Refer back to Lab 2.9 and check that the Celiac Trunk dissection is complete before moving on to Superior Mesenteric Artery. It will be best to utilize images from the Atlas for this portion of dissection.

Superior Mesenteric Artery: (2nd ed Fig. 14.4, 14.11, 14.20) (3rd ed. Fig. 16.4, 16.11, 16.20)

1. Reflect the greater omentum and transverse colon superiorly over the costal margin.
2. Position the coils of the jejunum and ileum to the left side of the abdomen so the right side of the mesentery faces anteriorly.
3. Make a small incision through the anterior layer of the peritoneum on the right side of the mesentery, and use forceps to peel the peritoneum away to expose the branches of the superior mesenteric a.
4. Remove the parietal peritoneum from the posterior abdominal wall on the right side of the mesentery as far laterally as the ascending colon.
5. Use blunt dissection to trace the superior mesenteric a. proximally, and observe that it crosses anterior to the third part of the duodenum.
6. Use blunt dissection to clean the branches of the superior mesenteric a., which are embedded in a variable amount of mesenteric fat.
7. Identify the superior mesenteric v. positioned along the right side of the superior mesenteric a. Posterior to the pancreas, the superior mesenteric v. joins the splenic v. to form the hepatic portal v.
8. Dissect a few of the branches of the superior mesenteric a. with the 15 to 18 intestinal aa., originating from the left side of the superior mesenteric a. and supplying the jejunum and ileum. Intestinal aa. end in straight terminal branches called vasa recta (straight aa.), which are interconnected by arterial arcades.

Small Intestine: (2nd ed. Fig. 13.5-13.12) (3rd ed. Fig. 15.5-15.12)

1. Palpate the small intestine, and note that the wall of the jejunum is thicker than the wall of the ileum and that the overall diameter of the diameter of the jejunum is larger.
2. Identify the termination of the ileum where it empties into the cecum at the ileocecal junction.

Inferior Mesenteric Artery: (2nd ed. Fig. 14.5, 14.12, 14.21) (3rd ed. Fig. 16.5, 16.12, 16.21)

1. Reflect the transverse colon and greater omentum superiorly over the costal margin to expose the posterior surface of the transverse mesocolon.
2. Move the small intestine to the right so the descending colon is visible from the left colic flexure to the sigmoid colon.
3. Identify the inferior mesenteric a. where it arises from the abdominal aorta, commonly posterior to the third part of the duodenum. If you have trouble finding it, find one of its branches in the sigmoid mesocolon, and trace that branch back to the main vessel and then proceed with the dissection of the peripheral branches.

The left ureter could be mistaken for the inferior mesenteric a. or one of its branches because the inferior mesenteric a. and v. and the ureter all lie in the retroperitoneal space. The vessels can be differentiated from the ureter because they descend in the abdominal cavity anterior to the ureter.

1. Use a probe to clean some of the branches of the inferior mesenteric a. beginning with the left colic a., which supplies the descending colon and the left third of the transverse colon.
2. Observe that the tributaries of the inferior mesenteric v. correspond to the branches of the inferior mesenteric a. The inferior mesenteric v. ascends on the left side of the inferior mesenteric a. and passes posterior to the pancreas, where it joins either the splenic v. or the superior mesenteric v. as a tributary of the hepatic portal v.

Large Intestine: (2nd ed. Fig. 13.13-13.16) (3rd ed. Fig. 15.13-15.16)

1. Beginning in the right lower quadrant, identify the various components of the large intestine, starting with the cecum.
2. Identify the appendix (vermiform appendix) attached to the end of the cecum in one of several positions.
3. Identify the ascending colon extending from the cecum to the right colic flexure and the transverse colon from the right colic flexure to the left colic flexure. Observe that the left colic flexure lies at a more superior level than the right colic flexure due to the location of the liver.
4. Trace the descending colon from the left colic flexure to the left lower quadrant, and recall that it is a secondarily retroperitoneal organ.
5. In the left lower quadrant, find the sigmoid colon and where it ends in the pelvis and is continuous as the rectum. Observe that the sigmoid colon has a mesentery (sigmoid mesocolon) that is mobile.
6. The rectum and anal canal are contained entirely within the pelvic cavity and will be dissected with the pelvic viscera.
7. On the external surface of the large intestine, observe three features that distinguish it from the small intestine:
   1. Teniae coli: three narrow bands of longitudinal muscle running the length of the large intestine
   2. Haustra: outpouchings of the wall of the colon
   3. Omental appendices (epiploic appendages): small accumulations of fat covered by visceral peritoneum

Pancreas: (2nd ed. Fig. 13.29, 13.30, 13.32, 14.10) (3rd ed. Fig. 15.29, 15.30, 15.32, 16.10)

1. Identify the pancreas within the bend of the duodenum. Note that it is a secondarily retroperitoneal organ that lies across the midline
2. Identify the head of the pancreas adjacent to the descending duodenum. Observe that the inferior vena cava lies posterior to the head of the pancreas.
3. Identify the body of the pancreas, extending from right to left and slightly superiorly as it crosses the abdominal aorta.
4. Identify the tail of the pancreas, the narrow left end of the gland, which lies in the splenorenal ligament and contacts the hilum of the spleen.
5. Return to the celiac trunk, and follow the splenic a. as it passes to the left along the superior border of the pancreas.

