The anus is the opening at the end of the large bowel where we pass stools (bowel motions) out of the body. The bowel (colon and rectum) and anus are part of the digestive system.
Food passes down the gullet (oesophagus) into the stomach to be digested. It then moves into the small bowel, where important nutrients are removed from it and absorbed into the body.
Digested food moves into the large bowel, where water is absorbed. The waste product (stools) remains in the back passage (rectum) until it can be passed out of the body through the anus.
The anal canal is about 3 to 4cm (1 to 1½in) long. It connects the anus to the rectum. The area where the anus opens at the lower end is called the anal margin or anal verge.
The anus also has a ring of muscle called the external sphincter. This muscle helps to control when you pass bowel motions.
The walls of the anal canal are lined with cells called squamous cells. Nearly all anal cancers develop in these cells. We have more information about different types of anal cancer.
Where the anal canal meets the rectum (transitional zone), the walls are lined with squamous cells and glandular cells. Glandular cells make mucus that helps the bowel motions to pass through the anus.