|External anal sphincter|
|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|
|Latin||sphincter ani externus|
|Anatomical terms of muscle|
The external anal sphincter (or sphincter ani externus) is an oval tube skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus. The sphincter exhibits a resting state of tonical contraction.
The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.
Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcontinuous), however, this is not supported by current anatomical knowledge. Some sources still describe it as consisting of two layers: deep (or proximal) superficial (or distal or subcutaneous).
Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.
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The sphincter receives innervation from the bilaterally paired inferior anal nerve (each a branch of the pudendal nerve which is derived from ventral rami of S2-S4). It may also receive additional motor innervation from the nerve to levator ani.
(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.[clarification needed]
Anatomy of the human anus.
Muscles of male perineum.
Muscles of the female perineum.
Sagittal (vertical) section of bladder, penis, and urethra.