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[L. colon, fr. Gr. kolon, large intestine]
The large intestine from the end of the ileum to the anal canal that surrounds the anus. It is about 59 in (1.5 m) long and is divided into the ascending, transverse, descending, and sigmoid or pelvic colon. Beginning at the cecum, the first part of the large intestine (ascending colon) passes upward to the right colic or hepatic flexure, where it turns as the transverse colon passing ventral to the liver and stomach. On reaching the spleen, it turns downward (left colic or splenic flexure) and continues as the descending colon to the brim of the pelvis, where it is continuous with the sigmoid colon and extends to the rectum.
COLON AND RECTUM
COLON Normal colon, seen endoscopically
Mechanical: The colon mixes the intestinal contents. Chemical: The colon does not secrete digestive enzymes. The products of bacterial action that are absorbed into the bloodstream are carried by the portal circulation to the liver before they enter the general circulation. More water is absorbed in the colon than in the small intestine. In this way, body fluids are conserved, and despite the large volumes of secretions added to the food during its progress through the alimentary canal, the contents of the colon are gradually dehydrated until they assume the consistency of normal feces or even become quite hard.
SEE: absorption, colon; SEE: defecation
BACTERIA OF THE COLON
The normal microbial flora in the colon, some of which may produce vitamins, esp. vitamin K; metabolize proteins and sugars; produce organic acids and ammonia; and deconjugate bile acids. Several conditions, such as use of antibiotics, corticosteroids, or dieting, may alter the normal flora. Although Escherichia coli is the most widely known bacterium that inhabits the colon, it is not the most common, being outnumbered by anaerobic Bacteroides species by a very wide margin.