Digestive system, organs in the body that play a major role in the digestion of food, including the mouth, esophagus, stomach, and bowels. The pancreas and liver secrete juices that assist in the digestive processes. The muscular activity of the digestive tract disturbs the daily existence of humans. Hunger and the desire to defecate may arise from awareness of the movement of a part of the tract. Hunger contractions are a stimulus for food intake and arise when the stomach is empty. A few minutes after a meal, food, broken down by enzymes (digestive juices) into chyme, begins to leave the stomach, which is usually empty within 3 hours. On leaving the stomach, the food passes fairly rapidly through the small intestine, where it is further broken down, and within 3–4 hours the unabsorbed remnants begin to reach the colon, the residues of the various meals taken every day lying in the descending colon until expulsion. Defecation usually takes place once a day, but it is not abnormal for it to occur 3 times daily or to be withheld for up to 3 days. The muscle of the digestive tract regulates the passage of the contents to allow adequate time for digestion and absorption. Mastication (chewing) is a voluntary movement, and swallowing can be initiated voluntarily, so the muscles of the mouth and pharynx are all striated, and the esophagus contains both striated and smooth (involuntary) muscle. From the stomach to the anus there is only smooth muscle, except for the external anal sphincter, which is again striated.