THE PLEURA AND MEDIASTINUM 407 



space between the visceral and parietal layers is the pleural 

 cavity (cavum pleurae), and contains a small amount of clear 

 fluid. Their is no cavity when the pleurae are in a healthy 

 condition, the two layers being in contact. 



The two pleurae are distinct from each other, and do not 

 meet in the median line except behind the second piece of the 

 sternum. At the root of the lung the visceral and parietal 

 layer of the same side are continuous, and at the lower part 

 of the root, a fold, the ligamentum latum pulmonis, runs down 

 to the diaphragm. 



The blood supply of the pleura is derived from the inter- 

 costal, the internal mammary, the musculophrenic, thymic, 

 pericardiac, and the bronchial. The veins correspond to 

 the arteries. The nerves, are the phrenic and sympathetic. 

 (Luschka.) The lymphatics of the visceral layer empty into 

 the superficial pulmonary trunks ; the lymphatics of the parietal 

 pleura empty into the intercostal, the diaphragmatic trunks 

 and the posterior mediastinal nodes. 



The structure of the pleura is composed of a fibroelastic 

 connective tissue, its free surface covered with a single layer 

 of flat endothelial cells. Subserous tissue fastens the pleurae 

 to the parietes and the lung respectively. 



The mediastinum is the space between the two pleural sacs, 

 and extends anteroposteriorly from the sternum to the spine; 

 it is divided into a superior mediastinum, above the upper 

 level of the pericardium; the anterior, in front of the pericardium; 

 the middle, containing the pericardium; and the posterior 

 mediastinum, behind the pericardium. 



The superior mediastinum is bounded by the manubrium 

 sterni in front, the upper four dorsal vertebrae behind, and 

 below by a plane passing from the lower border of the manu- 

 brium to the lower part of the fourth dorsal vertebra. It 

 contains the lower part of the sternohyoid and thyroid and 

 longus colli muscles, the transverse aorta, innominate, left 

 carotid, and subclavian arteries, the superior cava (upper half), 

 the two innominate and the left superior intercostal veins, 

 the vagus, cardiac, phrenic, and left recurrent nerves, trachea, 

 esophagus, thoracic duct, thymic remains, and lymphatics. 



The anterior mediastinum is bounded by the sternum and 

 the pericardium before and behind, by the pleurae laterally. 

 It runs toward the left, is broader below than above, and 



