The liver is the largest gland in the body, representing between 1% and 5% of total body weight in adult mammals (see Appendix 1) and up to 6% in newborn animals. The relative size of the liver is related to the percentage of protein and fat in the animal’s diet, since the liver plays a critical role in metabolism of these molecules. Normally, the liver lies under the rib cage, sandwiched between the diaphragm and the other abdominal viscera (stomach, intestines, kidneys). Larger livers, like those in carnivores, span the entire cranial abdomen, while the ruminant liver is confined to the right half of the abdominal cavity. Liver lobation varies by species (see Table 9.1 and Figure 9.1) and is determined more by branching of blood vessels than by external appearance. Carnivores and pigs have six liver lobes: Right lateral, right medial, left lateral, left medial, caudate, and quadrate. In addition, the caudate lobe is divided into a papillary and a caudate process, which are connected by an isthmus. Horses, rabbits, and camelids have a single right lobe, ruminants have a single right and a single left lobe, and rodents have a left and right lateral lobe and a single medial lobe. The liver lobes are fused near the central porta hepatis, but the degree of fusion varies between animals. Table 9.1 Species-specific features of the liver and pancreas. Yes Cervidae have a hepatic diverticulum that is not capable of storing and releasing bile Figure 9.1 Lobation patterns of livers from different species. The diaphragmatic surface of the liver is shown. The carnivore and pig livers have six lobes: Right lateral (RL), right medial (RM), left medial (LM), left lateral (LL), quadrate (Q), and caudate (C). The gallbladder (GB) is located between the right medial and quadrate lobes. Horses, camelids, and rabbits have a single right lobe (R). Cattle have a single right and single left (L). Sheep and goats (not shown) have only three lobes: left, right, and caudate. Horses and camelids lack a gallbladder. The liver lobes are attached to the diaphragm by ligaments. The falciform ligament, a remnant of the umbilical vein derived from the ventral mesentery, extends caudally from the liver and diaphragm to the umbilicus. In adults, the falciform ligament is filled with fat. The hepatorenal ligament is variable in occurrence but when present extends from the medial aspect of the renal fossa in the caudate process to the ventral surface of the right kidney. The normal liver is uniformly dark red-brown, smooth, and firm but friable. Careful inspection reveals a diffuse, finely mottled appearance due to the contrast in color between the darker hepatic parenchyma and the paler connective tissue that outlines the hepatic lobules, which are approximately 1 mm in diameter. Pig livers contain an extensive network of fibrous connective tissue separating the lobules and giving the capsular surface a slightly roughened appearance (Figure 9.2). This normal feature can be mistaken for pathologic fibrosis. Figure 9.2 Pig liver. The pig liver has more interlobular connective tissue than other species, creating a roughened appearance to the capsular surface. The portal vein (Figure 9.3) is formed by the confluence of the cranial and caudal mesenteric veins and the gastrosplenic vein. It originates in the root of the mesojejunum and runs cranially with the hepatic artery to form part of the ventral boundary of the epiploic foramen. The common hepatic artery arises from the celiac artery and branches into proper hepatic arteries that enter the hilus of the liver. Figure 9.3 The portal vein arises in the mesojejunum and passes over the angle of the pancreas before entering the liver. The gallbladder, when present, is located between the right medial (or right) and quadrate lobes. In cats, the gall bladder is occasionally bilobed (Figure 9.4). Domestic mammals lacking a gallbladder include equids, camelids, and rats. In evolution, the loss of a gallbladder in some herbivore species is believed to be caused by a mutation in a gene coding for Sox17, a transcription factor that regulates development of the gallbladder. Herbivores and omnivores can survive without a gallbladder; however, a similar mutation in a carnivore would be fatal. Figure 9.4 A bilobed gall bladder is a relatively common incidental finding in a cat. The cranial, rounded, blind end of the gall bladder is referred to as the fundus, the large middle portion is the body, and the slender, tapering extremity is the neck. The cystic duct extends from the neck of the gall bladder to the junction with the first bile duct tributary from the liver. From here the excretory channel becomes known as the common bile duct (Figure 9.5). The distal end of the common bile duct enters the duodenum obliquely and forms a low, longitudinal ridge that represents the intramural course. The bile duct terminates on a small mound of tissue at the end of the ridge termed the major duodenal papilla. To one side is the slit-like opening of the minor pancreatic bile duct.
Chapter 9
The Liver and Pancreas
9.1 Anatomy Review and Species Differences
9.1.1 The Liver
Carnivore
Ruminant
Equid
Camelid
Pig
Rodent/Rabbit
Liver lobes
Left lateral Left medial Right lateral Right medial Quadrate Caudate
Cattle: Left Right Quadrate Caudate Sheep and goats: Left Right Caudate
Left lateral Left medial Right Quadrate Caudate
Left lateral Left medial Right Quadrate Caudate
Left lateral Left medial Right lateral Right medial Quadrate Caudate
Rodent: Left lateral Median Right lateral Caudate Rabbit: Left lateral Left medial Right lateral Right medial Quadrate Caudate
Gallbladder?
Yes
No
No
Yes
Rat: no Other rodents and rabbits: yes
Main pancreatic duct(s): PD = Pancreatic duct; APD = Accessory pancreatic duct
Dog: APD (Eichhorn and Boyden 1955) Cat and ferret: PD
Cattle: APD Sheep and goats: PD
PD
PD
APD (Ferrer et al. 2008)
Rodent: PD (Higashiyama et al. 2018) Rabbit: APD 
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