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The rectum is a chamber that begins at the end of the large intestine immediately flowing the sigmoid colon and ends at the anus. The rectum is normally empty because stool is tored higher in the descending colon. Eventually the descending colon becomes full and stool passes into the rectum causing an urge to move the bowels (defecate). The anus is the opening at the far end of the digestive tract through which stool leaves the body. The anus is formed partly from the surface layers of the body including the skin and partly from the intestine. The anus is lined with a continuation of the external skin. A muscular ring keeps the anus closed until the person has a bowel movement.