musculus sphincter ani externus

Sphincter ani externus muscle

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.

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.


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 canal and 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.


Anismus is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in obstructed defecation and constipation. Abnormal function of this muscle is also seen in anorectal malformation and after certain surgeries, including coccygectomy.

Additional images

See also

External links

  • - "The Male Perineum and the Penis: The External Anal Sphincter"
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