Location and structure
The anal canal is the perineal part of the rectum (rectum). The entire rectum is 14–18 cm long, widening in its middle part (ampulla) and narrowing toward the anal canal.
Topographically, the canal lies below the pelvic diaphragm. Unlike the upper parts of the rectum, this subperitoneal part is not covered by peritoneum. It has a prominent visceral fascia known as the capsule of Amussat.
The longitudinal muscle layer of the rectum is evenly developed. 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 part.
Sphincter apparatus
- External anal sphincter (m. sphincter ani externus) is formed by striated muscle, is voluntary, and surrounds the outlet.
- Internal anal sphincter (m. sphincter ani internus) consists of smooth muscle and is involuntary. It is formed by annular muscle bundles located 3–4 cm from the anal opening.
- Third sphincter (m. sphincter ani tertius or Houston's muscle) is a local thickening of longitudinal muscle fibers located 10 cm from the opening.
Blood supply
Blood supply to the rectum and anal canal is provided by a system of five arteries:
- Superior rectal artery (a. rectalis superior) is an unpaired vessel, the terminal branch of the inferior mesenteric artery, passing in the root of the mesentery.
- Middle rectal artery (a. rectalis media) is a paired branch originating from the internal iliac artery (a. iliaca interna).
- Inferior rectal artery (a. rectalis inferior) is a paired branch originating from the internal pudendal artery (a. pudenda interna).
Clinical significance
Anogenital warts (condylomas) The anal canal and perianal region are common sites for human papillomavirus (HPV) infection, specifically serotypes 6, 11, 16, and 18. Histological findings include hyperkeratosis, acanthosis, papillomatosis, and koilocytic atypia—perinuclear vacuolation of cells. Detection of nuclear hyperchromasia, pleomorphism, and high mitotic activity indicates intraepithelial neoplasia or carcinoma in situ.
Perianal fistulas These are abnormal tracts connecting the lower portion of the 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, a leukocyte wall, and fecal masses.
Surgery and rehabilitation Surgical interventions involving the anorectal region may lead to purulent-septic complications, varying degrees of pain, and impaired fecal continence (incontinence). Medical rehabilitation is indicated following surgery involving the anorectal region.
Class and exam questions
Is the anal canal covered by the peritoneum?
No, the perineal part of the rectum lies subperitoneally and is not covered by the peritoneum. It features a prominent visceral fascia—the capsule of Amussat.
What is the internal sphincter formed by?
The internal sphincter (m. sphincter ani internus) is formed by smooth muscle, is involuntary, and is arranged in annular bundles 3–4 cm from the anal opening.
What is koilocytic atypia in the anal canal?
This is a specific epithelial cell change—perinuclear vacuolation—detected during histological examination of HPV-induced anogenital warts.
What structures are located on the sides of the anal canal?
Ischioanal fossae are located on both sides of the perineal part of the rectum.
What else to study on this topic
- Rectum
- Anal sphincters
- Ischioanal Fossa
- Human papillomavirus (HPV)
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