anal sphincter

noun

: 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