THE GREAT VESSELS 319 



arch is much more pronounced than its inclination to the left, 

 and, as a result, almost the whole of the vessel lies behind 

 the manubrium sterni (Fig. 106). 



At its commencement, the aortic arch occupies the interval 

 between the two pleural sacs, but, in most of its course, it is 

 covered by the left mediastinal pleura. As the left vagus and 

 phrenic nerves descend through the thorax, they cross the 

 vessel and intervene between it and the pleural sac. The 

 left innominate vein crosses the branches of the aortic arch 

 close to their origins, and it is therefore closely related to the 

 upper border of the arch. 



The signs produced by an aneurism of the aortic arch 

 depend partly on the direction in which it enlarges. When it 

 does so in a forward direction, it compresses the left lung and 

 comes into contact with the manubrium. The area of super- 

 ficial dulness in this region becomes increased in size, and, as 

 the aneurism enlarges, it may erode the sternum. At an early 

 stage, however, it may be difficult to determine whether the 

 dulness is due to aneurism or to a mediastinal tumour. The 

 left vagus and phrenic nerves are more liable to be stretched 

 than to be compressed, but they usually slip backwards over 

 the aneurism, and their involvement can rarely be determined 

 from the physical signs. 



Posteriorly, the aortic arch comes into contact successively 

 with the trachea, the left recurrent nerve, the oesophagus, the 

 thoracic duct and the vertebral column (Fig. 113). Any or all 

 of these structures may be compressed, when an aneurism of 

 the aortic arch enlarges in a backward direction. Pressure on 

 the trachea results in respiratory discomfort and is indicated 

 by the association of rales with the breath sounds. This sign 

 may be accompanied by difficulty in swallowing, since the 

 oesophagus passes downwards between the trachea and the 

 vertebral column. The left recurrent nerve leaves the vagus 

 at the lower border of the arch and hooks backwards and 

 upwards behind the vessel to gain the groove between the 

 trachea and the oesophagus. It is very commonly affected in 



