Sphincter ani externus muscle
3042621
226031800
2008-07-16T14:58:18Z
Una Smith
3349439
/* Pathology */ expand
{{Muscle infobox |
Name = Sphincter ani externus muscle |
Latin = musculus sphincter ani externus |
GraySubject = 120 |
GrayPage = 425 |
Image = Anorectum.gif |
Caption = |
Image2 = Gray1079.png |
Caption2 = 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]]. |
Origin = |
Insertion = |
Action = keep the [[anal canal]] and [[anal orifice|orifice]] closed|
Blood = |
Nerve = branch from the fourth sacral and twigs from the inferior hemorrhoidal branch of the [[pudendal nerve]] |
MeshName = |
MeshNumber = |
DorlandsPre = m_22 |
DorlandsSuf = 12550801 |
}}
The '''Sphincter ani externus''' ('''external anal sphincter''') is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the integument surrounding the margin of the [[anus]].
It measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm broad opposite the anus.
It consists of two strata, superficial and deep.
* The ''superficial'', constituting the main portion of the muscle, arises from a narrow tendinous band, the [[anococcygeal raphé]], which stretches from the tip of the [[coccyx]] to the posterior margin of the anus; it forms two flattened planes of muscular tissue, which encircle the anus and meet in front to be inserted into the [[central tendinous point of the perineum]], joining with the [[Transversus perinæi superficialis]], the [[Levator ani]], and the [[Bulbocavernosus]].
* The ''deeper'' portion forms a complete sphincter to the [[anal canal]]. Its fibers surround the canal, closely applied to the [[Sphincter ani internus]], and in front blend with the other muscles at the central point of the [[perineum]].
In a considerable proportion of cases the fibers [[decussate]] in front of the anus, and are continuous with the [[Transversi perinæi superficiales]].
Posteriorly, they are not attached to the [[coccyx]], but are continuous with those of the opposite side behind the anal canal.
The upper edge of the muscle is ill-defined, since fibers are given off from it to join the [[Levator ani]].
==Actions==
The action of this muscle is peculiar.
(1) It is, like other muscles, always in a state of tonic contraction, and having no [[Antagonist (muscle)| antagonistic muscle]] it keeps the [[anal canal]] and [[anal orifice|orifice]] closed.
(2) It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with [[defecation]].
(3) Taking its fixed point at the [[coccyx]], it helps to fix the central point of the [[perineum]], so that the Bulbocavernosus may act from this fixed point.
==Pathology==
[[Anismus]] is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in [[obstructed defecation]] and [[constipation]].
==Additional images==
<gallery>
Image:illu_intestine.jpg|Intestines
Image:Hemorrhoid.png|Schematic demonstrating the anatomy of hemorrhoids.
Image:Gray406.png|Muscles of male perineum.
Image:Gray408.png|Muscles of the female perineum.
Image:Gray1156.png|Vertical section of bladder, penis, and urethra.
</gallery>
==External links==
* {{MuscleLoyola|spha}}
* {{SUNYAnatomyLabs|42|13|01|00}} - "The Male Perineum and the Penis: The External Anal Sphincter"
* {{NormanAnatomy|perineum}} ({{NormanAnatomyFig|analtriangle3}})
* {{NormanAnatomy|pelvis}} ({{NormanAnatomyFig|rectum}})
{{Gray's}}
{{Muscles_of_trunk}}
{{muscle-stub}}
[[pl:Mięsień zwieracz zewnętrzny odbytu]]