(From hollow). It is the first and most considerable of the large intestines, called also enteron. From the coecum it reaches in the form of an arch above the umbilical region, and extends to the lower part of the left hypochondrium, running down before the left kidney, to which it is connected, and below which it turns towards the spine; then forming two opposite convolutions, called the sigmoid flexure, it terminates in the rectum. When it has passed below the last vertebra of the loins to the inside of the os sacrum, it is bent backwards on the concave side, to which it is joined; and running in the direction of the os coccygis, bends a little forwards, and terminates in the extremity of that bone.

The continuity of the colon is a little interrupted by the intestinum ilium, which advances into the cavity of the colon; not at its extremity, but at the distance of about two inches, penetrating through its longitudinal fibres. It thus forms the valve of the colon, which is shut only when the colon is distended. From this structure, it is evident that no sphincter is necessary to prevent the feces returning to the ilium: yet this sometimes happens from a retrograde peristaltic motion; and even clysters are thrown up by vomiting.

The whole convex side of the colon is divided longitudinally into three parts, by three ligamentous bands, continued from those of the coecum, and of the same structure with them. Two of these bands run on each side along the great curvature of the colon, and the third along the small curvature. This last was first noticed by Morgagni. Between these ligaments the intestine is formed into cells, from the ligaments being shorter than the duct, called the cells of the colon. The cellular coat does not differ from that of the small intestines; the internal coat is not villous, but papillous; the rugae are waved and irregular.

From the apparent course of this intestine, warm clysters may, it is said, be applied to almost all the abdominal viscera; for it begins under the right kidney, and runs up on its fore part, passes under the gall bladder, then runs upon the first curvature of the duodenum, to all which it is contiguous; from thence it runs before the great convexity of the stomach, then touches the spleen, and goes on to the left kidney, as above mentioned. This representation is, however, the offspring of theory and ignorance; for no clyster, except when thrown up by an injecting syringe of strong powers, furnished with a lateral pipe, can pass even the sigmoid flexure. De Haen, in this way, has sometimes filled the colon of a dog, and forced the valve; but he often failed.

The colon on its upper part receives arteries from the mesenterica superior and inferior: the lower portion is supplied from the mesenterica inferior, one of which forms the internal haemorrhoidal artery. The veins are from the vena portae ventralis, the mescraica major and minor, or haemorrhoidalis interna. The nerves of the arch of the colon are the two mesenteric plexuses.

When pain is in the colon, it is less acute, and with a sense of weight; when in the small guts, there is not any sense of weight, but an acute pain. If fever attends pain in the colon, the pain extending to the ribs gives a suspicion of pleurisy, though the colon in reality is only affected. The colon is narrower in the right side than elsewhere, whence colic pains arise more frequently, and are more acute in this part. The excrements are long retained in the colon, and often much indurated before they pass on.