THOEACIC ANEUEYSMS. 273 



the mediastinal aspect of the upper lobe of the left lung. 

 The lung ma} 7 be directly compressed by aneurysms, and both 

 the left vagus and the left phrenic nerves may be found adherent 

 to the sac. Compression of the phrenic nerve causes paralysis 

 of the corresponding half of the diaphragm. Apart from recur- 

 rent laryngeal paralysis it is difficult to correlate pressure on the 

 vagus with definite symptoms. Eupture of the aneurysm into 

 the left pleural cavity is not uncommon. 



The lower concave surface of the aortic arch overhangs the 

 root of the left lung, being placed astride the left bronchus and 

 bifurcation of the pulmonary artery. The tracheal tug is best 

 developed when an aneurysm springs from this aspect of the 

 arch, the impulse being communicated directly downwards to the 

 left bronchus, which drags on the trachea and the larynx. The 

 left bronchus may be obstructed or opened, and the left pulmonary 

 artery more or less occluded. Compression of the left recurrent 

 laryngeal nerve, which turns round the aortic arch beyond the 

 remnant of the ductus arteriosus, will produce at first abductor 

 and finally complete laryngeal paralysis on one side. The upper 

 surface of the aortic arch is in relation with the left innominate 

 vein and three great arteries spring from it, the innominate and 

 left common carotid arising close together and the left sub- 

 clavian further back. Pressure on the innominate veins will 

 give rise to enlargement of the veins of the head, arms, neck, 

 and upper part of the chest, whilst implication of the origin of 

 the arteries mentioned, or pressure on them by the aneurysmal 

 sac, or involvement of their lumina by the accompanying 

 endarteritis will modify their pulses in various ways. 



Eupture into the pericardium is an unlikely accident to occur 

 in connection with aneurysms of the aortic arch, but aneurysms 

 at the junction of the arch and the descending aorta have been 

 known to rupture in this way after working forwards above the 

 left bronchus. Eupture into the mediastinal tissues gives rise 

 to extravasation of blood around the oesophagus and pharynx, 

 extending into the neck beneath the deep cervical fascia. 



The descending thoracic aorta is closely applied to the back 



C.A.A. 18 



