Sphincter ani internus muscle

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Sphincter ani internus muscle
Coronal section through the anal canal. B. Cavity of urinary bladder V.D. Ductus deferens. S.V. Seminal vesicle. R. Second part of rectum. A.C. Anal canal. L.A. Levator ani. I.S. Sphincter ani internus. E.S. Sphinear ani externus.
Coronal section of rectum and anal canal.
Latin musculus sphincter ani internus
Gray's subject #120 426
Nerve: pudendal nerve
Action: keep the anal canal and orifice closed, aids in the expulsion of the feces
Dorlands/Elsevier m_22/12550809

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 S1, S2, and S3. It is not innervated by the pundendal nerve, which is a somatic nerve.

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This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.