The large intestine is divided into a number of regions. Flexures accommodate the five-foot length and final position of the large intestine. It is significant that every other part of the GI tract, beginning with the esophagus, is suspended from the posterior body wall via mesentery.
The blind-ended Cecum lies inferior to a horizontal line at the level of the ileocecal junction. The Appendix arises from the posteromedial aspect of the cecum inferior to the ileocecal junction. The position of the appendix is variable, but is usually retrocecal. The Ascending Colon begins superior to a horizontal line at the level of the ileocecal junction. The ascending colon lacks a mesentery and is attached to the posterior body wall by fusion fascia. It ascends to the level of the right colic (hepatic) flexure and continues as the longest portion of the large intestine, the Transverse Colon. The Transverse Mesocolon attaches the transverse colon to the posterior body wall. The Descending Colon begins at the variable level of the left colic (splenic) flexure and the phrenicocolic ligament. The descending colon lacks a mesentery and is attached to the posterior body wall by fusion fascia. The Sigmoid Colon, as the continuation of the descending colon, varies in length and attaches to the pelvic brim by the Sigmoid Mesocolon. The Rectum begins at the vertebral level of S2 or S3. The Anal Canal begins where the puborectalis portion of levator ani clasps the external anal sphincter. This region is the anorectal sling, and is palpable on rectal examination. The anal canal is 2.5 - 3.5 cm long and presents intermediate and cutaneous zones. The intermediate zone is characterized by smooth hairless skin. True mucous membrane begins at the level of the anal valves. The anal valves and anal columns characterize the upper 1.5 - 2cm of the anal canal. The anal columns are from five to ten vertical folds of mucous membrane, separated by grooves. The columns are connected by mucosal folds, the anal valves under which pass communicating veins. At the underside of the valves the mucous membrane becomes continuous with the smooth skin of the transitional zone along the Pectinate Line.