the different stages of digestion; others of a yellow color seem connected with the coloring matter of the bile; and a third variety, less refracting and colorless, is said to be related to the glycogen.

Section of the Liver of the Newt, in which the bile ducts have been injected.

Fig. 75. Section of the Liver of the Newt, in which the bile ducts have been injected, and can be seen through the transparent liver cells to form a network of fine capillaries.

Between the cells of the lobules there can be demonstrated very fine anastomosing canals, which appear to be formed by the juxtaposition of grooves which lie in the middle of the flat surface of two neighboring cells. Every liver cell is related to such a canal, and consequently a very dense network with peculiarly regular polygonal meshes is present, each mesh corresponding in size to one cell.

These fine intercellular canals are called lobular bile capillaries, and must not.be confounded with lobular blood capillaries, the diameter of which is about ten times as great as the former, and which have a definite boundary wall, while the bile capillaries have no other boundary than the substance of the liver cell, and therefore are not really vessels.

These fine intercellular bile passages are described as communicating with the interlobular ducts directly, opening into the ducts without any marked increase in the size or change of arrangement. The interlobular ducts which follow the course of the artery and portal vein are composed of a delicate basement membrane lined with a thin layer of epithelium which in the larger vessels shows a cylindrical character. The large bile ducts have a firm fibro-elastic coat lined with a definite mucous membrane covered with cylindrical epithelium lying upon a vascular submucosa, in which are scattered numerous mucous glands of saccular form.

The amount of connective tissue in the liver of man and most domestic animals is very small, but in the pig, bear, giraffe, and some others, it is easily recognized around the lobules, sending delicate supporting processes between them. This connective tissue passes into the organ with the portal system of vessels forming a loose sheath derived from the capsule of Glisson, and is distributed with the subdivisions of those vessels to the various parts of the gland.

The lymphatics are known to be very plentiful, and in intimate relation to the blood vessels.