Chapter 31

The rectum, endodermal in origin, is the dorsal component of the cloaca, which is partitioned by the anorectal septum. The anal canal is an invagination of ectodermal tissue. The anorectum develops from fusion of the rectum and the anal canal, which occurs at 8 weeks, when the anal membrane ruptures. The dentate line marks the point of fusion and the transition from endodermal to ectodermal tissue.

The rectum is 12–15 cm long. It extends from the rectosigmoid junction, marked by the fusion of the tenia, to the anal canal, marked by the passage into the pelvic floor musculature (Figure 31–1). The rectum lies in the sacrum and forms three distinct curves, creating folds known as the valves of Houston. The proximal and distal curves are convex to the left and the middle curve is convex to the right. The middle curve roughly marks the anterior peritoneal reflection, which is 6–8 cm above the anus. The rectum gradually undergoes transition from intraperitoneal to extraperitoneal beginning 12–15 cm from the anus and becoming completely extraperitoneal 6–8 cm from the anus. The rectum is fixed posteriorly, laterally, and anteriorly by the presacral (Waldeyer) fascia, the lateral ligaments, and Denonvilliers fascia, respectively.

Figure 31–1.

Anatomy of the anorectal canal.

The anatomic anal canal starts at the dentate line, the junction of colorectal mucosa and anal mucosa, and ends at the anal verge, the junction of the anal mucosa with the perianal skin. However, for practical purposes, the surgical anal canal extends from the muscular diaphragm of the pelvic floor to the anal verge. The anal canal is a collapsed anteroposterior slit 3–4 cm long. The anal canal is supported by the surrounding anal sphincter mechanism, composed of the internal and external sphincters. The internal sphincter is a specialized continuation of the circular muscle of the rectum. It is an involuntary muscle that is normally contracted at rest. The structure and function of the external sphincter is the subject of some controversy, but it acts as a spout on a funnel of one continuous circumferential functional muscle mass that includes the external sphincter caudally and extends cranially to the conical puborectalis and levator ani muscles. The external sphincter is composed of voluntary striated muscle. The conjoined longitudinal muscle separates the internal and external sphincters. This intersphincteric plane is created by the continuation of the longitudinal muscle of the rectum joined by fibers from the levator ani and puborectalis, forming the conjoined muscle. Some fibers from this muscle become the corrugator cutis ani and insert on the perianal skin, creating rugal folds and a puckered appearance. Other fibers traverse the internal sphincter and support the internal hemorrhoids as the mucosal suspensory ligaments.

Familiarity with the histology of the rectum and anal canal is important in order to understand the disease processes of these areas. The rectum ...

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