The esophagus is involved in the processes of swallowing and peristalsis to move substances from the mouth to the stomach. The swallowing food begins in the mouth and continues with the contraction of skeletal muscles in the pharynx and esophagus. The upper esophageal sphincter dilates to permit the swallowed substance to enter the esophagus. From this point, waves of muscle contraction called peristalsis move food toward the stomach. In peristalsis, regions of the esophagus closer to the stomach open to permit food to pass through while the region just above the food contracts to push the food onward. Peristalsis works so well that food can be swallowed even while the body is lying down, upside down, or even in zero-gravity.