Hepatic Portal Vein: (2nd ed. Fig. 14.15, 14.18, 14.19) (3rd ed. Fig. 16.15, 16.18, 16.19)

1. Identify the splenic v. where it courses posterior to the pancreas and inferior to the splenic a. Use a probe or blunt dissection to isolate the splenic v. posterior to the body of the pancreas. Observe that the splenic v. is typically flatter and straighter than the more tortuous thicker splenic a.
2. Follow the splenic v. to the right where it is joined by the superior mesenteric v. to form the hepatic portal v.
3. Return to the field of distribution of the inferior mesenteric v., and follow it superiorly. Note that the inferior mesenteric v. may join the superior mesenteric v., the splenic v., or the junction of the superior mesenteric and splenic vv.

Removal of the Gastrointestinal Tract:

1. Use your fingers to separate the fascia and peritoneum surrounding the distal end of the sigmoid colon and rectum, and gently pull them anteriorly away from the sacrum.
2. Tie two strings 4 cm apart around the distal end of the sigmoid colon, close to the rectum, or as far inferiorly as possible in the pelvic cavity. Make an effort to make the knots tight but do not tighten the string so much as to sever the colon.
3. Use scissors to cut the sigmoid colon between the strings to ensure as little remaining fecal matter as possible enters the dissection field.
4. Cut any fascia preventing the sigmoid colon from being elevated out of the pelvic cavity.
5. Inferior to the thoracic diaphragm, tie one string around the esophagus, making sure to not pull the knot so tight as to sever it, and cut the esophagus superior to the string.
6. Use scissors to cut the celiac trunk close to the abdominal aorta, and attempt to leave a short stump of the artery.
7. Use scissors to cut both the superior mesenteric and inferior mesenteric aa. near the aorta, leaving a 1-cm stump on each.
8. Free the stomach by cutting through any peritoneal attachments it may still have to the posterior abdominal wall.
9. Grasp posterior to the spleen, and carefully free the splenic vessels, tail of the pancreas, and body of the pancreas from the posterior abdominal wall as you gently pull anteriorly and medially.
10. Use scissors to cut the parietal peritoneum lateral to the ascending and descending colon, and use your fingers to free the colon from the posterior abdominal wall. Roll the ascending and descending colon toward the midline and use your fingers to loosen its blood vessels from the posterior abdominal wall.
11. The gastrointestinal tract, liver, pancreas, and spleen should now be free of attachments. Remove them from the abdominal cavity en bloc. Support the liver, and be careful not to twist or tear the structures in the hepatoduodenal ligament.
12. Arrange the abdominal viscera on a dissecting table or large tray in anatomical position, and study the parts from the anterior view.
13. Trace the branches of the celiac trunk, superior mesenteric a., and inferior mesenteric a. to their areas of distribution.
14. Observe the formation and termination of the hepatic portal v., noting the differences between the branching pattern of the arteries and the veins.
15. The viscera may be stored in a large plastic bag or in the abdominal cavity. Wet these specimens frequently with mold-inhibiting solution.

Clinical Correlations:

Portal Hypertension: The hepatic portal system of veins has no valves. When the hepatic portal v. becomes blocked, blood pressure increases in the hepatic portal system (portal hypertension) and its tributaries become engorged. Portal hypertension causes hemorrhoids and varicose gastric and esophageal vv. Bleeding from ruptured gastroesophageal varices is a dangerous complication of portal hypertension.

Four portosystemic (portacaval) anastomoses exist within the abdomen to allow for alternative routes of venous return: the gastroesophageal (left gastric v./esophageal vv./azygos v.), the anorectal (superior rectal v./middle and inferior rectal vv.), the paraumbilical (paraumbilical vv./superficial epigastric vv.), and the retroperitoneal (colic vv./retroperitoneal vv.).

Important Structures:

Small Intestine

Duodenum

Jejunum

Ileum

Large intestine

Cecum

Ileocecal junction

Appendix

Ascending colon

Right colic flexure

Transverse colon

Left colic flexure

Descending colon

Sigmoid colon

Teniae coli

Haustra

Omental appendices

Rectum

Pancreas

Head, Body, Tail

Vaculature

Superior mesenteric a.

Right colic a.

Middle colic a.

Left colic a.

Intestinal aa.

Vasa recta

Inferior mesenteric a.

Superior mesenteric v.

Hepatic portal v.

Inferior mesenteric v.

Splenic v.