Liver, an organ characterized by the presence of cells secreting bile, and found in some form or other throughout almost the whole animal series. These cells may be scattered over the intestinal canal, restricted within its follicles, contained in elongated branching tubes or caeca, or collected in loosely lobulated masses, as in invertebrates; or they may be clustered together in lobules and consolidated into a firm and compact organ, as in man and other vertebrates. The liver in man occupies the right hypochondriac and epigastric regions, extending partly into the left hypochondrium, below the diaphragm; it is above the stomach, duodenum, arch of the colon, gall bladder, and right kidney, and in front of the aorta and lower vena cava. Its size is large, and its normal weight from 3 to 4 lbs.; its form is irregular, being elongated transversely, flattened from above downward, very thick behind and thin in front; its tissue is dense and of a reddish brown color. The upper surface is convex, in contact with the diaphragm, and divided by the suspensory ligament or fold of peritoneum into two unequal parts, of which the right lobe is considerably larger than the left.
The lower surface is irregularly concave, presenting from left to right a superficial depression corresponding to the upper wall of the stomach; the antero-posterior or longitudinal fissure, which lodges in the foetus the umbilical vein and the ductus venosus, shrunk into mere fibrous cords in the adult; the transverse fissure, at right angles to the preceding, in which are situated the vena porta3, the hepatic, artery and biliary duct, and numerous nervous filaments and lymphatic vessels; the short fissure for the vena cava, near the posterior border; the small lobe of Spigelius, an irregularly triangular portion behind the transverse fissure; the lobus quadratus, in front of the transverse fissure, the gall bladder lying between it and the right lobe; and on the right lobe, depressions corresponding to the right portion of the transverse colon, and to the right kidney and supra-renal capsule. In the carnivora and rodents, portions of the liver rudimentary in man are highly developed; in these there are five distinct parts, a central or principal lobe, and a right and left lateral lobe, each with a lobular appendage.
The liver is in great part covered with a shining peritoneal or serous envelope; an investment of areolar tissue also is spread over the organ, extending into the interior, and forming thin but dense sheaths to the vessels and canals, called the capsule of Glisson. - The blood vessels of the liver are the hepatic artery and veins and the vena portae; in the foetus the maternal blood is brought to the liver by the umbilical vein; the lymphatics are numerous, and the nervous filaments are supplied from the pneumogastric nerve and the hepatic plexus of the sympathetic. The proper tissue of the liver is composed of a great number of polygonal masses, about 1/12 or 1/16 of an inch in diameter, generally called lobules or acini, of a foliated appearance from the branching distribution of the hepatic veins in the centre of each; in the spaces left between the polygonal lobules lie the branches of the vena portae, hepatic artery, and duct, each lobule giving the characteristic structure of the organ. The vena portoe, which receives the venous blood from the digestive-organs, divides and subdivides in the liver like an artery, till it reaches the interlobular spaces, forming a freely anastomosing network throughout the organ, and constituting the intralobular veins; after ramifying on the capsules they enter the lobules and become lobular veins, their terminal branches ending in the intralobular or hepatic vein.
The hepatic artery, a branch of the great cce-liac axis from the aorta, sends its branches to all parts of the organ, supplying the walls of the vessels and ducts, and the lobules through the interlobular spaces. The lobules or acini are made up of a great number of minute glandular cells, the "liver cells;" transparent rounded or polygonal bodies, 1/1200 of an inch in diameter, slightly granular in texture, and each one containing a round or oval nucleus and nucleolus. These cells often contain granules of yellow coloring matter, and one, two, or three oil globules of various sizes. - An abundance of minute capillary blood vessels penetrate into the substance of the lobule, ramify around and among the liver cells, and thus bring the blood into intimate relation with the glandular tissue of the organ. The blood coming from the alimentary canal by the portal vein reaches the hepatic vein only after having passed through the capillary circulation of the liver itself; and it is during this passage that various important changes take place having for their object partly the modification of the blood itself, and partly the production of new substances in the tissue of the liver.
In the first place, the blood, in passing through the liver, loses a great portion of its fibrine (a fact fully established by the observations of Simon, Lehmann, Bernard, and Brown-Sequard), so that the blood drawn from the hepatic vein always has less fibrine than that taken from the portal vein, and sometimes has so completely disappeared that the hepatic blood will not coagulate at all. What becomes of this fibrine is not positively known; but it is doubtless transformed into some other substance, required either for the nutrition of the liver itself, or for use in some other part of the circulation. Secondly, a kind of animal sugar is formed in the substance of the liver, and this independently of any vegetable or saccharine materials taken with the food. This sugar appears first in the solid substance of the hepatic tissue, where it exists on an average, at the moment of death, in the proportion of at least 2 1/2 parts in 1,000. The sugar, however, is not formed directly from the albuminous ingredients of the liver tissue, but is produced by the catalytic transformation of a peculiar substance termed "glycogene," which is itself produced in the hepatic tissue and afterward converted into sugar by a kind of fermentation.
Thirdly, certain ingredients of the bile, such as cholesterine, and the various mineral salts, already existing in the blood, are separated from it by the action of the liver, and exuded, together with the watery parts of the secretion, into the biliary ducts, there to take part in the constitution of the bile. Fourthly, there are other ingredients of the bile, and these the most important ones, such as the tauro-cholate and glyco-cholate of soda, which do not preexist in the blood, but are formed by the liver itself, in the substance of its glandular tissue. Thence they are exuded, with the other constituent parts of the biliary fluid accumulated in the bile ducts, and are either at once discharged into the intestine, or stored up in the gall bladder for subsequent use. The bile is collected from the glandular tissue of the liver by a great number of minute biliary ducts, which converge in such a way as to form larger and larger branches. Two of these main branches, coming one from the right, the other from the left lobe of the liver, emerge into the great transverse fissure of the organ, and there unite to form a single duct known as the " hepatic duct." The hepatic duct descends for about two inches toward the small intestine, when it is joined at an acute angle by another duct coming from the gall bladder, and termed the "cystic duct." The main canal formed by the union of the two, and called the "common biliary duct," then pursues its course, and penetrating obliquely the walls of the duodenum, or upper portion of the small intestine, terminates by a rounded orifice upon the internal surface of the duodenum, about four inches below the pyloric extremity of the stomach. - The liver is an organ which is continuously active, discharging its secretion during the intervals of digestion, as well as while that process is going on.
Its activity, however, according to the experiments of Bidder and Schmidt and others, increases perceptibly several hours after digestion has commenced, and continues at its height for a certain period, again to diminish, though not entirely to cease, until the next digestive period comes round. It is liable to various diseases, such as inflammation and cancerous growths; a peculiar degeneration termed " cirrhosis," in which it becomes contracted, hard, and irregular in shape; a fibrinous or waxy infiltration; and a fatty,degeneration, in which the proportion of fat globules naturally existing in the hepatic cells is so much increased that the whole organ loses its glandular texture and fails to perform the necessary functions which belong to it as a secreting organ.
The Liver viewed from below. - a, vena cava; b, vena portae; c, bile duct; d, hepatic artery; I, gall bladder.
A Section of part of the Liver to show the hepatic vein (H. V.), with the lobules or acini (L,.) of the liver, seated upon its walls, and sending their intralobular veins into it.
Glandular Cells from the Human Liver.