Anorectal Diseases - General Anatomic Considerations

Embryologic considerations are important when considering the epithelium, innervation, vascular supply, and venous and lymphatic drainage of the anorectum. The rectum, endodermal in origin, is derived from the dorsal component of the cloaca, which is partitioned by the anorectal septum. The anal canal arises from an invagination of ectodermal tissue. The anorectum develops in the 8-week embryo from the fusion of the rectum and the anal canal, which occurs when the anal membrane ruptures. The dentate line marks the point of fusion and the transition from endodermal to ectodermal tissue.

The rectum is approximately 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. The rectum lies in the sacrum and has three distinct curves resulting in folds that are visible on endoscopy and 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 generally 6-8 cm above the anus.

Viewed from the abdomen, the rectum is seen to gradually transition from intraperitoneal to extraperitoneal beginning posteriorly at 12-15 cm from the anus. It becomes completely extraperitoneal at 6-8 cm from the anus. The rectum is "fixed" posteriorly, laterally, and anteriorly by the presacral or Waldeyer's fascia, the lateral ligaments, and Denonvilliers' fascia, respectively.

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, the surgical anal canal is more clinically relevant. It extends from the muscular diaphragm of the pelvic floor to the anal verge.

The anal canal is a 3- to 4-cm-long collapsed slit that tilts posteriorly from the anal verge. 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 are controversial; however current evidence suggests that the external sphincter is the spout on a muscular funnel of 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 sphincter. It is created by the aggregation of fibers from the longitudinal muscle of the rectum, fibers from the levator ani, and fibers from the puborectalis. 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.