Dissector Answers - Peritoneal Cavity & Intestines

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Describe the basic organization of the peritoneum and peritoneal cavity, including subdivions, mesenteries, and ligaments.
  2. Describe the organization of the digestive tract and its parts.
  3. Describe the basic anatomy of the large and small intestines, including blood supply and internal structure.

Learning Objectives and Explanations:

1. Describe the basic organization of the peritoneum and peritoneal cavity, including subdivisions, mesenteries, and ligaments. (W&B 436-446, N 264,267,273,276,335,337, TG 5-12A, 5-12B, 5-13, 5-14, 5-15, 5-16B, 5-16C, 5-29, 5-31, 5-42, 5-43)

The peritoneum, like the pericardium and pleura, is a serous membrane that invests viscera. It is comprised of parietal and visceral peritoneum. There are many specializations of the peritoneum. All of the special structures that will be covered here are composed of two layers of peritoneum (much like the pulmonary ligament). They differ in location and what they connect. (Greek, peritonaion = stretch around)

Mesenteries: result from the invagination of "intraperitoneal" organs into the sac. The mesenteries connect viscera to the posterior abdominal wall and are VERY important in that they conduct blood vessels and nerves. (There are no vessels within the peritoneal cavity, of course.) The mesentery of the colon is usually called the "mesocolon". For example, we speak of the "transverse mesocolon" and the "sigmoid mesocolon". (The other parts of the colon are not completely invested by peritoneum, and are therefore "retroperitoneal".) Also, often "the mesentery" refers specifically to the mesentery of the small intestine. (Greek, mes = in the midst of, enteron = intestine)

Omenta: generally refers to a free fold of peritoneum. This is exemplified by the greater omentum, which attaches to the stomach, droops far down into the abdominal cavity, and comes back up to attach to the transverse colon. The lesser omentum, on the other hand, is not really "free". It connects the stomach to the liver, and its membranous portion is called the hepatogastric ligament.

Ligaments: connect organs to one another or to the abdominal wall. Usually, in this case, we are referring to the stomach or the liver.

There are many folds and fossae that can be described in the peritoneum. Some have already been discussed (see Abdominal Wall Dissector Answers). Others are inconstant and not extremely important (see W&B 445-446). A few structures should be noted, due to their clinical importance. (Appendectomy anyone?) They are found in the vicinity of the cecum, are listed here superior to inferior:

  • anterior cecal fold (constant): connects "the mesentery" to anterior cecum. It contains the anterior cecal artery.
  • superior ileocecal fossa (constant): between the anterior cecal fold and ileocecal junction.
  • ileocecal fold (variable): connects terminal ileum to cecum. It crosses the root of the vermiform appendix and is avascular. (Latin, vermiform appendix is literally "worm-shaped appendage")
  • inferior ileocecal fossa (variable): between ileocecal fold and mesoappendix.
  • mesoappendix: a continuation of "the mesentery" that carries the blood supply for the appendix - the appendicular artery.

The peritoneal cavity, analogous to the pleural cavity, is a space between the two layers of peritoneum. Fluid in this space provides lubrication, allowing the abdominal viscera to move around. The peritoneal cavity contains no viscera. There are two major subdivisions of the peritoneal cavity:

Greater sac: the main part of the peritoneal cavity. It is divided into two sections. The supracolic and infracolic compartments are divided by the transverse mesocolon. The infracolic compartment is further divided into right and left infracolic spaces by "the mesentery".

Lesser sac (omental bursa): lies posterior to the stomach. It is divided into two sections, a superior and an inferior recess.

The passage between the greater and lesser sacs is the omental (epiploic) foramen, posterior to the hepatoduodenal ligament.

2. Describe the organization of the digestive tract and its parts. (W&B 446-448, N 228,261,276,290,302, TG 5-12A, 5-12B, 5-13, 5-14, 5-15, 5-16B, 5-16C, 5-29, 5-31, 5-42, 5-43)

3. Describe the basic anatomy of the large and small intestines, including blood supply and internal structure. (W&B 477-488, N 291,295,296,300,301,302, TG 5-12A, 5-12B, 5-13, 5-14, 5-15, 5-16B, 5-16C, 5-29, 5-43)

The small intestine comprises the duodenum, jejunum, and ileum. The first and shortest portion of the small intestine is the duodenum, but due to its intimate relationship with the liver and pancreas, it will be considered with those structures. The remainder of the small intestine is divided into the jejunum and the ileum. The border between them is not distinct, but the jejunum is usually thought to lie mostly in the left upper quadrant, comprising about 40% of the total length, while the ileum lies mostly in the right lower quadrant, providing the remaining 60%.

