THE ESOPHAGUS, TRACHEA, AND THYMUS (See also pp. 119-121, 157-158, and 265-267.)
The esophagus enters the thoracic cavity to the left of the trachea but gradually assumes a median position above the trachea within the cranial mediastinum, where it is related to the left subclavian artery, which intervenes between it and the left lung (see Figure 13-14).
It continues dorsal to the trachea and subsequently to the left principal bronchus, where it crosses the heart before passing between the aorta and the azygous vein. Inclusion between these vessels and perhaps also the slight rise over the tracheal bifurcation predispose this part to obstruction by foreign bodies. A potentially more serious interference with the passage of food may be provided by the anomaly in which the right aortic arch persists as part of a constricting ring composed of the aorta to the right, the ligamentum arteriosum dorsally, and the pulmonary trunk and right pulmonary artery to the left (see Figure 7-2, D). More caudally, the esophagus rests on the left atrium and then on the accessory lobe of the right lung before reaching the hiatus in the diaphragm below the tenth thoracic vertebra. A slight narrowing here provides another site for obstruction. The chief blood supply from the bronchesophageal artery is supplemented by direct branches from the aorta; the most caudal stretch is supplied by branches of the left gastric artery.Cranial to the heart, a surgical approach to the esophagus is easier from the left; level with the heart, the approach from the right is favored because the azygous vein may be ligated with impunity, unlike the aorta to the left. The caudal section is equally approachable from either side.
The muscle is striated throughout the length of the esophagus in both dog and cat; only the caudal section is extensively covered by serosa. Glands are present in the submucosa only in the dog. The mucosa is thrown into ridges; these are predominately longitudinal throughout the length of the esophagus of the dog but acquire an oblique orientation in the caudal part of the esophagus of the cat. These differences are responsible for the radiographic appearance after a barium swallow: longitudinal streaks are replaced caudal to the heart in cats by a herring-bone pattern (Figure 13-19, B).
The relationship of the trachea to the esophagus has been mentioned. The shift to a position ventral to the esophagus at the level of the aortic arch produces a caudally open angle that is a very prominent feature of lateral radiographs (Figure 13-17Zin the formation, course, or distribution of the phrenic, vagus, and sympathetic nerves.