Location and Structure
The anal canal is the perineal portion of the rectum (rectum). The entire rectum is 14–18 cm long, expanding in its middle portion (ampulla) and narrowing toward the anal canal.
Topographically, the canal lies inferior to the pelvic diaphragm. Unlike the upper portions of the rectum, this subperitoneal segment is not covered by peritoneum. It features a distinct visceral fascia known as the fascia of Amussat.
The longitudinal muscular layer of the rectum is distributed evenly. At the level of the pelvic diaphragm, fibers of the levator ani muscle (m. levator ani) blend into it. Ischioanal fossae are located on both sides of the perineal segment.
Sphincteric Apparatus
- External anal sphincter (m. sphincter ani externus) — formed by skeletal (striated) muscle, is under voluntary control, and surrounds the orifice.
- Internal anal sphincter (m. sphincter ani internus) — consists of smooth muscle, is involuntary, and forms circular muscular bundles located 3–4 cm from the anal verge.
- Third anal sphincter (m. sphincter ani tertius or Houston's muscle/rectal valve fibers) — a local thickening of longitudinal muscle fibers located about 10 cm from the orifice.
Blood Supply
The blood supply to the rectum and anal canal is derived from a system of five arteries:
- Superior rectal artery (a. rectalis superior) — an unpaired vessel, which is the continuation of the inferior mesenteric artery running within the root of the sigmoid mesocolon.
- Middle rectal artery (a. rectalis media) — a paired branch arising from the internal iliac artery (a. iliaca interna).
- Inferior rectal artery (a. rectalis inferior) — a paired branch arising from the internal pudendal artery (a. pudenda interna).
Clinical Significance
Anogenital Warts (Condylomata Acuminata) The anal canal and perianal region are frequent sites for human papillomavirus (HPV) infection, notably serotypes 6, 11, 16, and 18. Histological findings include hyperkeratosis, acanthosis, papillomatosis, and koilocytic atypia (perinuclear halo and nuclear clearing). When nuclear hyperchromasia, pleomorphism, and high mitotic activity are identified, intraepithelial neoplasia or carcinoma in situ is diagnosed.
Perianal Fistulas These are pathological tracts connecting the lower rectum or anal canal to the perianal skin. The most common type is the intersphincteric fistula. Fistulas share a common etiology with anorectal abscesses. Histological examination reveals granulation tissue, fibrin, leukocytic infiltration, and fecal debris.
Surgery and Rehabilitation Surgical interventions involving the anorectal region carry risks of purulent-septic complications, varying degrees of pain, and fecal incontinence. Postoperative medical rehabilitation is indicated following anorectal surgery.