Anus

Abstract

The anal canal (anus) is 4 cm long and is continuous with the rectum above the pelvic floor. The mucous membrane of the upper half of the anal canal is lined by columnar epithelium and supplied by autonomic nerves, being sensitive only to stretch. The lower half is lined by strati- fied squamous epithelium and has a somatic nerve supply, being sensitive to pain, touch, etc. There is a transition zone with a sharp demarcation between the two types of mucosa, termed the dentate line. The circular muscle layer is thickened around the upper anal canal to form the internal (involuntary) sphincter. A sheath of striated muscle encloses this — the external (voluntary) sphincter. The longitudinal muscle coat descends between the internal and external sphincters. The ischiorectal fossa is a fat-filled space on either side of the anal canal between it and the bony pelvis (Figure 7.1).