internal anal sphincter(redirected from Sphincter ani internus muscle)
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in·ter·nal a·nal sphinc·ter[TA]
a smooth muscle ring, formed by an increase of the circular muscle fibers of the rectum, situated at the upper end of the anal canal, internal to the outer voluntary external anal sphincter. This sphincter is maximally contracted when the rectal ampulla is "at rest"-empty or relaxed to accommodate a distending fecal mass. It is inhibited with filling of the ampulla, increased distension, and peristalsis.