External Anal Sphincter

Origin

The external anal sphincter muscle originates from the anococcygeal ligament, which attaches to the apex of coccyx.

Insertion

The fibers of the external anal sphincter muscle travel anteriorly and insert onto the perineal body.

Key Features & Anatomical Relations

The external anal sphincter muscle is found in the anal triangle of the perineum. It is a large, voluntary, circular type of skeletal muscle that surrounds the anal canal.

For descriptive purposes, it is divided into three parts:

- a superiorly located deep part;

- a centrally located superficial part;

- an inferiorly located subcutaneous part.

The external anal sphincter muscle is located:

- anterior to the anococcygeal ligament;

- posterior to the perineal body and the superficial and deep transverse perineal muscles;

- inferior to the levator ani muscle.

Actions

The external anal sphincter muscle constricts the anal canal, keeping it closed and contributing to fecal continence. Its fibers are capable of maintaining a tonic contraction at rest, which relaxes during defecation (Moore, Dalley and Agur, 2009).

List of Clinical Correlates

- Fecal incontinence

References

Moore, K. L., Dalley, A. F. and Agur, A. M. R. (2009) Clinically Oriented Anatomy. Lippincott Williams & Wilkins.

Actions

The external anal sphincter muscle constricts the anal canal, keeping it closed and contributing to fecal continence. Its fibers are capable of maintaining a tonic contraction at rest, which relaxes during defecation (Moore, Dalley and Agur, 2009).