- Sphincter ani externus muscle
Name = Sphincter ani externus muscle
Latin = musculus sphincter ani externus
GraySubject = 120
GrayPage = 425
Caption2 = Coronal section through the
anal canal. B. Cavity of urinary bladderV.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.
Action = keep the
anal canaland orifice closed
Nerve = branch from the fourth sacral and twigs from the inferior hemorrhoidal branch of the
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
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 coccyxto 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
decussatein 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
The action of this muscle is peculiar.
(1) It is, like other muscles, always in a state of tonic contraction, and having no antagonistic muscle it keeps the
anal canaland 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
(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.
Anismusis a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in obstructed defecationand constipation. Abnormal function of this muscle is also seen in anorectal malformationand after certain surgeries, including coccygectomy.
* - "The Male Perineum and the Penis: The External Anal Sphincter"
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