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The colon begins at the ileocecal valve and ends at the rectum, spanning 140 cm (5 feet) in length. The cecum and ascending and descending colon are retroperitoneal, whereas the transverse colon and sigmoid are intraperitoneal (Figure 34–1). The diameter of the lumen is greatest at the cecum (approximately 7 cm) and decreases distally. As a result, mass lesions of the cecum are least likely to cause obstruction. However, the wall of the colon is thinnest in the cecum and is therefore the most vulnerable to ischemic necrosis and perforation.
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There are four layers of the colon wall: mucosa, submucosa, muscularis propria, and serosa (Figure 34–2). 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. 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 taenia coli. The appendix can be found at the point on the cecum where the taenia 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 the haustra. The epiploic appendages are fatty appendages on the serosal surface.
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The rectum begins at the sacral promontory and ends at the anorectal ring. It is approximately 12-16 cm in length and lies between the sigmoid and anus. The rectum is differentiated from the colon by its lack of tenia, epiploic appendices, and haustra. The anterior and lateral walls of the upper rectum are intraperitoneal, and the posterior wall is extraperitoneal. The anterior peritoneal reflection extends low into the pelvis, approximately 5-8 cm above the anal verge, and lies between the rectum and the bladder in men and the uterus in women (the pouch of Douglas). Tumors or abscesses in this location can be palpated on digital rectal or vaginal exam. The rectoprostatic fascia (Denonvilliers’ fascia) is the extension of the peritoneum that lies in the plane between the anterior rectum and the seminal vesicles in men and the cervix in women. The rectum has three major folds ...