Sphincter ani internus muscle
|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|
|Action:||keep the anal canal and orifice closed, aids in the expulsion of the feces|
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.
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.
Schematic demonstrating the anatomy of hemorrhoids.
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