Cancer of the Anus Anatomy
To better understand anal cancer, it helps to understand the anatomy of the colon, rectum and anus.
The intestine is a long, continuous, tube inside the body. It lets the body absorb nutrients from food and liquids. The intestine is about 22 feet long in an adult. It includes the large intestine (colon) and the small intestine.
The large intestine is also known as the colon. It is the last portion of the intestine. The colon has several parts, including:
- Ascending colon: the first section after the small intestine, located on the right side
- Transverse colon: sits horizontally across the upper abdomen
- Descending colon: located on the left side of the abdomen
- Sigmoid: a short, S-shaped section above the rectum
- Rectum: the lowest internal part of the colon
The anus is the opening on the gastrointestinal tract, where the stool exits. The anus marks the transition from the lining of the gastrointestinal tract to the external skin. The anal canal is a tube, 1-2 inches long that leads to the anus. The anal canal is surrounded by circular muscles, called sphincters, and blood vessels. The sphincters control defecation.
- Clark MA, Hartley A, Geh JI. Cancer of the anal canal. Lancet Oncol. 2004 Mar;5(3):149-57. 
- Sato H, Koh PK, Bartolo DC. Management of anal canal cancer. Dis Colon Rectum. 2005 Jun;48(6):1301-15. 
- Welton ML, Sharkey FE, Kahlenberg MS. The etiology and epidemiology of anal cancer. Surg Oncol Clin N Am. 2004 Apr;13(2):263-75.