In the digestive system, undigested food is converted into a form that can be absorbed into the circulatory system for distribution and utilization by the various tissues of the body. This is accomplished, physically, by the mouth chewing and the stomach churning, and mentally by secretions and enzymes of the gastrointestinal tract. Beginning at the mouth, all food passes through the alimentary canal, which includes the, pharynx, stomach, intestines and esophagus, before it reaches the anus, where the remaining undigested food passes to be removed as waste. The outer walls of the digestive tract are composed of layers of muscles and tissues. The inner lining holds glands that secrete enzymes that are necessary to break down food.