The small intestine contains circular folds of tissue that are covered with villi. (This will become important in histology.) Added to the fact that the long tube is coiled in the abdominal cavity, there is A LOT of surface area over which nutrients can be absorbed. It is completely supplied by the superior mesenteric artery (SMA), via 15 or so intestinal (jejunal and ileal) arteries. These smaller branches eventually turn into arterial arches (arcades), which resemble a colonnade, providing collateral circulation to different sections of the organ. Straight arteries (vasa recta) then jump from the arcades to the tissue.

The colon is considered in five sections. From the cecum, where the small intestine attaches, the ascending colon travels up to the right colic (hepatic) flexure. From there, the transverse colon moves across to the left colic (splenic) flexure. Then, the descending colon travels down, becoming the sigmoid colon which ends in the rectum.

The colon, or large intestine, has three features that distinguish it from the small intestine:

  1. Its longitudinal muscle forms three teniae coli, instead of being continuous all the way around.
  2. The teniae coli are actually shorter than the colon, so the large intestine forms haustra coli (bulges, sacculations).
  3. The surface of the colon has fat-filled tabs, called omental appendages (appendix epiploica).
    4. Note: The teniae coli come together at the base of the appendix, which can help in locating the appendix on the cecum.

The colon receives blood from both the superior mesenteric artery and the inferior mesenteric artery (IMA). The SMA supplies the colon up to the splenic flexure, via the following branches:

The IMA supplies the rest of the colon via:

If you draw pretty pictures like this on occasion, especially when connectivity is important, you'll be a happy medical student! Thanks to Susana Gonzalez!

Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless.

The Small Intestine - The Large Intestine - The Abdominal Aorta - Surface Anatomy of the Abdomen -Surface Markings of the Abdomen

Questions and Answers:

5. Differentiate between the abdominal cavity and the peritoneal cavity. (N 335,336,337, TG 5-42A, 5-42B, 5-43A, 5-43B)

The peritoneal cavity is within the abdominal cavity, which is continuous with the pelvic cavity. The peritoneal cavity is a potential space, like the pleural and pericardial cavities, between the parietal and visceral layers of the peritoneum. It allows the gut to move with a minimum of friction.
6. What is the suspensory ligament of the duodenum? (N 262, TG 5-26)
The suspensory ligament of the duodenum (more properly called the suspensory muscle of the duodenum or the Ligament of Treitz) is the structure that suspends the duodenojejunal flexure from the diaphragm. It is continuous with the right crus of the diaphragm.
7. Locate the ileocecal junction. At what level is it found? (N 260,273, TG 5-15)
The ileocecal junction is just posterior to the anterior superior iliac spine, in the iliac fossa, which is at L4-L5. Another way to say that is that it is right on or just inferior to the transtubercular line, in the right inguinal region.
8. Examine "the mesentery" (of the jejunum and ileum), noting its body wall attachment. How long is this attachment? What structures does it cross? (N 263,295, TG 5-13, 5-14A, 5-29)

The root of the mesentery is approximately 15cm long. It passes inferiorly and to the right, from the duodenojejunal junction to the iliocecal junction, crossing:

9. Review the development of "the mesentery" from the primitive dorsal mesentery. (Learning Module)

Initially the entire primordial gut is suspended in the center of the abdominal cavity by a "dorsal mesentery", which is attached to the midline of the posterior body wall. As development progresses, some things start twisting around, some become retroperitoneal, and some do both! When all is said and done, most of the duodenum is retroperitoneal, as is the ascending and descending parts of the colon. Between these two points we have an intraperitoneal structure, the small intestine, that is attached to the body wall via "the mesentery". (That is why the root of "the mesentery" runs from the duodenojejunal junction to the ileocecal junction.) Although the attachment does not change much in length after that, the small intestine grows an enormous amount. That is why "the mesentery" has to "fan out" to keep up with it.

