External anal sphincter muscle

The external anal sphincter is a striated, voluntary muscle that forms a circular band around the anal canal. It is the principal muscle involved in conscious anal closure, providing control over fecal expulsion in coordination with the internal anal sphincter and surrounding perineal structures.

Origin: Perineal body and surrounding connective tissue

Insertion: Encircles the anal canal; some fibers insert into the anococcygeal ligament or coccygeal fascia

Nerve:Caudal rectal nerve, a branch of the pudendal nerve

Action: Voluntary constriction of the anal canal; maintains fecal continence

Artery: Caudal rectal artery, a branch of the internal pudendal artery

Species-Specific Anatomy

1. Canine (Dog)

• Origin & Insertion: Encircles the anal canal and anchors into the perineal body and surrounding connective tissue. Some fibers blend with the levator ani and coccygeus muscles.

• Structure: Tripartite in some descriptions — subdivided into subcutaneous, superficial, and deep parts.

• Innervation: Caudal rectal branch of the pudendal nerve (somatic).

• Function: Maintains fecal continence and contracts during voluntary defecation efforts.

2. Feline (Cat)

• Similar to the canine, though relatively less robust.

• Also innervated by the pudendal nerve and participates in both voluntary and reflex control.

3. Equine (Horse)

• Well-developed muscle due to large anal diameter and substantial fecal volume.

• Strongly attached to the coccygeal vertebrae via fascia.

• Important during defecation; easily palpable in rectal examination or when evaluating tail tone.

4. Ruminants (Cattle, Goats, Sheep)

• Slightly more fibrous in cattle.

• Performs similar function across species — voluntary control of anal closure.

• Often examined in clinical practice for neuromuscular tone during neurological assessment.

5. Porcine (Pig)

• Structurally and functionally analogous to the dog.

• Plays a lesser clinical role due to differences in management and handling, but still important in anorectal integrity.

Functional and Clinical Relevance

• Innervation: The external anal sphincter is innervated by the caudal rectal nerve, a terminal branch of the pudendal nerve. This somatic nerve provides the voluntary motor control necessary for fecal continence.

• Vascularization: Supplied by branches of the caudal rectal artery, a branch of the internal pudendal artery.

• Lymphatic drainage: Primarily to the superficial inguinal and medial iliac lymph nodes.

• Clinical Importance:

  • Loss of function can occur with pudendal nerve damage (e.g., following dystocia or pelvic trauma).

  • Palpation of EAS tone is used in neurologic exams to assess sacral spinal cord function.

  • EAS tone contributes to continence; dysfunction can result in fecal incontinence, often seen in spinal cord injuries or neuropathies.