External anal sphincter


The external anal sphincter is an oval tube of skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus. It exhibits a resting state of tonic contraction and also contracts during the bulbospongiosus reflex.

Anatomy

The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.

Structure

Historically, the sphincter was described as consisting of three parts. This is not supported by current anatomical knowledge. Some sources still describe it in two layers, deep and superficial.
Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.

Function

The external anal sphincter keeps feces retained inside the rectum and prevents them from coming out of the rectum involuntarily.

Attachments

The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.

Innervation

The sphincter receives innervation from the bilaterally paired inferior anal nerve. It may also receive additional motor innervation from the nerve to levator ani.

Histology

The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.