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Large intestine. Coloured barium X-ray of a section of a healthy large intestine (blue), the final section of the digestive tract. The intestine provides a large surface area for the absorption of water, vitamins and mineral salts, and produces mucus to assist faecal movement and to protect its wall from mechanical damage. This is an air, or double, contrast barium enema. Water containing radio-opaque barium sulphate powder was introduced into the colon, which was then distended by air. This technique can be used to diagnose various intestinal disorders. (KEYSTONE/SCIENCE PHOTO LIBRARY/DU CANE MEDICAL IMAGING LTD)