ABSTRACT
The rectum (1) lies posteriorly within the pelvic cavity, following the concavity of the sacrum (2). It is the continuation of the sigmoid colon and holds faeces immediately prior to evacuation. It is about 12 cm long, starts at S3, and becomes wider inferiorly, dilating as the rectal ampulla. It takes a sinuous course of left, right, left curves. The sigmoid enters it from the left, creating a concavity to the left at the start of the rectum. The middle concavity is on the opposite side, the right, and there is a smaller, less consistent concavity, again to the left, at the lower end of the rectal ampulla, just above the anorectal junction. At the concavities the rectal wall tends to push into the lumen creating three ‘shelves’ or rectal valves (of Houston) that may be palpable on rectal examination and visible on proctoscopy and sigmoidoscopy. The middle, right-sided ‘shelf’ is the most prominent.
