If the food has been properly cooked and masticated, gastric digestion will be completed in from one and one-half to three hours. If not properly cooked and masticated, the stomach digestion may continue from one to two hours longer. It should, however, be completed in three hours. It will be seen that the evening meal is ordinarily digested before sleep, as one does not retire for from three to five hours after eating. If, through imperfect mastication, or a disordered condition of the stomach, the digestion is not completed in about three hours, the food is likely to be retained in the stomach and by its weight may cause prolapsus of that organ if the supporting tissues are weak. Fermentation may ensue and give rise to gases which may cause acute distress. Animal foods, which are readily digested, remain in the stomach for a shorter time. Meat, as a rule, is easily digested, because the digestive juices of the animal have converted the starches and sugars. The white meat of chicken is digested in a shorter time than the red or the dark meat. Corn, beets, peas, beans, etc., take about three and a half hours to digest; baked potatoes about two and a half hours. Raw vegetables and fruits remain about the same length of time as potatoes. Sugar is usually absorbed within an hour. The cereals, if well cooked, take but two hours. Coarse or badly masticated food, tough meats, unripe fruits, and much fat hinder digestion. Undigested food passing into the intestine may fail to be acted on there and will sometimes produce diarrhea. Fluids leave the stomach more rapidly than solids. Seven ounces of water entirely leaves the stomach in one and one-half hours, seven ounces of boiled milk in about two hours. Water and buttermilk almost immediately begin to pass out of the stomach; milk begins to pass out in about fifteen minutes. The flow of gastric juice, as the flow of saliva, is governed by the nerves; the sight, taste, and smell of food, and the attitude of mind toward it, to a certain extent, regulate its flow. After the food has accumulated, during the progress of a meal, the stomach begins a series of wave-like movements called peristaltic waves.[7] These waves propel the food through the length of the stomach towards its lower opening, known as the pyloric orifice. During this process the food is thoroughly mixed with the gastric juice.