Nerve: Pudendal nerve
Action: Keep the anal canal and anus closed, aids in the expulsion of the feces
The Sphincter ani internus (Internal sphincter ani) is a muscular ring which surrounds about 2.5 cm. of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus. It is about 5 mm. thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm. from the orifice of the anus.
Actions.—Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces.