There are several anatomical arrangements that prevent feces spilling out of the rectum at inappropriate times. The anal canal is surrounded by two circular muscle bands, known as the sphincter muscles. One of these (the internal anal sphincter) is not under voluntary control, but maintains constant contraction except when bowel emptying is occurring. The external anal sphincter can be contracted voluntarily for a very short time to prevent an unwanted catastrophe. It also contracts automatically when abdominal pressure is suddenly raised (as in coughing or sneezing) A third continence mechanism is the anorectal angle, maintained at less than 90 degrees by a sling-like muscle, the puborectalis (see Figure 2), which arises from the back of the pubic bone and divides into two parts to encircle the anorectal junction—the two parts then fusing together posteriorly. Fecal incontinence may occur if the anal sphincters are damaged (e.g., in childbirth) or if the anorectal angle is maintained at more than 90 degrees—as by a constipating mass of feces lying in the rectum (see Figure 3). This may also impair awareness of the nature of rectal contents, particularly in confused elderly persons, and feces may be expelled when the individual simply attempts to pass flatus (gas).
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