THE THORAX 935 



the arch, and then ascends behind it. Within the concavity of the 

 aortic arch the trunk of the pulmonary artery breaks up into its right 

 and left divisions, and the ligamentum ductus arteriosi extends from 

 the root of the left pulmonary artery to the back part of the con- 

 cavity of the aortic arch, immediately beyond the origin of the left 

 subclavian artery. The gangliated cord of the sympathetic lies very 

 deeply, and descends over the heads of the ribs close behind the 

 parietal pleura, and the two, or three, splanchnic ner\''es lie obliquely 

 on the sides of the bodies of the lower thoracic vertebrae. 



The oesophagus lies in contact with the front of the vertebral 

 column, and the trachea is anterior to it. The descending thoracic 

 aorta lies very deeply, being situated at first on the left side of the 

 vertebral column, but subsequently in front of it. The thoracic 

 duct ascends on its right side, and the right azygos vein ascends 

 on the right side of the thoracic duct, both structures being under 

 cover of the oesophagus. The two left azygos veins, upper and 

 lower, as well as the two transverse azygos veins, upper and lower, 

 are closely related to the vertebral column. 



The thoracic cavity contains the following sets of lymphatic 

 glands : the anterior mediastinal, in front of the pericardium ; the 

 superior mediastinal, above the pericardium, along the arch of the 

 aorta and innominate veins ; the posterior mediastinal, behind 

 the pericardium ; the posterior intercostal, in the back parts of the 

 intercostal spaces ; and the bronchial, in the interval between the 

 diverging bronchi, and also at the root of each lung. In early 

 life a portion of th? thymus body, which is then of large size, lies 

 behind the upper part of the sternum, whence it extends into the 

 lower part of the neck. 



Pleurae. — ^The pleurae are the two serous sacs which invest the 

 lungs, and line the adjacent parietes. Each forms a shut sac, which 

 is quite distinct from its fellow. Like other serous membranes, 

 the pleura consists of two portions — parietal and visceral — which, 

 however, are continuous with one another. 



The parietal pleura lines the parietes, and is divisible into five 

 portions, namely, costal, diaphragmatic, pericardial, cervical, and 

 mediastinal. The costal pleura lines the inner surfaces of the ribs 

 and internal intercostal muscles. The diaphragmatic pleura covers 

 the upper surface of one-half of the diaphragm. Th& pericardial 

 pleura is in intimate contact with the pericardium. The cervical 

 pleura forms what is known as the dome, and rises into the neck for 

 about I inch above the clavicle, being a little higher on the right 

 side than on the left, on account of the projection formed by the liver. 

 The mediastinal pleura of each side bounds the media'^tinum thoracis. 



The visceral pleura closely invests the lung, and is known as the 

 pulmonary pleura. It is intimately connected with the limg sub- 

 stance, and extends into the fissure, or fissures, which map out the 

 lung into lobes. Below the root of each lung it forms a fold, called 

 the ligamentum latum pulmonis, which descends to the diaphragm, 

 and is connected with the pericardium. 



