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The Ruminant Gastrointestinal Tract 3: Liver, pancreas, lesser omentum. Development, Topography, and Function. General. The digestive system develops in the embryo as a simple tube extending the length of the body. Its epithelial lining is formed by the endoderm, its coats of muscle, its blood and lymphatic vessels and the connective tissues supporting them arise from the mesoderm. The nerves which innervate these tissues have their origin in the neural ectoderm. Liver and pancreas develop as branching, tubular extensions from the duodenum. The form of the digestive tube at an early embryonic age looks like this: Drawn by David Stewart Geary Liver. The liver of the newborn calf develops initially within the transverse septum of the embryo. and, as it enlarges, pulls away from the septum as a separate organ. It 1 remains caudal to the diaphragm, with a narrow linear strip of its diaphragmatic surface remaining adherent to the diaphragm. Initially, the liver’s long axis is in a transverse plane. In this position, it most resembles the human liver. The left lobe is left of the quadrate lobe; the right lobe is to the right. The quadrate lobe is essentially central in position, between the umbilical fissure and the fossa for the gall bladder. The caudate lobe is caudal to the right lobe. With growth of the rumen and reticulum, the liver is pushed to the right and rotated so that its left lobe is ventral and its right lobe is dorsal. The liver is displaced almost entirely to the right of the midline. On its caudal, visceral, face, the caudate lobe is disposed in an inverted “L” shape. Its caudate process forms the short arm of the inverted L, and the free end of the process is directed right. The main part of the caudate lobe is cranioventral and is medial to the porta of the liver; its craniomost extent reaches to the esophageal notch. The papillary process is small. Note: The division of the liver into lobes has comparative significance but is useful chiefly in permitting a more precise description of liver anatomy and the location of disease processes. Fig. A. Liver developing within septum transversum and separating from it. Diagr. Lehrbuch der Enwicklungsgeschichte der Haustiere, Zietzschmann and Krolling,1955: Verlag Paul Parey, Berlin and Hamburg Fig. B. Bovine embryo, 34 days, 12 mm length. Body wall cut away in part. Fig. 1. Lehrbuch der Enwicklungsgeschichte der Haustiere, Zietzschmann and Krolling,1955: Verlag Paul Parey, Berlin and Hamburg diaphragm pleural cavity stomach mesonephros heart s lung septum transversum liver dev’ng liver peritoneal Fig. 23. Bovine liver, mature, parietal (diaphragmatic) face and cavity visceral face. From Sisson, modified. Numbering of this figure and those following continues from Ruminant Gastrointestinal Tract 2. 2 Parietal Face Visceral Face esophageal impression portal vein, hepatic art., common bile duct umbilical fissure b diaphragm esophagus lesser omentum, line of hepatic reflection Biliary duct system. In cattle, right and left hepatic ducts receive the smaller bile ducts from the right and left halves of the liver, respectively. They join, forming the common hepatic duct which joins the cystic duct that leads from the gall bladder. The common bile duct (ductus choledochus) that is formed by their union then passes to the duodenum just beyond the sigmoid loop (Fig. 24). 3 Fig. 24. The biliary duct system in cattle. common bile duct right hepatic duct left hepatic duct common hepatic duct cystic duct gall bladder Pancreas. The right lobe of the pancreas lies in the mesoduodenum dorsal to the descending duodenum and the left lobe lies within the deep wall of the greater omenum, where the omentum is applied to and fused with the descending colon (see Fig. 14, 360o rotation, and Fig. 20a to see the close relationship of the deep wall of the greater omentum with the transverse colon; in ruminants and the horse, the deep wall (with the left lobe of the pancreas) is regularly fused with the cranial aspect of the transverse colon). Right and left lobes of the pancreas meet at the body of the pancreas, which is where the greater omentum is continuous with the 4 mesoduodenum of the descending colon. The body of the pancreas is perforated by the portal vein and forms the caudal margin of the omental foramen. The portal vein, with the hepatic artery, which passes ventral to the vein, forms always the ventral margin of the omental foramen in the domestic animals (Fig. 25). Fig. 25. Relations at the omental foramen of the horse (shown here) are very similar to the relations in ruminants. Fig. 26. Goat. Pancreas, portal vein, hepatic artery, common bile duct. Dorsal view. pancreas, right lobe pancreas, left lobe panc. duct 5 hepatic artery duodenum, sigmoid loop spleen rumen common bile duct The pancreas arises in two places in the embryo: 1) as a ventral pancreatic bud from a common origin with the hepatic bud (the hepatopancreatic bud of the dudodenum) and 2) as a dorsal pancreatic bud farther distally, directly from the duodenum. It is the branching of these two buds that gives rise to the duct system and the exocrine pancreatic acini. The endocrine islet cells separate from the proliferating cells and come to lie between the acini. The origin of the ventral pancreatic bud becomes the pancreatic duct; the origin of the dorsal pancreatic bud becomes the accessory pancreatic duct. Within the pancreas the duct systems usually communicate and in cattle, and in sheep and goats, ordinarily only one duct persists. In cattle, the pancreatic duct usually undergoes involution and only the accessory pancreatic duct is present. Occasionally a small pancreatic duct is also present, which probably is due to a failure of the two systems to unite completely. The accessory pancreatic duct passes from the caudal end of the right lobe of the pancreas and joins the descending duodenum distal to the common bile duct. In the small ruminants, it is the accessory pancreatic duct that undergoes involution. The pancreatic duct of the sheep and goat is present, well developed, and joins the common bile duct 2 – 3 cm proximal to that duct’s union with the duodenum just caudal to the sigmoid loop. The sigmoid loop itself is brought about by the greater growth of the cranial part of the duodenum relative to the limited growth of the common bile duct. Thus, to find the common bile duct in ruminants, one first finds the caudal end of the sigmoid loop. Fig. 27. 