Anal canal

The anal canal is the short, terminal segment of the large intestine that extends from the rectum to the anus, serving as the exit passage for fecal material. Its structure, location, and histological features are largely similar across domestic mammalian species, though some species-specific differences exist.

1. Location and Extent

• The anal canal begins at the anorectal junction, where the rectum terminates and the pelvic diaphragm (notably the levator ani muscle) marks the start of the perineum.

• It ends at the anal orifice (anus), the external opening surrounded by the external anal sphincter.

• Its length varies among species:

  • Dogs and cats: approximately 2–4 cm.

  • Horses and ruminants: slightly longer, reflecting body size.

2. Anatomical Relations

• Dorsally: Coccygeal vertebrae and associated muscles.

• Ventrally: Perineal body and, in females, the vulva; in males, the root of the penis.

• Laterally: Ischiorectal fossae, filled with fat and neurovascular structures.

3. Layers of the Anal Canal

• Mucosa: Changes from simple columnar (rectum) to stratified squamous non-keratinized and eventually to keratinized epithelium at the anus.

  • In carnivores, prominent anal sacs (paranal sinuses) open into the canal at the level of the intermediate zone.

• Submucosa: Contains venous plexuses (site of potential hemorrhoids in humans).

• Muscularis:

  • Internal anal sphincter: Smooth muscle (continuation of the circular layer of the rectum).

  • External anal sphincter: Skeletal muscle under voluntary control.

4. Species-Specific Features

• Dogs and cats: Have well-developed anal sacs (paranal sinuses) located between the internal and external anal sphincters.

• Ruminants and horses: Lack prominent anal sacs but still have similar muscular and mucosal arrangements.

• Pigs: Possess intermediate characteristics, with moderately developed anal glands.

5. Vascular and Nerve Supply

• Arterial supply: Primarily from the caudal rectal arteries.

• Venous drainage: Via the caudal rectal veins, communicating with the internal vertebral venous plexus.

• Innervation:

  • Autonomic: Regulates internal sphincter (sympathetic and parasympathetic).

  • Somatic: Pudendal nerve controls the external anal sphincter.

The anal canal in domestic mammals is typically divided into three zones:

• Columnar zone (zona columnaris): closest to the rectum.

• Intermediate zone (zona intermedia or zona hemorrhagica).

• Cutaneous zone (zona cutanea): most distal, opening to the exterior.