I 3 2 



THORAX 



part of its extent it lies to the left of the vertebral column ; 

 but in the lower part of the thorax it lies anterior to the 

 column, in the median plane (Figs. 56, 54, 47, 48). 



Branches. Branches spring both from the anterior and the 

 posterior aspects of the descending thoracic aorta. Those 

 from the anterior aspect are the two left bronchial arteries, 

 four cesophageal branches, and some small and irregular media- 

 stinal and pericardial branches. The posterior branches are 

 nine pairs of aortic intercostal arteries and one pair of sub- 

 costal arteries. 



Relations. Anterior to the thoracic part of the descending 

 aorta, from above downwards, are the root of the left lung ; 



Pericardium 



Diaphragm 



phragm 

 (Esophagus 



Thoracic aorta 

 Left pleura 



Thoracic duct 

 Right pleura 

 Vena azygos 



FIG. 69. Tracing of a section through the lower part of the Posterior 

 Mediastinum, where its anterior wall is formed by the diaphragm. 



the upper part of the posterior wall of the pericardium, 

 separating the aorta from the left atrium ; the oesophagus, 

 separating the aorta from the lower part of the posterior wall 

 of the pericardium ; and the crura of the diaphragm, which 

 separate the lower portion of the thoracic aorta from the 

 omental bursa of the peritoneum and from the posterior sur- 

 face of the caudate lobe (O.T. Spigelian) of the liver (Fig. 28). 

 Posteriorly are the vertebral column, its own intercostal and 

 subcostal branches, the hemiazygos and accessory hemiazygos 

 veins ; and it is overlapped posteriorly in the upper part of its 

 extent by the left pleura and lung. Along its right side, in its 

 whole length, are the thoracic duct and the vena azygos, and 

 anterior to them, from the fifth to the lower part of the 

 seventh thoracic vertebra, lies the oesophagus. At a lower 

 level a mass of areolar tissue separates the aorta from the 



