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Gastrointestinal Tract

The Esophagus

Anatomically and functionally, the esophagus is the least complex section of the digestive tube. Its role in digestion is simple: to convey boluses of food from the pharynx to the stomach. The esophagus begins as an extension of the pharynx in the back of the oral cavity. It then courses down the neck next to the trachea, through the thoracic cavity, and penetrates the diaphragm to connect with the stomach in the abdominal cavity.

Like other parts of the digestive tube, the esophagus has four tunics, but important differences exist in the composition of these tunics in comparison to more distal sections of the tube. First, instead of the muscular tunic being entirely smooth muscle, as it is in the stomach and intestines, the wall of the esophagus contains a variable amount of striated muscle. In dogs, cattle and sheep, its entire length is striated muscle, whereas in cats, horses and humans, the proximal esophagus has striated muscle and the distal esophagus smooth muscle. Second, instead of the esophagus being free as it courses through the thoracic cavity, it is embedded in the connective tissue; thus, its outer tunic is referred to as adventitia instead of serosa.