THE THORAX 



329 



substance. The two layers are continuous through sheaths sur- 

 rounding the vessels which enter and leave the heart. The parietal 

 layer of the serous pericardium is applied to the inner surface of a 

 thicker and much tougher sac, the fibrous pericardium. This, in 

 turn, is partly attached by loose connective tissue to surrounding 

 organs and partly covered by the serous mediastinal pleurae, the 

 linings of the cavities for the lungs. 



The paired pleural cavities containing the lungs are broadly separated by a 

 longitudinal vertical partition, the mediastinum or mediastinal septum, the 

 «pace enclosed by the latter being largely occupied by the heart and by the cavity 

 oi the pericardium. For a considerable area ventrally the pericardium is loosely 



Fig. 112. Interior of the left pleural cavity with the lung removed to show the 

 mediastinum, lateral view, a, thoracic aorta; d, diaphragm; h, heart in peri- 

 cardium; 1, remnant of base of lung; Ip, pulmonary ligament; m, posterior 

 mediastinum; oe, oesophagus; th, thymus gland; vcs, left superior caval vein. 

 The phrenic nerve is visible crossing tlie pericardium and the mediastinal septum 

 and branching into the diaphragm, more dorsally the vagus nerve crosses the 

 lateral surface of the oesophagus, and the sympathetic trunk appears on the 

 dorsal wall. 



applied to the thoracic wall, the intervening space, which is bounded laterally 

 by the membrane lining the pleural cavities (pleura, p. 334), being known as the 

 anterior mediastinum. A corresponding dorsal space lying between the heart 

 and the bodies of the thoracic vertebrae, and also bounded laterally by the pleura, 

 is the posterior mediastinum. It is occupied by several structures, namely, 

 the oesophagus, the thoracic aorta, the bfonchi, and the pulmonary blood-vessels, 

 and caudally forms a thin septum similar to mesentery. In the rabbit and most 

 quadrupedal mammals, unlike man, the pericardium does not reach the diaphragm, 

 so that, in the rather narrow space between the caudal end of the pericardium and 



