The '''external anal sphincter''' (or '''sphincter ani externus''' ) is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the [[integument|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, when defecation occurs the sphincter muscle retracts.
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 body|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 [[Perineal body|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'' portion 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==
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
