Esophagus

The esophagus is a muscular tube extending from the pharynx to the stomach just caudal to the diaphragm. The end adjacent to the pharynx is kept closed by the m. cricopharyngeus, which passes from its origin on the cricoid cartilage over the dorsal aspect of the proximal esopha­gus.

From the pharynx, the esophagus runs dorsal to the trachea and usually inclines some­what to the left in the neck in the midcervical region. It again passes dorsal to the trachea when it enters the thorax and continues caudad between the trachea and the aorta through the mediastinum to pass through the diaphragm at the esophageal hiatus. Within the abdominal cavity, the esophagus joins the stomach.

The mucosa of the esophagus is thrown into prominent longitudinal folds that permit con­siderable cross-sectional dilation of the lumen to accommodate the passage of a bolus of food. The epithelium is of the stratified squamous type, being more or less keratinized in accordance with the roughness of the usual feedstuff.

The tunica muscularis of the esophagus con­sists of two layers that cross obliquely in the proximal esophagus, assume a spiral configura­tion in the midesophageal region, and form an inner circular and an outer longitudinal layer in the more distal parts. The muscle changes from striated to smooth in the caudal third of the esophagus in the horse and just cranial to the diaphragm in the pig; it is striated through­out its length in the ruminants. Esophageal muscles, both striated and smooth, are inner­vated by the vagus nerve.