Block 3: DISSECTION 2 CELIAC TRUNK, JEJUNUM/ILEUM, LARGE INTESTINE, DUODENUM, PANCREAS, PORTAL VEIN; MOBILIZATION OF THE LIVER

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1 1 Block 3: DISSECTION 2 CELIAC TRUNK, JEJUNUM/ILEUM, LARGE INTESTINE, DUODENUM, PANCREAS, PORTAL VEIN; MOBILIZATION OF THE LIVER Attempt to complete as much as you can of the dissection explained in the following three directions; identification of some of the structures may be easier later, once the liver is more mobile or even after the gastrointestinal tract has been removed from the abdominal cavity. Use a probe to trace the common bile duct superiorly; you may be able to identify the cystic duct and the common hepatic duct. Follow the common hepatic duct superiorly until you can see its formation from the right and left hepatic ducts. Clean the proper hepatic artery and trace it to the porta hepatis, where it divides into the right and left hepatic arteries (this branching pattern is highly variable). Follow the proper hepatic artery inferiorly and verify that it is a branch of the common hepatic artery. (Please see Figures 13 and 14 in Lab Guide 1.) Try to identify two additional arteries arise in the area the cystic artery and the right gastric artery. It may now be necessary to remove most of the left lobe of the liver to gain access to the celiac trunk, the artery that arises from the abdominal aorta just below the diaphragm. Prior to removing any of the liver, review your knowledge of the falciform and coronary ligaments of the liver. The attachment of the left lobe of the liver to the underside of the diaphragm, via the left triangular ligament, must be released. Extend your fingers between the superior surface of the left lobe of the liver and the underside of the diaphragm. Locate the left triangular ligament and, further to the right, the falciform ligament. Do not perform this portion of the dissection if you are able to access the celiac trunk without sectioning the liver. Transect the left triangular ligament. Make sure that you do not interrupt the falciform ligament in the process. Next, protect the portal triad by placing a forceps loosely around it one prong of the forceps in the foramen of Winslow and the other covering the anterior surface of the structures. Consult Figure A before cutting. Using a large spatula, and staying to the right of the ligamentum teres and ligamentum venosum, slice through the left lobe of the liver from anterior to posterior (see image for location of cut). Use scissors to finish the cut. You should be able to completely avoid cutting the diaphragm, stomach, and caudate and quadrate lobes of the liver. Block 3, Lab 2 Page 1

2 2 Incise the hepatogastric ligament (of the lesser omentum) and then the peritoneum of the posterior wall of the omental bursa and expose the celiac trunk. Identify the branches of the celiac trunk (Figure B). o Common Hepatic artery o Left gastric artery o Splenic artery Block 3, Lab 2 Page 2

3 3 Observe that the gastroduodenal artery is a branch of the common hepatic artery. Follow the gastroduodenal artery behind the first part of the duodenum and determine that it gives rise to the right gastro-omental artery and the superior pancreatico-duodenal artery. Now follow the common hepatic artery to the left towards its origin from the celiac trunk. The celiac trunk is a short, stout branch of the abdominal aorta just inferior to the diaphragm (at the level of the T12 vertebra). Note that the celiac trunk is surrounded by a very dense plexus of autonomic nerves, the celiac plexus. Try to identify the celiac ganglion on the celiac trunk. Follow the left gastric artery from the celiac trunk towards the gastro-esophageal junction (it may be difficult to find as it is usually quite deep); there, the artery turns sharply inferior to reach the lesser curvature. Follow it to the right (within the lesser omentum). o Note any esophageal branches of the left gastric artery. Follow the splenic artery to the left behind the stomach, as it courses toward the spleen against the posterior body wall. The artery seems to disappear, as it lies partly embedded in the superior border of the pancreas; do not separate the artery from the pancreas at this time. (The pancreas is somewhat delicate and can easily shred.) The splenic artery gives rise to short gastric arteries, try to identify these as well as the left gastro-omental artery. (You can find this latter artery in the greater omentum, about an inch from the greater curvature of the stomach.) Find the right gastro-omental artery near the right end of the greater curvature, and note the anastomosis between the two gastro-omental arteries within the greater omentum. Follow the right gastro-omental artery further to the right and notice, once again that it arises from the gastroduodenal artery. At the junction of esophagus and stomach, identify the anterior and posterior vagal trunks. Observe the hepatic branch of the anterior vagal trunk and the distribution of the anterior vagal trunk to the stomach. Note the termination of the posterior vagal trunk in the celiac plexus (Figure C). Block 3, Lab 2 Page 3

