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musculus sphincter ani internus

Sphincter ani internus muscle

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.

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