Internal anal sphincter
Sphincter internus ani
- Latin synonym: Musculus sphincter ani internus
- Synonym: Internal anal sphincter muscle
Definition
The internal anal sphincter (sphincter ani internus) 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 external anal sphincter (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 external anal sphincter to occlude the anal aperture and aids in the expulsion of the feces.
Nerve: Pudendal nerve
Action: Keep the anal canal and anus closed, aids in the expulsion of the feces
References
Text by Antoine Micheau, MD - Copyright IMAIOS
