anal sphincter complex
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)
- the aforementioned 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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