Medical Definition of anal sphincter
: either of two sphincters controlling the closing of the anus:a: an outer sphincter of striated muscle extending from the coccyx to the central tendinous part of the perineum and surrounding the anus immediately beneath the skin—called also external anal sphincter, sphincter ani externusb: an inner sphincter formed by thickening of the circular smooth muscle of the rectum—called also internal anal sphincter, sphincter ani internus
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