Colon, the portion of the large intestine extending from the caecum to the rectum, from the right to the left iliac region. It is divided into four portions: the ascending colon, on the right side, from the caecum to the edge of the ribs; the transverse, or arch of the colon, from one hypochondrium to the other, below the stomach and above the small intestine; the descending colon on the left side; and the sigmoid flexure, in the shape of the letter S, in the left iliac region, terminating in the rectum or last portion of the intestine. Along its course it presents prominences of its walls, interrupted by three fleshy longitudinal bands, and many fatty appendages formed in the folds of the peritoneum. The peritoneal or serous coat, after covering the intestine, fixes-it loosely to the vertebral column by the folds called mesocolon; the muscular coat consists of both circular and longitudinal bands, as in the caecum; the mucous coat presents a great number of mucous follicles. The arteries of the large intestine are derived from the superior and inferior mesenteric, proceeding directly from the aorta; the veins open into the portal vein of the liver; the nerves are furnished by branches of the solar plexus.
The colon in man will average about 4 1/2 feet in length, and about 2 inches in diameter, being about a quarter as long and twice as wide as the small intestine; though the capacity is nearly the same, the absorbing surface is scarcely half that of the smaller tube, and this difference is increased by the absence of folds in the large intestine. The ascending colon lies upon the right kidney and quadratus lum-borum muscle; above it is the duodenum; and in front the folds of the small intestine; the descending portion is on the left kidney and corresponding muscle, and is also covered by the small intestine. The sigmoid flexure is generally in contact with the abdominal walls, though, from its freedom of movement, it may assume a variety of curvature and position. The whole colon is very liable to displacement by the pressure of its own accumulated contents, by tumors from within, and by corsets and other articles of dress from without. It retains its sacculated shape throughout, but very gradually decreases in size toward the rectum; the fatty appendages (appendices epi-ploicce) appear to be small reservoirs of fatty matter, and are sometimes greatly increased in cases of remarkable obesity.
After the food has passed the caecum, little is left but excre-mentitious matter, which collects in the sacs of the colon, the forms of which it assumes and preserves even after having passed through the rectum. When, from want of tone in the bowel, or other causes, the faeces are delayed in these sacs, they often acquire extreme hardness and roundness, causing painful and even dangerous symptoms. Like the rest of the intestine, the colon is subject to inflammation, ulceration, and other diseases of mucous membranes; it is also the seat of dysentery. - The colon is separated from the small intestine in fishes by a slight constriction; this is the case with most reptiles. In birds the short and straight large intestine is continued from the small without a distinct separating valve, and ends in a cloaca common to the digestive, urinary, and generative organs. In mammalia there is generally a well marked colon, though in some of the edentata there is no distinction between large and small intestine; in carnivora it is short, wide, and cylindrical; in the herb-ivora, long and sacculated; in the horse, whose intestines are ten times as long as the body, the colon has a length of 19 feet, much curved and sacculated, and the lower portion attached loosely by a very long mesocolon; in rodents it is not much larger in diameter than the small intestine, but is provided with deep sacs; in the monotremata it gradually increases in size to the rectum; in the monkeys it is very similar to that of man.