Lab Instructions

The Liver, epi table (all species)

Please review wet liver specimens for study and comparison between species.

The liver of the pig, unlike that of other domestic mammals, does not contact the right kidney, so there is no distinct renal impression.

The lobe projecting caudally toward you, that is dorsal to the porta, is the caudate lobe, which in the ruminant has two parts, a caudate process and a papillary process. The caudate process is especially large in the ox. The horse and the pig do not have a papillary process.

The round ligament enters the liver between the left lobe and the quadrate lobe. The left lobe, in the pig and the horse is divided into medial and lateral parts. The gall bladder tucks in between the right lobe, (divided into medial and lateral parts in the pig), and the quadrate lobe. As you recall, the portal vein receives blood from the stomach, spleen, pancreas, and intestines that is laden with absorbed nutrients (except triglycerides), and may also contain infectious organisms and/or toxic material. The liver can metabolically convert these nutrients and store and release over time in the appropriate form to the general circulation. Similarly, the liver, functioning as a filter, can capture infectious organisms and help detoxify certain toxins. As a result, the liver is affected directly by many toxins, and abscesses of the liver are not uncommon, especially in feedlot cattle. The hepatic artery delivers the liver’s nutritional blood supply.

Blood that enters the liver through the portal vein and hepatic artery returns to the general circulation through the hepatic veins to the caudal vena cava. The parenchyma of the pig liver is supported by a pronounced fibrous stroma, which is evidenced on the surface by the more obvious lobulated appearance of the pig liver. Depending on the lifestyle and husbandry of the animal, the livers of domestic ungulates frequently bear the scars of parasitic larval migrations.

Horse liver

Ruminant liver

Pig liver

Pancreas, all species

Locate the pancreas near the portal vein, in the mesoduodenum. FYI – In the horse and pig, the pancreas completely surrounds the portal vein, forming the annulus pancreatic.  Like the liver, the exocrine part of the pancreas develops as an outgrowth of the embryonic gut (duodenum); however, in the case of the pancreas, there are two primordial buds, dorsal and ventral. The dorsal bud grows into the mesoduodenum (becoming the right lobe), while the ventral bud is closely associated with the developing liver in the ventral mesogastrium (becoming the left lobe). By definition, the dorsal bud (right lobe) forms the accessory pancreatic duct and the ventral bud (left lobe) forms the pancreatic duct. The two buds grow together near the cranial flexure of the duodenum to form the body of the pancreas, and their individual duct systems anastomose with one another. Because only one excretory duct is really needed, some animals choose to do away with the other one. If one day you need to know exactly which ducts remain and what papillae they empty at in the duodenum, please feel capable of looking that information up :). Note that a major duodenal papilla is expected in all species studied, for discharge of bile from the bile duct.

Spleen – all species

Although its shape differs considerably in our domestic mammals, its position is similar. Having developed in the dorsal mesogastrium, the spleen is closely associated with the greater curvature of the stomach (well I guess that makes sense and once again knowing a little developmental anatomy goes a long way).

In the horse, note/palpate the fibrous connection between the spleen and the left kidney. This is the nephrosplenic ligament, a site of entrapment of the ascending colon (i.e. the large colon). Also note the attachment of the spleen to the stomach – this is the gastrosplenic ligament, which can entrap and strangulate small intestine when it passes through a rent in this structure.

Clinical Application

Clinical relevance: The large colon of the horse can displace to the left dorsal abdomen and become hooked over the nephrosplenic ligament – i.e. nephrosplenic entrapment (NSE) aka left dorsal displacement of the large colon (LDDLC); the small intestine can become entrapped and strangulated through a rent in the gastrosplenic ligament.

.

Abdominal Topography

Now that we have an understanding of the abdominal cavity and its viscera, the last step is to picture it all together and visualize its relative location in the abdomen.  Before any procedures of the abdomen are performed, or imaging is interpreted, the veterinarian must know what is expected where.

Recall that the normal peritoneal cavity is a potential space, containing only a thin layer of peritoneal fluid, spread out around the organs. (How much fluid? Just enough.)

Left Side of the Horse

Most of the dorsal flank area is occupied by a mixture of jejunum (smooth wall) and descending colon (haustrated) resting on the ascending colon. The pelvic flexure of the colon may be noted near the pelvic inlet in the left flank. Part of the spleen may be seen projecting from under the last rib. With the diaphragm reflected, in the cranial aspect of the abdomen you should be able to see the spleen, the greater curvature of the stomach, and part of the left lobe of the liver.

Right Side of the Horse

The upper right flank area is occupied by the base of the cecum, which more ventrally is continuous with the body of the cecum. Part of the right ventral colon may also be noted in the concavity of the cecum, which inclines medially. With the diaphragm reflected, you should be able to see the descending duodenum, the right kidney, the right lobe of the liver, and more of the ascending colon.

Left Side of the Ruminant

In the adult, almost the entire left side of the abdominal cavity (and more) is occupied by the rumen, with the dorsal sac located in the paralumbar fossa. The ventral sac is covered by the superficial wall of the greater omentum. With the diaphragm reflected, you should be able to see the spleen, the cranial sac of the rumen, and the reticulum.

In the newborn calf, the rumen is small and deflated. Most of the left flank is occupied by small intestine that may be partly covered by the greater omentum, which attaches ventrally to the abomasum. Depending on the amount of sublumbar, retroperitoneal fat, you may also be able to identify the left kidney. With the diaphragm reflected, you should be able to see the spleen and the reticulum.

Right Side of the Ruminant

Typically, the only organ identifiable on initial reflection of the parietal peritoneum is the descending duodenum. Dorsal to the duodenum is the mesoduodenum, which contains fat and the right lobe of the pancreas; ventral to the duodenum is the superficial wall of the greater omentum, which typically contains a great deal of fat. The caudal free fold of the greater omentum may also be identified. With the diaphragm reflected, you should be able to see the liver and gallbladder, part of the abomasum, and the omasum, which is covered by the lesser omentum.

In the newborn calf, most of the right flank area is occupied by the intestines, but the large liver of the newborn projects caudal to the last rib. Ventral to the costal arch, you can see the abomasum, which is crossed by the large, flat round ligament of the liver contained in the free edge of the falciform ligament.  In the fetus, this is the umbilical vein, which transports oxygenated blood into the fetus from the placenta.  The notch for the entry of the round ligament (atrophied umbilical vein) into the liver marks the division between the quadrate and left lobes.

Left Side of the Pig

Unlike that of other adult domestic ungulates, the cecum of the pig is typically located near the dorsal part of the left flank and closely associated with the cecum is the spiral part of the ascending colon. More ventrally, you can see many loops of the jejunum (smooth) and more haustrated coils of the spiral colon. With the diaphragm reflected, you should be able to see the left kidney, the spleen, the stomach, and the liver.

Right Side of the Pig

In the non-pregnant animal, most of the caudal part of the abdominal cavity is filled with intestines, primarily jejunum. Part of the liver and the gallbladder project beyond the costal arch. With the diaphragm reflected, you should be able to see the right kidney and more liver.