3. Examine the abdominal contents, identifying organs, mesenteries, and their relationships.

Visceral and Parietal Peritoneum Next turn your attention to the peritoneum. Like the pleura, it lines the abdominal cavity with a parietal layer shown here to be very thin and lining the abdominal wall. A visceral layer forms an outer covering of the organs, the serosa, in this case of the liver. Below the liver, notice the large, fat filled omental apron hanging down over the intestines. It is attached to the anterior surface of the transverse colon. Below that, notice the intestine itself, the small intestine, and its mesentery, which runs from the upper left quadrant to the lower right quadrant. It is a double layer of peritoneum containing fat and blood vessels and nerves. Its root lies from the duodenojejunal junction of the upper left to the cecum of the lower right.
Small Intestine Next turn your attention to the small intestine itself. It begins at the distal end of the duodenum, which is retroperitoneal. The junction with the duodenum is called the duodenojejunal junction and represents a sharp bend which is made possible or due to a ligament attaching to the distal end of the duodenum to the body wall. This is called the ligament of Trietz and will be seen later. From this point the small intestine gains a mesentery. It projects downward towards the lower right quadrant. You will notice that as you run the bowel and follow along, it is initially quite large and has much intestinal contents. As you follow the bowel along you will notice it becomes smaller because the intestinal contents are being absorbed. Also you will notice there is less fat proximally, than distally in the mesentery itself. As you approach the lower right quadrant, you will come to the distal end of the small intestine, some 20 feet from its proximal end. At this point you reach the cecum of the large intestine at the ileocecal junction. Returning to the beginning of the small bowel, reflect the proximal end of the jejunum to the right. This exposes a cavity behind the duodenojejunal junction. Here we can see the distal end of the jejunum in its retroperitoneal position as it approaches the duodenojejunal junction. It is covered with peritoneum. To the left of this, and behind the duodenojejunal junction there is a cave-like depression. This is the retro-duodenal fossa. This is sometimes the site of intra-abdominal hernias. It is made by the duodenojejunal junction and the surrounding structures. To the left of this fossa, notice a ridge, which is sometimes fat filled called the para-duodenal fold due to the inferior mesenteric vein.
Large Intestine Next observe the large intestine which, recall, begins at the ileocecal junction. It projects upward towards the liver as the ascending colon. Behind the liver, it takes a sharp bend towards the left. This is the right colic flexure or the hepatic flexure. At this point, the large intestine once again gains a mesentery and becomes the transverse colon. It may be quite redundant and hang down into the pelvis. It's suspended by the transverse mesocolon. Next observe the external features of the large intestine. You will notice that the longitudinal muscle coat is restricted to three bands of muscle, the teniae coli, this one being the teniae libra, the other two are attached to the omental apron and to the mesentery and are not visible. Between them, the wall is weaker and forms sacculations. At the margins of the teniae notice the projections of mesentery called epiploic appendages. These little fat filled structures are characteristic of the large intestine only. Compare the wall of the small intestine to that of the large and notice that they are lacking in the small intestine. Continue following the large intestine to the left. You will notice it takes a sharp bend downwards at the spleen. This is called the splenic flexure or the left colic flexure. At this point, the colon loses its mesentery again and becomes retroperitoneal. It is very short and extends to the iliac fossa where the sigmoid colon begins. This is named by its shape and has a pronounced mesentery, the sigmoid mesocolon. We have changed the orientation to look down into the pelvis here. Notice the sigmoid colon continues down into the pelvis in a straight portion called the rectum. As the intestine goes into the pelvis it loses its mesentery and the rectum lacks a mesentery. In front of it you may see in this cadaver, the uterus.
Fusion Fascia Next turn your attention to the cecum and the ascending colon to demonstrate the fusion fascia. Recall that the ascending colon had a mesentery and it was subsequently lost or fused to the posterior abdominal wall. At the lateral margin of the ascending colon and the cecum, the peritoneum can be cut and the mesentery of the intestine recreated. The avascular plane between the ascending colon and posterior body wall, which results is the layer we call the fusion fascia. The fusion fascial plane may be dissected using blunt instruments or one's fingers only. This same plane exists behind the descending colon and duodenum and pancreas and is the result of the loss of the mesentery. Notice as you dissect on this plane, no blood vessels are crossed and this is the advantage of the plane.