Swallowing, or deglutition, is divided into three phases: The buccal phase occurs voluntarily in the mouth when the tongue forces the bolus of food toward the pharynx. The pharyngeal phase occurs involuntarily when food enters the pharynx, as follows: The soft palate and uvula fold upward and cover the nasopharynx to prevent the passage of food up and into the nasal cavity. The epiglottis, a flexible cartilaginous flap at the top of the larynx, folds down as the larynx rises. As a result, the opening to the trachea is covered, and food can pass only into the esophagus. The esophageal phase occurs involuntarily in the esophagus. The esophageal sphincter, normally closed, opens to allow food to pass when the larynx rises during swallowing. When food reaches the lower end of the esophagus, the lower esophageal sphincter opens to allow the food to enter the stomach.