The anus is the outlet to the gastrointestinal tract, and the rectum is the lower 10 to 15 cm of the large intestine. The dentate line divides the outer skin from the mucosa. Four to eight anal glands drain into the crypts of Morgagni at the dentate line; most rectal abscesses and fistulae originate in these glands. The area of the rectum above the dentate line does not contain pain nerves, which allows for surgical procedures to be performed without anesthesia. However, the area below the dentate line is highly sensitive. Bowel movements are controlled by the muscles of both the involuntary internal sphincter and the voluntary external sphincter.