External anal sphincter
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. Anatomy
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. Structure 
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. Attachments 

The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament. Innervation 
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. Histology 
The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction. Function
(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

See also
Internal anal sphincter
Puborectalis muscle

References

External links
  - "The Male Perineum and the Penis: The External Anal Sphincter"
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Perineum
Anus
Muscular system