Main Model
14 Sphincter ani externus muscle
The external anal sphincter is a large voluntary sphincter that forms a broad band on each side of the inferior two thirds of the anal canal. This sphincter is attached anteriorly to the perineal body and posteriorly to the coccyx via the anococcygeal ligament. It blends superiorly with the puborectalis muscle.
The external anal sphincter is described as having subcutaneous, superficial, and deep parts; these are zones rather than muscle bellies and are often indistinct. The external anal sphincter is supplied mainly by S4 through the inferior rectal nerve, although its deep part also receives fibers from the nerve to the levator ani, in common with the puborectalis, with which it contracts in unison to maintain continence when the internal sphincter is relaxed (except during defecation).