The thick dorsal part of the cranial mediastinum contains the esophagus and trachea, the vessels passing to and from the neck and forelimbs, an assembly of lymph nodes, the thoracic duct, and various nerves.

In older animals the ventral part is thin, containing only the internal thoracic vessels and a vestige of the thymus. The difference in thickness is less striking in younger animals, in which the thymus has yet to regress (Fig.

FIG. 27.3 Left lateral view of the bovine thoracic cavity. The left lung and part of the mediastinal pleura have been removed. 1, External jugular vein; 2, Sternocephalicus; 3, axillary artery; 4, axillary vein; 5, cervicothoracic ganglion; 6, esophagus; 7, vagus; 8, phrenic nerve; 9, one of the cardiac nerves; 10, trachea; 11, internal thoracic artery; 12, mediastinal pleura; 13, pericardium, reflected; 14, pulmonary trunk; 15, aorta; 16, left azygous vein; 17, sympathetic chain; 18, recurrent laryngeal nerve; 19, ventral vagal trunk; 20, dorsal vagal trunk; 21, caudal mediastinal lymph nodes; 22, cranial extent of diaphragm;

23, diaphragm; 24, internal intercostal muscle; 25, external intercostal muscle.

The middle mediastinum is occupied by the heart (within the pericardium) ventrally, and the esophagus, the termination of the trachea, the aortic arch, pulmonary vessels, left azygous vein, various lymph nodes, and the vagal trunks dorsally (Fig. 27.6). It is thus of very irregular thickness, being reduced in places to apposed pleural sheets. Ventral to the heart, it widens to contain the pericardiosternal ligament.

The caudal mediastinum is generally thin. The dorsal part contains the esophagus, aorta, vagus trunks, and caudal mediastinal nodes (Fig. 27.7). The septum is very short and level with the base of the heart but, below this, lengthens where it deviates to the left (Fig. 27.5).