The anal canal joins the rectum to the perineum. It is about 5 cm in length in adults. The canal starts where the rectum passes between the two levator ani muscles. The circular muscle of the rectum is thickened in the the anal canal to become the internal anal sphincter, which has a well defined lower extent. The longitudinal muscle of the rectum passes between the internal and external anal sphincters.
The epithelial lining of the rectum is a simple columanr lining and that of the skin in the buttock a startified squous lining. There is thus a transition in the lining epithelium of the anal canal from the columnar lining of the rectum to the squamous lining of the perineum. This transition zone is found just above the dentate line which gets its name from the tooth like appearance of its edge.