External anal sphincter

The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.

Sphincter ani externus muscle
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. Sphincter ani externus.
Details
NerveBranch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve
ActionsKeep the anal canal and orifice closed
Identifiers
LatinMusculus sphincter ani externus
TAA04.5.04.012
FMA21930
Anatomical terms of muscle

Anatomy

The external anal sphincter measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm opposite the anus, when defecation occurs the sphincter muscle retracts.

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 superficial transverse perineal muscle.

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 antagonistic muscle it keeps the anal canal and orifice shut.

(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 bulbospongiosus muscle may act from this fixed point.

Additional images

See also

References

This article incorporates text in the public domain from page 425 of the 20th edition of Gray's Anatomy (1918)

  • 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) (analtriangle3)
  • pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (rectum)
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