Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/248

230 ?30 DIGESTIVE ORGANS liver substance called pons hepatis. The under surface of the left lobe is smooth, and overlaps the anterior surface of the stomach. The under surface of the right lobe is divided into smaller lobes by fissures and fossae. Starting from about the middle of the longitudinal fissure is the portal or transverse fissure, which extends for from 3 to 4 inches across the under surface of the right lobe. It is the gate (porta] of the liver, the hihis or fissure of entrance into the organ of the portal vein, hepatic artery, hepatic duct, and hepatic nerves and lymphatics. A short distance to the right of that part of the longitudinal fissure in which the round ligament lies, is the fossa for the gall bladder, which is a depression on the under surface of the right lobe extending from the anterior border to the transverse fissure : in it the gall bladder lies. Extending somewhat obliquely from the posterior border of the liver, towards the transverse fissure, is a deep fossa for the inferior vena cava. Opening into the vena cava as it lies in this fossa are the trunks of the large hepatic veins from the substance of the liver. A portion of liver substance, which is bounded by the gall bladder, the longitudinal fissure, the transverse fissure, and the anterior border, forms a four-sided lobe called lobus quadratus. Another portion, bounded by the transverse fissure, the posterior border, the vena cava, and the longitudinal fissure, is the lobus Spigdii. A thin pro longation of liver substance continuous with the lobus Spigelii, and running obliquely between the fossa for the inferior cava and the transverse fissure, is the lob us caudatus. Structure of the Liver. The liver is a solid -organ, of a brownish-red colour. It is composed of the ramifications of the portal vein, of the portal capillaries, the hepatic vein, the hepatic artery, the hepatic duct, of secreting cells, nerves, and lymphatics. These several structures are bound together by connective tissue, and the organ is invested by the peritoneum. The liver possesses two coats, a serous and a fibrous. The serous or external coat is a part of the peritoneal membrane, and forms an almost complete investment for the liver. It is reflected from the transverse fissure as the gastro-hepatic omentum, and from the upper surface and the posterior border as the falciform, coronary, and right and left lateral ligaments of the liver. The fibrous coat, or tunica propria, is immediately sub jacent to the serous coat. When carefully raised from the liver delicate processes of areolar tissue may be seen to pass from its deep surface into the substance of the organ. At the transverse fissure it is prolonged into the liver as a very distinct sheath, enveloping the portal vein, hepatic artery, hepatic duct, nerves, and lymphatics. This sheath is named the capsule of Glisson, and is prolonged through out the substance of the organ, along the ramifications of the portal vein and the structures that accompany it. Lobules of the Liver. To the naked eye the substance of the liver doss not present a homogeneous aspect, but is mottled, and mapped out into multitudes of small areas or lobules, the hepatic lobules or leaflets. The lobules of the liver are irregular polygons, and vary in size from ^V^ 1 * -^ths of an inch. In man and the mammalia generally the lobules are imperfectly separated from each other by the interlobular vessels and duct, and a scarcely appreciable quantity of areolar connective tissue. In the pig, camel, and polar bear, each lobule is circumscribed by a definite capsule of connective tissue. As a lobule of the liver is a liver in miniature, and as the structure of the entire liver is the sum of the structure of its lobules, it will be necessary to examine with care the constituent parts of a lobule, and the arrangement of the vessels, duct, and nerves which pass to and from it. An hepatic lobule is composed of blood-vessels, secreting cells, and bile-ducts, with perhaps nerves and lymphatics. The blood-vessels will first be considered. The portal vein conveys to the liver the venous blood rom the stomach, spleen, pancreas, gall bladder, and small and large intestine. It ascends to the transverse fissure, and before it enters the liver divides into two branches, one for the right and one for the left lobe. In its course within the liver, the portal vein divides and subdivides after the manner of an artery. It is closely accompanied by the hepatic artery and duct, and, along with them, is invested by the fibrous sheath, cnlled Glisson s capsule. The terminal branches of the portal vein run between the lobules, and are named, from their position, the interlobular branches. The interlobular branches lie around the circumference of a lobule, and anastomose with each other. They partly terminate directly in a capillary network situated within the lobule, and partly give off fine branches, which enter the lobule before they end in the capillary network. The intralobular capillaries form a close network, and converge from the periphery of the lobule, where they spring from the interlobular branches of the portal vein, to the centre of the lobule, where they terminate in the intralobular or central vein, one of the rootlets of the hepatic vein. In man, where the lobules are not separ ated from each other by a distinct capsule, the capillaries of one lobule to some extent communicate with those of adjacent lobules. The hepatic artery closely accompanies the portal vein, and Fio.9.-Trrasvee section through the hepatic .. . f. lobules. T, i, ^, interlobular veins ending in the divides Intralobular capillaries ; r, c, central veins joined by the intralobular capillaries. At a, a the capillaries of one lobule communicate with those adjacent to it. . into two branches, for the right and left lobes. It is the nutrient artery of the liver, and gives off three series of branches :(a) vaginal branches, which are dis tributed to the walls of the portal vein, the hepatic duct, and to Glisson s capsule, probably also to the wall of the hepatic vein; they end in a capillary network in these structures, from which vaginal veins arise that terminate in the portal vein ; (i) capsidar branches, which are dis tributed to the fibrous coat of the liver, and end in a capillary network, from which arise capsular veins that join the portal vein ; (c) interlobular branches of the hepatic artery lie along with the interlobular branches of the portal vein, and end in the capillary network within the lobules. The hepatic vein arises within the substance of the liver from the intralobu lar capillaries. In the centre of each lobule is the intra lobular or central vein. It traverses the axis of the lobule, and leaves it to join a small vein running immediately under the bases of adjacent lobules, which, from its position, is named the Sublobular Vein. FIG. 10. Vertical section through two hop.itic snVilnVmlir lobules of a pig. c, c, central veins receiving SUDlODUiar t])e intralobular cai)i i larics. ,, suWo bular vein; veins then join to- ct, Interlobular connective tissue forming the ,1 j i capsules of the lobules : t, i, inte-rlobular veins. gether, and lorm larger vessels, which are the trunks of the hepatic vein, or