The Sphincter ani internus
(or internal anal sphincter
) 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.
Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces
. It is innervated by parasympathetic nerves from S2, S3, and S4. It is not innervated by the pudendal nerve, which is a somatic nerve.