External anal sphincter

External anal sphincter
External anal sphincter
	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
Nerve	Branch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve
Actions	Keep the anal canal and orifice closed
Identifiers
Latin	Sphincter ani externus
TA98	A04.5.04.012
TA2	2426
FMA	21930
	Anatomical terms of muscle

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.

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, the sphincter muscle retracts on defecating.

It consists of two layers: superficial and deep.

The superficial layer, constitutes the main portion of the muscle, and arises from a narrow tendinous band, the anococcygeal raphe, 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 superficial transverse perineal muscle, the levator ani, and the bulbospongiosus muscle also known as the bulbocavernosus.
The deeper layer forms a complete sphincter to the anal canal. Its fibers surround the canal, closely applied to the internal anal sphincter, 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 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

(1) Like other muscles, it is 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

Intestines
Anatomy of the human anus.
Muscles of male perineum.
Muscles of the female perineum.
Sagittal (vertical) section of bladder, penis, and urethra.

References

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

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

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