Anal sphincter

Last revised by Dr Calum Worsley on 28 Oct 2021

The anal sphincter is divided into internal and external anal sphincters. It surrounds the anal canal

Gross anatomy

Internal anal sphincter
  • continuation of inner rectal muscle
  • thickened, circular muscle fibers, up to 5 mm thick
  • composed of visceral muscle
External anal sphincter

Composed of skeletal muscle and classically categorized into deep, superficial, and subcutaneous parts:

  • deep part
    • circular muscle fibers
    • blends with puborectalis part of levator ani (posteriorly and laterally)
    • this region is called the anorectal ring and is palpable on rectal examination
  • superficial part
    • elliptical muscle fibers
    • attaches from the tip of the coccyx posteriorly to the perineal body anteriorly
    • only part of the sphincter with bony attachment
  • subcutaneous part
    • circular muscle fibers
    • lower ends curve inwards, lying below the end of the internal sphincter
      • the intersphincteric groove that results is palpable on examination

Related pathology

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Cases and figures

  • Figure 1: illustration
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