WHAT CONTROLS DEFECATION?
Defecation is a complex business that depends on the successful interaction of learned and involuntary behavior. The diagram below of the normal human colon indicates some of the involved parts. The rectum, the lowest part of the bowel, collects fecal (stool) material as it is formed and passed on from the upper colon. The fecal material is prevented from leaking out of the rectum by the action of two muscles, the internal anal sphincter and the external anal sphincter (the external sphincter is the muscle you can voluntarily squeeze shut when attempting not to defecate). Under normal conditions these sphincters are closed, but when stool enters the rectum, the internal sphincter relaxes. There is no control over this relaxation. Stool then presses on the external sphincter, creating the urge to defecate, or to have a bowel movement. During defecation, both sphincters relax and stool is evacuated by both muscle activity in the colon and voluntary forceful “bearing down”. If a child doe
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