4 4 Now that you have displayed and studied the arterial blood supply to the midgut (SMA) and hindgut (IMA), return to the jejunum, ileum and colon to identify distinctive features of each. Review the gross anatomical differences between the jejunum and ileum on the cadaver: 1. the wall of the jejunum is thicker than that of the ileum 2. the arterial arcades of the ileum are more complex than those of the jejunum 3. the vasa recta of the ileum are shorter than those of the jejunum. 4. Note also that the mesentery of the ileum contains more fat than the mesentery of the jejunum. At a site about 1/3 of the way along the length of the jejunum, make an incision in its wall. Make a similar incision in the wall of the ileum about 2/3 of the way along its length. Clean the inside of the jejunum and ileum at the incision points, and compare the number and arrangement of circular folds of the mucosa in each (Figures 1a and 1b). Block 3, Lab 2 Page 4

5 5 Large Intestine Review the components of the large intestine: o Cecum (with appendix), o Ascending colon o Transverse colon o Descending colon o Rectum o Anal canal Direct your attention to the right lower quadrant of the abdomen and to the ileocecal junction (Figure 2) (Latin, caecus = blind). Block 3, Lab 2 Page 5

6 6 Find the vermiform appendix. Approximately 60% of the time, the appendix is found posterior to the cecum, in the retrocecal recess. Look for an appendicular artery within the mesoappendix. Squeeze the cecum in order to propel its contents distally and tightly tie a string around it to prevent the contents from moving proximally. Open the cecum; clean it and note the ileocecal valve (Figure 2). Observe these external features of the large intestine, features that easily distinguish it from the small intestine o Taeniae coli o Haustra o Omental appendices (epiploic appendices) Duodenum and Pancreas Follow the jejunum proximally to identify, once again, the duodenojejunal junction. Observe the suspensory muscle of the duodenum (Ligament of Treitz) (Figure 3). Turn the transverse colon and its mesocolon upwards over the costal margin so that you may view the parietal peritoneum of the posterior body wall. Use blunt dissection to remove the peritoneum and connective tissue superficial to the duodenum and the pancreas, and identify the four parts of the duodenum (Figure 3): o The superior (first) part (at L1 vertebral level) o The descending (second) part (at L2 and L3 vertebral levels) o The horizontal (third) part (at L3 vertebral level) o The ascending (fourth) part (at L2 and L3 vertebral levels). Block 3, Lab 2 Page 6

7 7 Identify the pancreas within the curve of the duodenum (Figure 4). This secondarily retroperitoneal organ lies against lumbar vertebral bodies (mainly L2). Identify the parts of the pancreas: o head o uncinate process o neck o body o tail. Try to follow the SMA from the aorta just above the pancreas, behind the neck of the pancreas but in front of the uncinate process of the pancreas, and then anterior to the third (horizontal) part of the duodenum. Extend the incision you made in the anterior wall of the stomach into the first part of the duodenum and observe the: o pyloric antrum o pyloric canal o pyloric sphincter and pyloric orifice (Figure 5). Now, make a longitudinal incision in the anterior wall of the descending part of the duodenum. With the aid of a piece of damp cloth, clean the lumen. Locate the major duodenal papilla on the medial wall of this part of the duodenum. The hepatopancreatic ampulla opens here. Block 3, Lab 2 Page 7

