External anal sphincter

Location and anatomical relations

The external anal sphincter is located in the anal region of the perineum.

• Rectum (rectum) — the sphincter surrounds its outlet.
• Perineal body (centrum tendineum perinei) — the structure into which the sphincter muscles interweave.
• Levator ani muscle (m. levator ani) — represents the deepest layer of the pelvic floor muscles.

Sphincter apparatus of the rectum

The sphincter apparatus of the rectum includes:

• External anal sphincter (m. sphincter ani externus) — consists of striated muscle, is under voluntary control, and surrounds the outlet of the rectum.
• Internal anal sphincter (m. sphincter ani internus) — formed by smooth muscle, involuntary. Circular bundles are located 3–4 cm from the anus.
• Third sphincter / Houston's muscle / Hohl's sphincter (m. sphincter ani tertius) — thickening of the longitudinal muscle fibers located 10 cm from the anus. It corresponds to the perineal flexure of the rectum.

Topography and surrounding structures

The external anal sphincter serves as a topographical landmark of the perineum.

• Ischioanal fossae (fossa ischiorectalis) — located on both sides of the perineal part of the rectum.
• Pelvic floor muscles — at the level of the pelvic diaphragm, fibers of the m. levator ani interweave into the muscular layer of the rectum. The levator ani muscle includes the pubococcygeus, iliococcygeus, and puborectalis/ischiococcygeus muscles.
• Anococcygeal ligament (lig. anococcygeum) — listed among the landmarks of the perineum.

Clinical significance and birth injuries

The condition of the external sphincter is important in obstetrics and operative surgery:

• Perineal tears: in third-degree perineal tears, the external anal sphincter (m. sphincter ani externus) is listed among the damaged structures.
• Surgical repair: when repairing a third-degree tear, the rectum is repaired first: sutures are passed through the edges of the intestinal wound extramucosally, without piercing the mucosa. Then catgut sutures are placed on the edges of the m. sphincter ani externus. The result of this step is converting a third-degree tear into a second-degree tear. Next, the vagina, deep transverse perineal muscle, medial margins of the m. levator ani, and superficial transverse muscle along with connective tissue are sutured in layers.
• Central perineal tear: in this rare injury, the fetus is born through an abnormal opening in the center of the perineum. The rectal sphincter and posterior labial commissure remain intact, while severe damage to the perineal muscles is observed.
• Rectal surgeries: during the perineal stage, the anal opening is closed with a purse-string suture, and two elliptical incisions are made around the anus.

Class and exam questions

What else to study on this topic

• The Rectum and Its Parts
• Levator ani muscle (m. levator ani)
• Perineal body
• Internal anal sphincter (m. sphincter ani internus)

Other topics in the "Anatomical Structures" section

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