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The large intestine is divided into three main regions: the cecum, colon, and rectum. Extending from the ileocecal valve to the anus, it frames the small intestine on three sides.
The ileocecal sphincter, a mucous membrane fold, guards the opening from the ileum to the large intestine. This valve permits material from the small intestine to pass into the large intestine. Attached to the ileocecal valve is the cecum. This small pouch, approximately 6 cm long, has a twisted, coiled tube known as the appendix or vermiform appendix attached to it.
Following the cecum is the colon, a long tube divided into ascending, transverse, descending, and sigmoid portions. The ascending and descending colon are retroperitoneal, whereas the transverse and sigmoid colon are intraperitoneal. The ascending colon rises on the right side of the abdomen, reaches the liver's inferior surface, and then takes a left turn to form the right colic (hepatic) flexure. It continues across to the left side as the transverse colon, curves beneath the spleen's inferior end as the left colic (splenic) flexure, and descends to the level of the left iliac crest as the descending colon. The sigmoid colon, which is S-shaped, starts near the left iliac crest, projects medially to the midline, and ends as the rectum at the third sacral vertebra level.
The rectum, about 15 cm long, is positioned anterior to the sacrum and coccyx. It leads to the terminal end of the large intestine, known as the anal canal. The anal canal's mucous membrane is organized into longitudinal folds called anal columns, housing a network of arteries and veins. The anus, the external opening of the anal canal, is guarded by an internal anal sphincter comprised of smooth muscle and an external anal sphincter comprised of skeletal muscle. These sphincters typically keep the anus closed, opening only during the elimination of feces.
