The large intestine is divided into several regions. The first part is a pouch called the cecum. Then, the colon is divided into ascending, transverse, descending, and sigmoid (S-shaped) regions. Finally, the straight rectum ends in the anal canal that is the short passageway to the anus, the opening through which feces are evacuated.
The vermiform appendix is a worm-like appendage off the cecum. Teniae coli are longitudinal ribbons of smooth muscle that contract lengthwise to produce bulges (haustra). Omental (epiploic) appendages are 50-100 small pouches of fat along the teniae coli. The anal canal has internal, vertical folds in its mucosa called anal columns. Dual sphincters (internal and external) around the anus control passage of feces to the exterior.
Whithin the intestine, bacteria synthesize vitamin K and some B vitamins, ferment undigested foods, and prevent growth of harmful bacteria.
The large intestine undergoes mass peristalsis: these waves of peristalsis move feces into the rectum some 3-4 times per day. The large intestine completes absorption of water (by osmosis) to produce a relatively dry feces.
Feces (undigestible materials) are temporarily stored in the rectum before excretion through the anus. Defecation is controlled by two sphincters. The internal anal sphincter relaxes in response to pressure in a filled rectum (defecation reflex). The external anal sphincter is normally under voluntary control.
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