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 externus
b : 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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