The main organs of the digestive system include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. The mouth softens the food into a bolus, which is moved, via peristalsis, down the esophagus and into the stomach. Three layers of muscle churn the food into a uniform mixture called chyme. Parietal cells form HCl, which activates the enzyme pepsin and begins protein digestion.