External anal sphincter
Template:Short description Script error: No such module "Infobox".Template:Template otherTemplate:Main otherScript error: No such module "Check for unknown parameters". The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers.[1] Distally, it is adherent to the skin surrounding the margin of the anus.[2] It exhibits a resting state of tonical contraction[1] and also contracts during the bulbospongiosus reflex.[3][4][5][6]
Anatomy
The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.[1]
Structure
Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcutaneous). This is not supported by current anatomical knowledge. Some sources still describe it in two layers, deep (or proximal) and superficial (or distal or subcutaneous).[1]
Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.[1]
Attachments
Script error: No such module "Unsubst". The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.[1]
Innervation
The sphincter receives innervation from the bilaterally paired inferior anal nerve (each a branch of the pudendal nerve which is derived from ventral rami of S2-S4). It may also receive additional motor innervation from the nerve to levator ani.[1]
Histology
The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.[1]
Gallery
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Intestines
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Anatomy of the human anus.
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Muscles of male perineum.
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Muscles of the female perineum.
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Sagittal (vertical) section of bladder, penis, and urethra.
See also
References
Template:Reflist Public domain This article incorporates text in the public domain from Template:Wikidatathe 20th edition of Gray's Anatomy (1918) Template:Main other
External links
- Anatomy photo:42:13-0100 at the SUNY Downstate Medical Center - "The Male Perineum and the Penis: The External Anal Sphincter"
- perineum at The Anatomy Lesson by Wesley Norman (Georgetown University) (Template:NormanAnatomyFig)
- pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (Template:NormanAnatomyFig)
Template:Muscles of perineum Template:Digestive tract Template:Authority control Template:Portal bar