THOEACIC ANEUKYSMS. 275 



in this region and may be compressed, but this is rather too low 

 for the pupil fibres to be caught. Lower down in the thorax 

 the aorta lies in a more median position in front of the spine, 

 and behind the vertical part of the diaphragm. 



The left pleural sac is closely applied to the left aspect of the 

 vessel, and an aneurysm of this part of the aorta is more likely 

 to rupture into the left pleura than elsewhere. But the right 

 pleural sac is also in relation, being only separated by the 

 thoracic duct and the oesophagus, and even projects behind the 

 oesophagus in the lower part of the thorax in the form of a 

 cul-de-sac, which comes very close to the aorta. So it is not 

 surprising that the percentage of cases of rupture into the right 

 pleura is quite half of that of rupture into the left. 



An aortic aneurysm rarely produces symptoms of pressure on 

 the thoracic duct, although the latter is in close proximity ; the 

 pressure increases gradually, and probably a collateral lymphatic 

 circulation has time to establish itself. 



Aneurysms of the upper part of the descending thoracic aorta 

 have been known to rupture into the mediastinal or sub-pleural 

 tissue, surrounding the oesophagus in tubular fashion, invading 

 the connective tissue basis of the pericardium, burying the vagus 

 nerves and extending even under the serous coat of the stomach. 

 Anomalous symptoms may thus be produced, of which anaemia 

 and vomiting are the chief. 



An innominate aneurysm is generally associated with some 

 dilatation of the aortic arch itself, and often of the subclavian and 

 common carotid. From the position of the innominate artery 

 the tumour usually appears above the sternum and slightly to 

 its right, and tends to raise the right sterno-mastoid muscle near 

 its origin. The pressure it exerts upon the superior vena cava 

 may lead to considerable dilatation of the venous tributaries of 

 that vessel ; hence congestion of both upper extremities and of 

 the head and neck is liable to be in evidence. The right radial 

 pulse may be considerably altered in character, both in the way 

 of size and time. 



An aneurysmal sac upon the common carotid artery is 



182 



