TOPOGRAPHY OF THE THORAX AND ABDOMEN 21 



up to where its subclavian branch arches across the an- 

 terior surface of the lung. The phrenic nerve is anterior 

 to the lung above the first rib, but below this level it is be- 

 tween the lung and the vena cava superior. The vagus is 

 also anterior to the lung above but at the sternoclavicular 

 articulation it passes inward and backward between 

 the lung and the trachea. In this region the left lung is 

 deeply grooved by the arch of the aorta below and the sub- 

 clavian artery and vertebral vein above. The groove for 

 the subclavian extends from that for the aorta upward to 

 the subclavian groove across the anterior surface of the 

 apex. The crista pulmonis is in relation in this region with 

 the thoracic duct and oesophagus anteriorly and the verte- 

 bral column posteriorly. The left lung does not enter into 

 close relation with the trachea but is separated from it above 

 by the subclavian artery, and the oesophagus which lies to 

 the left of the midplane, and below by the arch of the aorta. 

 The vagus and phrenic nerves of this side like those of the 

 right side are at first anterior to the lung at the apex but 

 pass internal to it just below. On both sides these nerves 

 pass anterior to the arch of the subclavian. The left vagus 

 follows the anterior surface of the left subclavian artery 

 down to the arch of the aorta, the left side of which it 

 crosses between the pleura and the artery. The phrenic 

 nerve lies anterior to the vagus. It is between the lung 

 and the innominate vein above, and is embedded below, in 

 the mediastinal wall about half way from the sternum to 

 the vertebral column (Plates V, VI, and VII). 



In the region of the posterior mediastinum the internal 

 surface of the inferior lobe of each lung presents a slight 

 impression. The vena azygos major lies in the impression 

 in the right lung. The descending aorta makes the im- 

 pression in the left lung. This impression is not as deep 



