Three blood-vessels are concerned in the circulation of the liver. Two carry blood to it - the hepatic artery and the portal vein - while the third, the hepatic vein, returns the blood which has circulated through the gland to the posterior vena cava, which it joins just before that vessel perforates the diaphragm to discharge its blood into the right auricle of the heart. The hepatic artery divides and subdivides to form capillaries which join those of the portal vein in the lobules. The portal vein is a large trunk that contains the blood returning from the stomach and intestines, and from the spleen and pancreas. Having reached the inferior surface of the liver it penetrates into its substance, and, as an exception to the usual behaviour of veins, instead of joining a larger vein, which in this case would be the vena cava, it proceeds to break up as if it were an artery, into smaller and still smaller branches, which run between the lobules and are hence called interlobular veins (3, 3, fig. 90). From these, minute branches are given off which enter the lobules and there form a net-work of capillary vessels ramifying among the cells (2, 2, fig. 90), and then, after uniting and reuniting, form a blood-vessel which runs down the centre of the lobule and is known as the intralobular vein (1, fig. 90). From this the blood passes out of the lobules into a set of veins beneath it (sublobular veins), and then enters the hepatic vein. The blood as it traverses this close net-work of capillaries within the lobules comes into intimate relations with the gland cells which occupy the spaces between them.
Fig. 89.-Hepatic Cells.
1 Hepatic cell. - Nucleus. 3 Granules of fat, pigment, and glycogen. 4 Bile capillaries.
Fig. 90. - Section of Lobule of Human Liver.
1 Section of intralobular or central vein. 2 Its smaller tributaries, which receive the blood from 3 and convey it to 1. 3 Interlobular or peripheric branches of the vena porta.
The bile-ducts commence in the form of an extremely delicate network of tubes which ramify over and between the gland cells in the lobule, and take up from them the bile they have secreted. After uniting to form larger and larger tubes they at length terminate in the common bile-duct. This opens into the first part of the intestine just beyond the stomach in common with the duct of the pancreas. No gall-bladder is present in the horse.
The bile is a glairy fluid of yellow, green, or olive-brown colour, alkaline reaction, nauseous smell, and bitter taste. It has a specific gravity of about 1.030. It has been estimated that a horse secretes about 1/66 part of its weight of bile in twenty-four hours, or about 12 or 14 lbs. The flow of bile into the intestine increases about an hour after food has been ingested, and the quantity continues to be large for some hours, when it gradually declines till after the next meal.