=The Liver.=—The liver weighs from 3 to 3½ pounds. It lies beneath the diaphragm, more on the right side than on the left. Its posterior border, which rests against the last three ribs (separated from them by the diaphragm), is about 3 inches thick. Its anterior border is thin, and keeps close along the line of the ribs. If the organ is neither unduly enlarged nor squeezed out of its place owing to the use of a tight corset, it does not project below the ribs, save where it crosses the space between the rib-cartilages below the end of the breast-bone.
[Illustration: FIG. 7.—DIAGRAM OF A LOBULE OF THE LIVER DIVIDED VERTICALLY THROUGH ITS AXIS.
In its centre is a space, the intralobular vein, through which the blood falls into a branch of the hepatic vein, on its way to the heart. An interlobular branch of the portal vein, which brings the blood from the digestive organs, pours it by many smaller vessels over the surface of the lobule. It filters into the lobule through innumerable pseudo-capillary vessels, or spaces, between the radiating columns of liver-cells. Arterial blood is brought to the lobule by a twig of the hepatic artery. Bile is drained away from it by an affluent of the hepatic duct. In the lower part of the diagram seven liver-cells are shown, forming a divided column, magnified about 300 diameters. The cells are loaded with glycogen, and contain minute globules of fat. Red blood-corpuscles and two leucocytes are seen between the columns of liver-cells. One of the leucocytes has ingested two blood-corpuscles.]
The liver is supplied with blood by the hepatic artery. This vessel is small for so large an organ. Although responsible for the nutrition of the liver, it does not bring it the materials which are stored in its cells. A much larger supply of blood is derived from the portal vein, which breaks into capillaries, or, to speak more accurately, into sinuses, or pseudo-capillaries, in the liver. The blood, whether conveyed to the organ by the hepatic artery or by the portal vein, is drained away by the hepatic veins. The plan of structure of the liver is best understood when viewed with reference to the hepatic veins. These, if traced backwards, are found to break up into fairly straight vessels, each of which has a large number of lateral branches. Each of the lateral branches is in the centre of a mass of cells, which are packed round it in radiating columns. These masses, which have a diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual pressure the lobules are squeezed into a pentagonal or hexagonal form. The vein in the centre of the lobule is the intralobular vein. Turn now to the portal vein; this is seen to break up into branches which run between the lobules, and are therefore termed “interlobular veins.” The branches of the hepatic artery also run between the lobules, as do the radicles of the bile-duct. Each lobule is a liver in miniature. The blood of the portal vein, which has come from the spleen, in which red blood-corpuscles are destroyed, and from the stomach and intestines, from which it has absorbed the products of digestion, is poured over the surface of the lobule, to be filtered through into its central intralobular vein. In its passage from the interlobular veins (and branches of the hepatic artery) to the intralobular vein the blood is confined to radiating capillary channels; but since these merely prevent the escape of red blood-corpuscles without imposing any restrictions upon the exudation of blood-plasma, the portal blood is to all intents and purposes filtered through the columns of liver-cells. The body-substance of the liver-cells is soft, destitute of envelope, and capable, when free on the (warmed) stage of a microscope, of changing in form, somewhat after the manner of a leucocyte. Such cells have a great capacity for taking up the products of digestion. Possibly they take up and store fats and proteins, but undoubtedly it is their chief business to absorb sugar which accumulates as glycogen in their substance. The glycogen is handed out to the hepatic blood as required. The pigment which results from the disintegration of red blood-corpuscles in the spleen is secreted, along with the bile-salts, into minute channels, or canaliculi, which groove the flat surfaces of adjacent liver-cells. These canaliculi converge to the bile-ducts. The liver is therefore at the same time the storehouse of sugar which it takes up from the blood when it is in excess, and passes out to the blood when it is deficient, and an excretory organ which eliminates the refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and returns to the blood.