Anal sphincter

Last revised by Khalid Alhusseiny on 28 Aug 2023

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

  • continuous with the inner circular layer of rectal smooth muscle

  • thickened, circular fibers, up to 5 mm thick

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

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Cases and figures

  • Figure 1: illustration
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  • Case 1: transverse
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  • Case 1: longitudinal
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