The colon - The major part of the large intestine.

The colon is a segmented tube, about 1.3 m long and 6.5 cm wide, that forms a large loop in the abdomen. It consists of four sections: the ascending, transverse, and descending colons, and the S-shaped sigmoid colon, which connects with the rectum.

The colon consists of four layers. It has a tough outer membrane that protects it from damage. The next layer comprises muscles that contract and relax rhythmically to move the intestinal contents along (see peristalsis). Inside the muscular layer is a submucous coat containing blood vessels and lymph vessels (see lymphatic system). The innermost layer produces mucus, which helps to lubricate the passage of waste material.

The main functions of the colon are to absorb water and mineral salts from food residue and to concentrate the remaining waste products. The material that remains after digestion enters the colon from the small intestine. As this substance passes through the colon, the water and salts that it contains are absorbed into the blood vessels in the submucous coat. The waste material becomes increasingly concentrated and is finally expelled from the rectum in the form of faeces. (See also digestive system; intestine, disorders of.)


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