The esophagus is a muscular tube that passes from the pharynx through the thoracic cavity and diaphragm into the abdominal cavity, where it joins the stomach.Relaxation of the sphincter allows the bolus to pass into the stomach, while contractionprevents the acidic contents of the stomach from backing upinto the esophagus.When vomiting occurs, a contraction of the abdominal muscles and diaphragm apropels the contents of the stomach upward through theesophagus.The esophagus isordinarily collapsed, but it opens and receives the boluswhen swallowing occurs.A rhythmic contraction called peristalsis, pushes thefood along the digestive tract.Peristalsis begins in the esophagus and continues in all the organs of the digestive tract.