To get a good idea of what is going on with all of the "twisting around", check out the several gut rotation movies in Animations.

10. What is the location of the small intestine within the peritoneal cavity? (N 260,261, TG 5-16A, 5-42A, 5-42B, 5-43A, 5-43B)
Most of the jejunum is located in the left upper quadrant, while the ileum is mostly in the right lower quadrant.
11. What parts of the large intestine are peritoneal. What parts are retroperitoneal? (N 273,276, TG 5-29, 5-43A, 5-43B)
The ascending and descending colon are retroperitoneal. The cecum, transverse colon and the sigmoid colon are peritoneal, and are suspended by mesenteries.
12. Consider the derivation of the anterior cecal fold (vascular), the ileocecal fold, the mesoappendix, the transverse mesocolon, and the sigmoid mesocolon with regard to gut development. (N 273, TG 5-15)
Much like "the mesentery", these structures are derived from the primitive dorsal mesentery. Again, the gut rotates and some structures, like the ascending and descending colon, become retroperitoneal. The remaining intraperitoneal structures; the cecum, appendix, transverse colon, and sigmoid colon; are attached to the body wall via the anterior cecal fold (vascular) and ileocecal fold, the mesoappendix, the transverse mesocolon, and the sigmoid mesocolon respectively.
13. Why are some parts peritoneal or retroperitoneal?
As gut development proceeds some parts get squished against the posterior body wall and stick there.
14. What happens to the primitive mesentery of the retroperitoneal part of the large intestine? (N 276, TG 5-29)
The mesentery belonging to the parts that are squished against the wall becomes fused with the peritoneal lining of the wall, and is now called "fusion fascia".
15. What is the significance of fusion fascia?
Fusion fascia is a relatively avascular connective tissue plane through which nerves and vessels pass, usually parallel to its plane, to reach target structures. The clinical significance is that it allows retroperitoneal structures to be mobilized during surgery (as long as blunt dissection is performed, so that vessels and nerves are not sectioned). Very small veins may traverse across the plane of the fusion fascia. These so-called "veins of Retzius" may enlarge in cases of portal hypertension.
16. Where does the superior mesenteric artery terminate? (N 295,296, TG 5-13, 5-15)
The ileocolic artery is the terminal branch of the superior mesenteric artery. It supplies the structures near the ileocecal junction.
17. Describe the superior mesenteric vein and its branches. (N 300,301,302, TG 5-28)
Starting superior and moving inferiorly, the tributaries of the SMV are the gastroduodenal vein, the anterior and posterior inferior pancreatoduodenal veins, the middle colic vein and the right colic vein. Areas drained include the ileocecal junction, the ascending and transverse colon, the duodenum, the pancreas, and the greater curvature of the stomach. Behind the neck of the pancreas, the SMV unites with the splenic vein to form the portal vein.
17a. Is there a separate ascending branch of the left colic artery accompanying the inferior mesenteric vein? (N 296,301, TG 5-28)
The left colic artery usually splits into an ascending and a descending branch. Because the inferior mesenteric vein travels more superiorly to drain to the splenic vein, it is often accompanied for part of its course superiorly by the ascending branch of the left colic artery as this artery travels toward the splenic flexure.
18. Examine the arteriae rectae of the large intestine. How do they differ from those of the small intestine? (N 295,296, TG 5-14, 5-15)
The arteriae rectae of the large intestine are quite long, are fairly large in caliber, and are branches of the marginal artery. On the other hand, the arteriae rectae of the small intestine are shorter, smaller, and are branches of the arterial arcades of the small intestine.
19. What constitutes the marginal artery? (N 296, TG 5-14)
The marginal artery is the anastomosis of branches of the ileocolic, right colic, middle colic, left colic, and sigmoid arteries. It forms a collateral circulatory circuit for the large intestine.
20. Identify lymph channels, if possible. (N 258,305,306, TG 5-35A, 5-35B, 5-36A, 5-36B, 5-37)

The small intestine lymph drainage is primarily to the mesenteric lymph nodes embedded in the mesentery. They are located: