Chap, x.] THE THORAX. 161 



due to the spreading downwards of a retro-pharyngeal 

 or retro-cesophageal collection of matter. 



The azygos veins, commencing as they do below 

 in the lumbar veins, and having more or less direct 

 communications with the common iliac, renal, and 

 other tributaries to the vena cava, are able to a great 

 extent to carry on the venous circulation in cases of 

 obstruction of the main trunk. 



These veins are apt to be pressed upon by tumours 

 (such as enlarged gland masses) developed in the 

 posterior mediastinum, and to produce in consequence 

 some oedema of the chest walls by engorgement of 

 those intercostal veins that they receive. Tumours 

 growing in the posterior mediastinum may cause 

 trouble by pressing upon the trachea or gullet, or by 

 disturbing the vagus nerve or the cord of the sym- 

 pathetic (Figs. 33 and 35). 



