ESOPHAGUS

also spelled Oesophagus, relatively straight muscular tube through which food passes from the pharynx to the stomach. The esophagus can contract or expand to allow for the passage of food. Anatomically it lies behind the trachea and heart and in front of the spinal column; it passes through the muscular chest diaphragm before entering the stomach. Both ends of the esophagus are closed off by muscular constrictions known as sphincters; at the anterior, or upper, end is the upper esophageal sphincter, and at the distal, or lower, end is the lower esophageal sphincter. The upper esophageal sphincter is composed of circular muscle tissue and remains closed most of the time. Food entering the pharynx relaxes this sphincter and passes through it into the esophagus; the sphincter immediately closes to prevent food from backing up. Contractions of the muscles in the esophageal wall (see peristalsis) move the food down the esophageal tube. The food is pushed ahead of the peristaltic wave until it reaches the lower esophageal sphincter, which opens, allowing food to pass into the stomach, and then closes to prevent the stomach's gastric juices from entering the esophagus. Some animals, such as dogs and certain ruminants (cud-chewing animals), can bring the contents of their stomach back through the esophagus by reverse peristaltic movements. It is doubtful that humans have this ability. A bird's esophagus depends mainly on gravity for liquid and food propulsion; the bird must elevate its head to start the food's descent. The esophagus is a short, ill-defined area of alimentary tract of some fish and is elongated in land vertebrates. Disorders of the esophagus include ulceration and bleeding; heartburn, from gastric juices in the esophagus; achalasia, an inability to swallow or to pass food from the esophagus to the stomach, caused by destruction of the nerve endings in the walls of the esophagus; scleroderma, a collagen disease involving the esophagus; and spasms of the esophageal muscles. For information on esophageal speech, see pseudolaryngeal speech.