anus

The anus is the opening at the end of the digestive tract through which solid waste leaves the body. It lies at the distal end of the anorectal canal, between the rectum and the skin of the perineal region.

Anatomically, the anus includes the anal canal and the surrounding perianal tissues. The canal is a few centimeters long and is lined by mucosa; below it, the anal verge forms the boundary with the skin. The dentate line marks the transition between columnar and non‑columnar epithelium.

Two sphincters regulate continence: the internal anal sphincter, a thick ring of smooth muscle that is involuntary, and the external anal sphincter, a voluntary skeletal muscle. The puborectalis portion of the levator ani helps maintain the anorectal angle, aiding continence.

Nerve supply includes autonomic fibers to the internal sphincter and somatic nerves from the pudendal nerve to the external sphincter. Sensation can occur in the anal canal and surrounding skin, providing reflex and conscious control over defecation.

Blood supply comes from the superior, middle, and inferior rectal arteries, with venous drainage to the portal and systemic circulations via the superior rectal vein and other channels. Lymphatic drainage follows the pelvic nodes.

Functionally, the anus facilitates feces excretion and maintains continence. It is involved in reflexes for defecation and protects against accidental passage of stool through sensory feedback and sphincter control. Common conditions include hemorrhoids, fissures, infections, and malignancy.