| Sphincter ani internus muscle | |
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| 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. | |
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| Coronal section of rectum and anal canal. | |
| Latin | musculus sphincter ani internus |
| Gray's | subject #120 426 |
| Origin | |
| Insertion | |
| Artery | |
| Nerve | Pelvic Splanchnic (S4) |
| Actions | 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.
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. It is innervated by parasympathetic nerves from S2, S3, and S4. It is not innervated by the pudendal nerve, which is a somatic nerve.
Additional images
 Intestines |
 Schematic demonstrating the anatomy of hemorrhoids. |
External links
- LUC spha
- pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (rectum)
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 As such, some of the information contained herein may be outdated.
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