8mm sheep embryo. Diagrammatic. From Lehrbuch der Entwicklungsgeschichte der Haustiere, O. Zietzschmann, O. Krölling, 1955; Verlag Paul Parey. dorsal pancreas ventral pancreas liver gut heart 6 accessory pancreatic duct from caudal end of right lobe of pancr. caudate process of caudate lobe caudal vena cava Fig. 28. Visceral face of bovine liver. The probe is in the omental foramen. From Sisson. portal vein common bile duc at end of sigmoid umbilical vein remnant Lesser omentum. 7 Fig. 29. The mammalian embryo at an early stage of development. Dorsally, a dorsal mesogastrium, which becomes the greater omentum (green), passes to the greater curvature of the stomach; it is continuous caudally with the mesentery (orange. Ventrally, the ventral mesogastrium (yellow) passes from the lesser curvature of the stomach and the first part of the duodenum to the abdominal face of the diaphragm and the ventral body wall up to the umbilical stalk. It ends caudally in a free border. The heavy dashed line indicates the position of the septum transversum, which forms the ventral part of the diaphragm and is a cranial attachment of the ventral mesogastrium. 8 septum transversum caudal free border Fig. 30. From the hepatopancreatic bud, the hepatic (liver) and ventral pancreatic buds grow into the part of the ventral mesogastrium that extends to the first part of the duodenum. Actually, different from the diagram above, most of the liver growth takes place within the septum transversum and then gradually separates from the septum. The growth of the liver separates the ventral mesogastrium into the ventral falciform ligament, which conveys the umbilical vein from the umbilicus to the liver, and the more dorsal lesser omentum, which extends from the liver to the lesser curvature of the stomach and the first part of the duodenum. Both the falciform ligament and the lesser omentum end caudally in a free border. After birth, the umbilical vein becomes non-functional in most animals and in mature ruminants and the dog and cat usually is lost entirely. In 80% of horses, it remains patent, small and draining veins of the ventral body wall. In ruminants, the falciform ligament itself is also usually lost except for a narrow line on the of the liver leadsthe from the omentum umbilical fissure. On diaphragmatic the completion surface of development, wethat expect lesser to extend from the liver to the stomach and first part of the duodenum and to end in a free caudal border. Fig. 31. Omentums and omental foramen. In the lower right figure, the double arrow is in the omental foramen. lesser omentum line of atttachment to the stomach atttachment to liver free caudal border (at omental foramen) 9 lesser omentum greater omentum Fig. 32. Relations at the omental foramen of the horse (shown here) with the liver in place are similar to the relations in ruminants. 10 The falciform ligament containing the remnant of the umbilical vein is usually entirely absent but for a thin line on the parietal face of the liver. If intact, we would expect to see the ligament extending from the liver’s umbilical fissure (see Fig. 23) and parietal face to the body wall from the esophagus to the umbilicus. Like this: Fig. 33. The position of an intact falciform ligament. The writer has seen a completely intact falciform ligament in the sheep and only once. Fig. 28, from Sisson, shows a (rare) remnant of the umbilical vein in the ox. Displaced abomasum and abomasal torsion. The abomasum joins the cranial part of the duodenum at the pylorus. Together, these two parts of the gastrointestinal tract form a continuous tube, held on the one end by the omaso-abomasal junction and, on the other end, by the common bile duct. The duct is strong and does not stretch. The result is that with displacement of the abomasum, these two parts are relatively fixed in position but the tube of smooth muscle of abomasum and cranial part of the duodenum is more mobile. The same is true in abomasal torsion. Left displacement of the abomasum and torsion of the abomasum are shown in Figs. 33a – c. Fig. 33a. Ruminant stomach, left view. omasum abomasum 11 Fig. 33b. Left abomasal displacement. Fig. 33c. As 33b, left abomasal displacement, in color. Fig. 33d. One form of abomasal torsion. 12 Addendum: A common procedure for surgical correction of displaced abomasum is to put the animal in dorsal recumbency. This frees the displaced abomasum, allowing it to “float” ventralward toward the surgical incision, which is a linear ventral abdominal one. Freed of the weight of the rumen, gas may be removed and the abomasum begins to contract, reducing its size. The incision is closed with the surgeon grasping the abomasal wall with the suture, which fixes the abomasum in a fairly normal, ventral, position as the abdomen is closed. This usually effectively avoids future displacement. Understanding the anatomy and physiology of the ruminant gastrointestinal tract is useful, necessary, in clinical diagnosis and in a number of clinical procedures, including surgical correction of abomasal displacement and torsion, traumatic gastritis, relief of bloat, and grain overload. The developmental aspects of ruminant anatomy are presented chiefly to give an understanding of how things “got this way.” This may also help to explain those occasional circumstances in which development has been incomplete or altered. 13 14