External anal sphincter

At a glance

English

External anal sphincter

Latin (TA2)

Sphincter externus ani

System

Muscle

Region

Trunk

Side

Paired, left and right

About the external anal sphincter

The anal canal is closed by two muscular rings. The inner one, the internal anal sphincter, is smooth muscle continuous with the bowel wall and works without conscious control. Around it lies the external anal sphincter, a tube of skeletal muscle under voluntary control that blends at its upper edge with the puborectalis part of levator ani. It is oval in outline and its lowest fibres sit just beneath the skin at the anal margin.

It is usually described in three parts that merge into one another. The subcutaneous part is a thin ring around the anal opening with no bony attachment. The superficial part is elliptical and is anchored behind to the tip of the coccyx through the anococcygeal body and in front to the perineal body, the fibrous centre of the perineum where several muscles meet. The deep part is a thicker ring around the upper anal canal; it has little attachment to bone and fuses with puborectalis above. Between the internal and external sphincters runs a thin layer of longitudinal fibres and a groove that can be felt at the anal verge.

The muscle is kept in a state of constant mild contraction, which keeps the canal shut at rest. It can be tightened strongly at will, for example when the urge to empty the bowel must be resisted, or reflexly during a cough. With puborectalis it forms the main barrier that holds back gas and faeces, and it relaxes when the bowel is emptied. Its nerve supply comes from the inferior rectal (anal) branch of the pudendal nerve, from the second to fourth sacral segments, with a contribution from the perineal branch of the fourth sacral nerve. Blood reaches it through the inferior rectal artery, with help from the middle rectal artery.

Damage to the sphincter by injury, surgery for anal fistula or fissure, or nerve damage leads to faecal incontinence, so anal surgeons plan their cuts with great care. Its tone is assessed on digital rectal examination and with anal ultrasound.

Attachments, action and supply

Origin

Anococcygeal body and tip of the coccyx (superficial part); the subcutaneous and deep parts form rings without bony attachment.

Insertion

Perineal body, in front of the anal canal.

Action

Keeps the anal canal and anus closed at rest and can close them more firmly at will; relaxes during defaecation.

Nerve supply

Inferior rectal (anal) nerve from the pudendal nerve (S2-S4) and the perineal branch of S4.

Blood supply

Inferior rectal artery, with contributions from the middle rectal artery.