The colon extends from the ileo-caecal junction to the anus and is approximately 125 cm long in vivo. The large intestine is wider and shorter than the small intestine. The lumen progressively diminishes from a maximum diameter at the caecum (about 8.5cm) to the sigmoid segment (about 2.5 cm). It can be divided into the caecum, ascending, transverse, descending, and sigmoid colon, rectum and anus (Figure 7.1).
The caecum is the widest part of the colon and is a downward pointing blind pouch approximately 8.5 cm long with the appendix attached to its apex, and its base at the ileocaecal junction. It is attached to the floor of the right iliac fossa by the peritoneum, within the folds of which lies the appendix. It receives undigested food material from the small intestine and is considered the first region of the large intestine. It is separated from the ileum (the final portion of the small intestine) by the ileocecal valve (also called Bauhin's valve). The ileocaecal valve limits the rate of food passage into the caecum and may help prevent material from returning to the small intestine. In humans, the caecum's main functions are to absorb fluids and salts that remain after the completion of intestinal digestion and absorption, and to mix its contents with mucus for lubrication. The internal wall is composed of a thick mucous membrane beneath which is a deep layer of muscle tissue that produces churning and kneading motions.
The ascending colon is approximately 20 cm long and extends from the caecum to the hepatic flexure, which lies lateral to the right kidney and in contact with the inferior surface of the liver. The transverse colon is normally over 45 cm in length and hangs loosely between the hepatic and the splenic flexures, often following the greater curvature of the stomach. The splenic flexure is usually located higher than the hepatic flexure. The descending colon
extends downwards from the splenic flexure to the pelvic brim and is approximately 30 cm long. The colon then turns towards the midline to form the coiled sigmoid colon, which is about 40 cm in length. This in turn joins the rectum in front of the third part of the sacrum and travels for approximately 12 cm before joining the anal canal. This is 3 cm long and its diameter is narrower than that of the rectum to which it connects. It is identified by the presence of the anal sphincters which replace the muscular coats of the rectum. The sling of the puborectales muscle supports the ano-rectal junction.
In humans the rectum is formed by the last 15 to 20 cm of the large intestine. The internal cavity of the rectum is divided into three or four chambers; each chamber is partly segmented from the others by permanent transverse folds (valves of Houston) that apparently help to support the rectal contents. A sheath of longitudinal muscle surrounds the outside wall of the rectum, making it possible for it to shorten in length.
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