» The Liver
The liver of the adult animal lies almost entirely within the right half of the abdomen, related to the caudal face of the diaphragm and under cover of the ribs (Fig. 28.9/9). Its projection extends between the ventral third of the sixth intercostal space and the upper part of the last (Fig.
28.19/4). The visceral surface is related to the reticulum, atrium ruminis, omasum, duodenum, gallbladder, and pancreas, most of which leave impressions on the living organ. The impressions are retained by the specimen hardened in situ (Fig. 28.23). The thick dorsal border extends farthest caudally and is partly fashioned by the blunt caudate process; this is separated from the main mass by a recess into which fits the cranial pole of the right kidney. The medial (originally dorsal) border follows the midline rather closely; toward its lower end it is marked by an impression that gives passage to the esophagus, and below this a small part spreads across into the left half of the abdomen. The caudal vena cava (Fig. 28.23/8) tunnels through this edge of the liver and in its course receives its hepatic tributaries (Fig. 28.9/10).The thin lateral border is marked by the fissure that divided the right and left "halves" of the fetal organ, and in most adult cattle this provides entrance for the round ligament, the remains of the umbilical vein (Fig. 28.23/5). The blind vertex of the piriform gallbladder (Fig. 28.23/14) projects beyond the lateral margin of the right lobe. It lies against the diaphragm opposite the ventral part of the 10th or 11th rib.
The liver is retained in position by certain ligaments attaching it to the diaphragm and, more important, by visceral pressure. Its position may be verified by dullness on percussion over an area centered on the dorsal part of the 11th rib and 11th intercostal space. The percussion area is small in relation to the size of the organ and corresponds to the area of direct contact with the body wall (Fig.
28.10/10). A detectable increase in its extent generally signifies a disproportionate enlargement of the organ.The relationship of the liver to the right pleural sac should be noted to reduce the risk during collection of biopsy specimens (Fig. 28.19/2 and 4). The preferred site for taking a liver biopsy is to puncture through the 11th intercostal space in the plane of the lower part of the coxal tuber. The trocar is directed to meet the diaphragm and thus the liver at right angles so that a clean puncture is ensured and to avoid the larger vessels. The relatively larger size of the liver of the young calf may allow the organ to be palpated behind the last rib.
The ruminant liver has no significant species-specific features. It is enclosed within a tough fibrous capsule, but the extensions into the parenchyma do not outline obvious lobules as in the liver of the pig. The hepatic ducts join together in the portal region to form a single channel from which the cystic duct branches to the gallbladder. The continuation beyond this junction constitutes the bile duct, which enters the duodenum. The most superficial hepatic ducts may be visible through the covering liver tissue, especially when thickened by disease. The cattle liver may show fluke infestation (distomiasis) in many countries.
The liver receives blood from the hepatic artery and portal vein, which enter at the porta. Blood from both sources returns to the general circulation through the hepatic veins, which enter the embedded portion of the caudal vena cava. The openings of the major hepatic veins are arranged in two widely separated clusters. Intrahepatic anastomoses between the two sets provide a potential collateral pathway that becomes important when the intervening stretch of the caudal vena cava is obstructed.
The efferent lymphatic vessels pass mainly to the hepatic group of nodes scattered about the porta; the lymph thence drains into the visceral radicle of the cisterna chyli. Some lymph is routed via accessory hepatic (on the caudal vena cava) and caudal mediastinal nodes.
Although the livers of the sheep and the goat generally resemble that of cattle, size alone prevents confusion of the adult organs. They are distinguished from the liver of the calf by the much deeper umbilical fissure, narrower and less bluntly shaped caudate process, more elongated gallbladder, and absence of the sizable vestige of the umbilical vein that is evident on the liver of the young calf. An extensive contact with the abomasum is retained throughout life.