The liver is an extremely vascular organ, and receives its blood supply from two distinct vessels, the portal vein and the hepatic artery, while the blood is returned from it into the inferior vena cava by the hepatic vein. Its secretion, the bile, is conveyed from it by the hepatic duct, either directly into the intestines, or, when digestion is not going on, into the cystic duct, and thence into the gall bladder, where it accumulates until required. The portal vein, hepatic artery, and hepatic duct branch together throughout the liver, while the hepatic vein and its tributaries run by themselves. At the transverse fissure it is merged into the areolar investment called Glisson’s capsule, which surrounds the portal vein, hepatic artery, and hepatic duct, as they enter at this part, and accompanies them in their branches through the substance of the liver.
The liver is made up of small roundish or oval portions called lobules, each of which is about 1⁄20 of an inch in diameter, and composed of minute branches of the portal vein, hepatic artery, hepatic duct, and hepatic vein; while the interstices of these vessels are filled by liver cells. These cells, which make up a great portion of the substance of the organ, are of rounded or polygonal form; about 1⁄800 to 1⁄1000 of an inch in diameter.
The function of the liver is the secretion of bile. The bile is a somewhat viscid fluid of a yellow, or greenish-yellow, color, a strongly bitter taste, and when fresh a scarcely perceptible odor. It has a neutral or slightly alkaline reaction, and its specific gravity is 1.020.
The composition of human bile is:
| Water, | 859.2 |
| Solids, | 140.8 |
| 1,000 |
The solids are:
| Biliary acids combined with alkalies (bilin), | 91.5 |
| Fat, | 9.2 |
| Cholestrin, | 2.6 |
| Mucus and coloring matter, | 29.8 |
| Salts, | 7.7 |
| 140.8 |
Bile is distinguished from the other alimentary secretions by the entire absence of proteids. The chemical composition of bilin, as compared with the organic parts of blood, is:
| Carb. | Hyd. | Nitr. | Oxy. | Sul. | |||
| Bilin atoms, | 76 | 66 | 2 | 22 | |||
| Blood, | 48 | 36 | 6 | 14 | |||
| Coloring matter, |  | Biliverdin, | 16 | 20 | 2 | 5 | |
| Glycocholic acid, | 26 | 43 | 1 | 6 | |||
| Taurocholic acid, | 26 | 45 | 1 | 7 | 1 | ||
There seems to be some relationship between the coloring matters of the blood and bile; and it may be added, between these and that of the urine also; so that it is possible they may be, all of them, varieties of the same pigment, or derived from the same source.