Anatomy And Physiology
The colon, or large bowel, is configured like an inverted U and comprises three parts: the ascending colon, starting from the cecum where it is connected to the small bowel; the transverse colon; and the descending colon, which ends in an S shaped part known as the sigmoid colon (see Figure 1). The sigmoid connects to the rectum, which terminates in the anus.
The upper end of the anal canal contains nerve endings, which can distinguish between fluid, flatus, and feces, thus allowing a sampling of contents if the rectum is suddenly distended— an important social safeguard. The main functions of the colon are storage of feces and reab-sorption of water and some electrolytes. The colon looks like a series of small bladders joined together to form a wide tube. Circular bands of muscle serve to move the contents back and forth between the bladder-like segments (known as shuttling motility) which promotes absorption, and a number of longitudinal muscle bands the whole length of the colon cause onward movement of feces into the rectum (mass peristalsis). This movement occurs several times a day, prompted particularly by food entering the stomach and by a person moving about. Thus, it occurs particularly after breakfast and is less likely to occur in immobile people (e.g., those who are bedridden).
Filling, and thus distention, of the rectum produces the desire to void, which may be responded to or, if inconvenient, may be voluntarily suppressed and the feces may then be returned to the colon. This effect of food entering the stomach is called the gastrocolic reflex. This used to be regarded as a neural reflex via the spinal cord and colonic nerves, but is now regarded as a chemical reaction to substances secreted by the stomach as it is filled with food.