External anal sphincter muscle

The external anal sphincter muscle is a ring-shaped muscle composed of striated skeletal fibers that encircles the anal canal. It functions to voluntarily control the passage of feces by maintaining closure of the anal orifice. This muscle is part of the perineal musculature and is innervated by the caudal rectal nerve, a branch of the pudendal nerve.

Anatomy and structure

The external anal sphincter lies external to the internal anal sphincter, which is composed of smooth muscle. The two work together to maintain fecal continence. In many species, the external anal sphincter appears as a continuous muscular band. In some anatomical descriptions, especially in humans, it is divided into three parts:

• Subcutaneous part: located immediately beneath the perianal skin.

• Superficial part: may attach anteriorly to the perineal body and posteriorly to the anococcygeal ligament.

• Deep part: lies closest to the anal canal and may blend with fibers of the pelvic diaphragm (levator ani).

In domestic animals such as the dog, cat, and horse, this tripartite division is less distinct, and the muscle is generally described as a single, circular structure.

Species-specific characteristics

In carnivores like dogs and cats, the external anal sphincter also plays a role in compressing the paranal sinuses, aiding in their expression during defecation or stress. In large herbivores such as horses and ruminants, it serves primarily in maintaining anal closure. The muscle is under voluntary control, and its tonic contraction keeps the anal canal closed except during defecation.

Function

• Provides voluntary control over defecation.

• Maintains tonic contraction at rest to ensure anal continence.

• Assists in reflex closure of the anal canal during increased intra-abdominal pressure.

• Participates in the expression of anal sac contents in species that possess them.

Clinical relevance

Dysfunction of the external anal sphincter can result in fecal incontinence. This may occur due to damage to the pudendal nerve or trauma to the perineal region. In dogs, the external anal sphincter is often involved in perineal hernias, particularly in intact males. Surgical procedures near the anal region require care to avoid compromising this muscle and its innervation.