Anal sphincter

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

Gross anatomy

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

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

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

Abdominal and pelvic anatomy
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Article Information

rID: 33134
Section: Anatomy
Synonyms or Alternate Spellings:
  • Anal sphincters
  • Anal sphincter complex

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