Caring for mother and child during pregnancy
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 The health information contained herein is provided for general informational and educational purposes only and is not intended to replace discussions with a healthcare provider. All decisions regarding patient care must be made with a healthcare provider, considering the unique characteristics of each patient.
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Anatomy of the anorectal area (1)
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The rectum is the lower 10 to 15 cm of the large intestine. The anatomic anal canal is the outlet of the digestive system. It is a tube about 3.8 cm long running from the perianal skin of the buttocks to the mucosal lining of the rectum. Its external opening is the anus, which is tightly shut except during stool evacuation by two strong but sensitive rings of muscles: the internal sphincter and external sphincter. The action of the internal sphincter is controlled subconsciously (it is always contracted to prevent accidental loss of stool), whereas the action of the external sphincter is voluntary.
The sphincters are well supplied with blood vessels and nerves. Where the anal canal meets the rectum there is a ring of folds called the dentate line. Among these folds are the anal crypts, small tube-like depressions opening into the anal canal. The dentate line delineates where nerve fibers end. Above this line, this area is relatively insensitive to pain. Below the dentate line, the anal canal and anus are extremely sensitive.