The anus is the last 4 cm of the digestive tract. The function of the anus is a sphincter to hold the bowel motion in the rectum until you wish to go to the loo. The most common type of anal cancer (more than 90%) arises from the cells lining the anal canal and is called squamous cell carcinoma.
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There are some rarer types of anal cancer arising from the cells lining the rectum called adenocarcinomas and occasionally melanomas may occur.