The rectum is the lower 10 to 15 cm of the large intestine. The anal canal is the last segment of the digestive system. It is a tube about 3.8 cm long running from the perianal skin of the buttocks to the wall 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 supplied by blood vessels and nerves.
There is a ring of folds called the dentate line where the anal canal meets the rectum.
Among these folds are the anal crypts, small tube-like depressions opening into the anal canal.
The dentate (or pectinate) line delineates where nerve fibres end. Above this line, this area is relatively insensitive to pain. Below the dentate line, the anal canal and anus are extremely sensitive.