ANUS, the terminal aperture of the intestinal tract. This is not to be confused with the rectum, of which it is the final portion. In the embryo the anus is closed by a membrane which normally ruptures shortly before birth. Encircling the anal canal are the muscle fibres which control the evacuation of faeces. When con tracted they throw the skin and mucous membrane into folds, giv ing a wrinkled appearance. The common disorder called haemor rhoids or piles is due to dilated veins which project from the anus and often bleed. Cancer of the rectum or anus may also cause bleeding.