338 A MANUAL OF ANATOMY. 



tion of the under surface of the arch is opposite the articu- 

 lation between the first and second portions of the sternum. 

 From its upper surface it gives off the innominate, left com- 

 mon carotid, and subclavian arteries. 



Relations. In front, pleurae and lungs (in inspiration), 

 left phrenic and pneumogastric and cardiac nerves, and the 

 left superior intercostal vein ; behind, trachea, oesophagus, 

 thoracic duct, deep cardiac plexus, left recurrent laryngeal 

 nerve ; above, its three branches and the left innominate 

 vein ; below, left bronchus, bifurcation of pulmonary 

 artery, left recurrent laryngeal nerve, remains of ductus 

 arteriosus, some lymphatic glands. 



The Descending Aorta. Diag. 22. 



This continues the arterial vessel from the ending of the 

 transverse aorta (see above) to the level of the lower bor- 

 der of the fifth dorsal vertebra, where it becomes the 

 thoracic aorta. 



Relations. In front, left lung and pleura covering it ; 

 behind, left side of the (third), fourth, and fifth vertebrae ; 

 at the right, oesophagus, thoracic duct, and the above 

 vertebrae ; at the left, lung covered by its pleura. 



The Thoracic Aorta. Diags. 22, 23. Figs. 72, 74. 



Is the continuation of the aorta from its descending por- 

 tion (see above) along the front of the dorsal vertebrae to 

 the twelfth, where it leaves the thorax by passing through 

 the aortic opening in the diaphragm into the abdomen. 

 At its beginning it is a little to the left of the middle line, 

 but at its ending is over the middle line. It is one of the 

 constituents of the posterior mediastinum. 



Relations. In front, root of left lung, oesophagus, dia- 

 phragm ; behind, the dorsal vertebrae from the fifth to the 

 twelfth inclusive, vena azygos minor (at the seventh or 