8 8 A minor, or accessory, pancreatic duct usually opens separately into the lumen of the duodenum, about 2 cm. superior to the major papilla. Try to observe this as well but it may be very difficult to see. Review the manner of emptying of the common bile duct and the main pancreatic duct into the lumen of the duodenum. Remember, the shared opening of these two ducts lies at the major duodenal papilla. Observe the interior of the duodenum and the plicae circulares, or circular folds distal to the major duodenal papilla (Figure 5). Use a probe to dissect into the anterior surface of the head of the pancreas to find the main pancreatic duct (Figure 6). Trace it to the left for a short distance into the neck and body of the pancreas. Then follow the main pancreatic duct towards the second part of the duodenum, and observe that is joined by the common bile duct. The two ducts, embedded in the wall of the duodenum, empty into the hepatopancreatic ampulla (of Vater); this short, dilated chamber protrudes into the lumen of the duodenum as the major duodenal papilla (Figure 5). Block 3, Lab 2 Page 8

9 9 Return to the celiac trunk, SMA and their branches to the duodenum and pancreas, which could not be examined earlier because both the duodenum and pancreas are retroperitoneal in position (Figure 7). Identify these arteries: o superior pancreaticoduodenal artery (from the gastroduodenal artery) o the greater pancreatic artery (from the splenic artery) o the dorsal pancreatic artery (from the splenic artery) o the inferior pancreaticoduodenal artery (from the SMA) Block 3, Lab 2 Page 9

10 10 Portal Vein Find the portal vein in the free border of the lesser omentum, behind the hepatic artery and the bile ducts. In order to observe the formation of the portal vein, posterior to the head and neck of the pancreas, lift the stomach, then lift and reflect the body and tail of the pancreas upwards. Identify the veins that form the portal vein: the splenic vein and the superior mesenteric vein. Try to observe the inferior mesenteric vein that drains into the splenic vein, or into the junction of the superior mesenteric vein with the splenic vein, or directly into the superior mesenteric vein (Figure 8). Recall that the portal vein carries venous blood to the liver from the abdominal portion of the G-I tract and from the spleen and pancreas. Liver To study the surface features of the liver, it must be detached from the diaphragm. With scissors, cut the falciform ligament along its attachment to the anterior abdominal wall. Extend the cut superiorly and also cut the right triangular ligament along the inferior surface of the diaphragm. Extend your fingers between the superior surface of the liver and the underside of the diaphragm and transect any remaining connections between the liver and the diaphragm. Locate the inferior vena cava posterior to the liver (Figure 9). Transect the inferior vena cava (IVC) between the liver and the diaphragm (the inferior vena cava passes through the diaphragm to enter the right atrium of the heart). Elevate the inferior border of the liver so that you can reach the inferior vena cava just below the liver, and transect the vessel again, as close to the inferior surface of the liver as possible. These two cuts through the IVC will leave a short portion of it within the liver. The liver should now be relatively mobile, but remain attached to the other abdominal viscera by the hepatic artery, the portal vein and the bile duct. Move the liver carefully so that these structures are not torn. Examine the liver and note its right lobe. Block 3, Lab 2 Page 10

11 11 Identify the sharp inferior border that separates the diaphragmatic surface from the visceral surface. Observe the bare area of the liver (Figure 9) on its diaphragmatic surface, and, around the bare area, note the cut edges of the coronary ligament (including its right and left triangular portions). On the visceral surface of the liver, note, in addition to the right and left lobes, the caudate lobe and the quadrate lobe. The outlines of these four lobes of the liver are established by an H-shaped set of fissures and structures. Note that the porta hepatis constitutes the horizontal arm of the H, and that the quadrate and caudate lobes lie within the vertical members of the H. The quadrate lobe is adjacent to the gallbladder, while the caudate lobe is adjacent to the IVC (Figure 9). (The lobes are demarcated by surface features but do not correspond to the hepatic segments that are established by the branching of the bile ducts, the hepatic arteries and the portal vein.) Identify the falciform ligament and the round ligament of the liver; note the ligamentum venosum. Examine the small segment of the IVC in its groove on the liver; note that two or three large hepatic veins drain directly into it. (Be sure you can differentiate between hepatic veins and the hepatic portal vein.) Study the impressions of the various abdominal organs on the visceral surface of the liver (Figure 10). Block 3, Lab 2 Page 11

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