n :  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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