PNEUMOTHOBAX. 327 



region, and sometimes it travels downwards between the vertebral 

 and costal attachments of the diaphragm. 



Sudden death sometimes follows aspiration or irrigation 

 of the pleural sac. This is attributed to reflex inhibition of 

 the heart tli rough the vagus filaments which are distributed 

 to the lung. 



PNEUMOTHOBAX. 



The parietal and visceral layers of the pleural sacs are 

 normally in intimate apposition and coherent. When air by 

 any means gains access to the pleural sac, or exceptionally when 

 gases are generated in it by certain gas-producing bacteria, 

 the pleural layers become separated from each other, and 

 pneumo thorax results. 



The visceral pleura is in the more direct relation with the air, 

 and it is by perforation of this layer that pneumothorax is com- 

 monly produced. In perhaps ninety per cent, of the cases the 

 perforation is due to pulmonary tuberculosis, but it is sometimes 

 caused by an empyema which has eroded the lung and dis- 

 charged into the air passages, and is occasionally the result of 

 abscess, gangrene or septic infarction of the lung. It is con- 

 jectured that the thin wall of an emphysematous air cell may 

 sometimes rupture when the alveolar air pressure is suddenly 

 raised by violent muscular effort. Pneumothorax has also 

 been caused by wound of the visceral pleura and lung during 

 paracentesis, but this is . rare. The visceral pleura is often 

 wounded by the ends of fractured ribs. Laceration of the lung 

 or the tearing of a bronchus near the lung root may allow air to 

 enter the sac. The application of a crushing force to the chest 

 is usually responsible for such accidents, and fractures of the 

 ribs usually coexist, but in children the thorax is so elastic that 

 the lung may be lacerated, or even a bronchus torn across 

 without any fracture taking place. 



The parietal pleura is sometimes opened in its costal portion by 

 perforating wounds, and pneumothorax may be brought about in 



