The external anal sphincter is the ring of skeletal muscle surrounding the distal anal canal. Its contraction helps maintain faecal continence; its motor neurons allow voluntary squeezing as well as reflex control. It works alongside the smooth muscle of the internal anal sphincter, so continence depends on coordinated structures rather than one isolated ring.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 23.5, The Large Intestine, Anal Canal
Branches of the pudendal nerve supply the external anal sphincter. Dissection of 28 cadavers found a local nerve plexus near the sphincters in five, illustrating why a schematic nerve route need not capture every individual's branching. Preserving muscle alone therefore does not guarantee preservation of its motor supply.[ref]
Anatomy of the pudendal nerve and its terminal branches: a cadaver study by Steven E. Schraffordt and others. Abstract, Results: inferior rectal and perineal interconnections
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