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 (Figure 28-9/9). Its projection extends between the ventral third of the sixth intercostal space to the upper part of the last (Figure 28-19/4).
The visceral surface is related to the reticulum, atrium ruminis, omasum, duodenum, gallbladder, and pancreas, most of which impress their form on the living organ; the indentations are retained by the specimen hardened in situ (see Figure 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 (Figure 28-23/8) tunnels through this edge of the liver and in its course receives its hepatic tributaries (Figure 28-9/79).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 (Figure 28-23/5). The blind vertex of the piriform gallbladder (Figure 28-23/74) projects beyond the lateral margin of the right lobe; it lies against the diaphragm opposite the ventral part of the tenth or eleventh rib.
The liver is retained in position by certain ligaments attaching it to the diaphragm and, more importantly, by visceral pressure. Its position may be verified by dullness on percussion over an area centered on the dorsal part of the eleventh rib and eleventh 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 (Figure 28-10/70). 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 so that biopsy specimens may be obtained with the least risk (Figure 28-19/2,4). The preferred site for puncture is through the eleventh 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; this route avoids 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 structure of the liver shows no species-specific features of importance. The organ 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; in many countries, most ostensibly normal animals show this evidence of fluke infestation (distomiasis).
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 prohibits 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.