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ANATOMY

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The colon begins at the ileocecal valve and ends at the rectum, spanning 140 cm (5 feet) in length. The colon has both intraperitoneal and retroperitoneal components. The cecum, ascending, and descending colon are retroperitoneal, whereas the transverse colon and sigmoid are intraperitoneal (Figure 30–1). The diameter of the lumen is greatest at the cecum (~ 7 cm), and decreases distally. As a result, mass lesions of the cecum are least likely to cause obstruction and the thin wall of the cecum is most vulnerable to ischemic necrosis and perforation from large bowel obstructions. There are four layers of the wall—mucosa, submucosa, muscularis propria, and serosa (Figure 30–2). The mucosa is composed of three layers—a simple columnar epithelium organized to form crypts, lamina propria, and muscularis mucosa. The submucosa is the strength layer of the colon because it has the highest concentration of collagen. Therefore, this layer is especially important to incorporate during anastomoses. The muscularis propria is composed of an inner circular layer and an outer longitudinal layer that thickens into three bands around the circumference to form the taeniae coli. The appendix can be found at the point on the cecum where the taeniae converge. At the rectosigmoid, these bands fan out to form a uniform layer, marking the end of the colon and the beginning of the rectum. The forces of these muscular components of the wall result in shortening of the colon to form sacculations called haustra (Figure 30–3). These are not fixed structures, but can be observed to move longitudinally. The appendices epiploicae are fatty appendages on the serosal surface.

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Figure 30–1.

The large intestine: anatomic divisions and blood supply. The veins are shown in black. The insert shows the usual configuration of the colon.

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Figure 30–2.

Cross-section of colon. The longitudinal muscle encircles the colon but is thickened in the region of the taeniae coli.

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Figure 30–3.

Barium enema of normal colon. Note the appearance of haustra and the location of the splenic and hepatic flexures.

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The rectum begins at the sacral promontory and ends at the anorectal ring. It lies between the colon and the anus and is 12-16 cm in length. There are no taeniae as the longitudinal muscle fans out and encompasses the circumference of the rectal wall. The rectum can be further differentiated from the colon by its lack of appendices epiploicae and haustra. The rectum is both an intra- and extraperitoneal organ. Posteriorly, the rectum is only intraperitoneal at the level of the rectosigmoid. However, the anterior and lateral walls of the upper rectum are intraperitoneal. The anterior peritoneal reflection extends low into the pelvis, approximately 5-8 ...

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