ABSTRACT
Surgical anatomy The rectum begins where the taenia coli of the sigmoid colon join to form a continuous outer longitudinal muscle layer at the level of the sacral promontory. The rectum follows the curve of the sacrum, to end at the anorectal junction. The puborectalis muscle encircles the posterior and lateral aspects of the junction, creating the anorectal angle (normally 120°). The rectum has three lateral curvatures: the upper and lower are convex to the right, and the middle is convex to the left. On the luminal aspect, these three curves are marked by semicircular folds (Houston’s valves) (Fig. 68.1). That part of the rectum that lies below the middle valve has a much wider diameter than the upper third and is known as the ampulla of the rectum.
