Thoracic Aneurism. 159 



tortuous. These abnormal appearances would not be 

 very marked, if the part of the vessel within the pericar- 

 dium were the only portion involved, since the vena azygos 

 major offers a free channel for the collateral circulation; 

 and as this vessel arises from the lumbar veins, which are 

 radicles of the inferior vena cava, and empties into the 

 superior vena cava, the blood can find its way backward 

 through the azygos veins to the inferior vena cava and 

 thence into the heart. Should, however, the whole of the 

 superior vena cava be compressed the blood could not find 

 it's way back to the. heart through the above mentioned 

 channels, since the termination of the azygos vein in the 

 superior cava would be obliterated, but the circulation can 

 still be carried on through the following channels: (1) 

 The internal mammary veins which communicate with the 

 phrenic and the deep epigastric vessels branches of the 

 inferior vena cava; (2) the left superior intercostal vein, 

 which brings the left innominate into connection with the 

 left azygos veins, and thence into the inferior cava. Sud- 

 den swelling of the arm, or, the arm and one side of the 

 head and neck, without any local inflammatory symptoms, 

 should lead the surgeon to examine for an aneurism com- 

 pressing the vena innominate, right or left, as the case 

 may be. 



6. Pressure on the Nerves. When the pneu- 

 mo gastric is irritated, i.e., when the pressure of the aneur- 

 ism is not sufficient to completely destroy the functions of 

 the nerve, the rhythm of the heart may be disturbed from 

 irritation of the cardio-inhibitor fibres contained in the 

 nerve ; when, however, the degree of pressure is greater 

 than the above, then the functions of this nerve may be 

 entirely suspended, so that acceleration of the heart's ac- 

 tion results, because of the unopposed activity of the in- 

 tact accelerator branch of the sympathetic nerve. Should 



