MEDIASTINAL EMPHYSEMA. 329 



or negative pressure in the sac, is diminished, and the lung, by 

 virtue of its elasticity, immediately retracts, unless adhesions 

 prevent it. The mediastinum is drawn over to the opposite side, 

 and the lung of this side also retracts somewhat in consequence. 

 The dislocation of the cardiac impulse is an index of the displace- 

 ment of the mediastinum. The inflation of the pleura causes the 

 chest to assume an inspiratory position, and its resonance 

 encroaches on the cardiac, hepatic or splenic areas of dulness. 

 When the pressure in the sac rises above that of the atmosphere 

 the diaphragm becomes depressed, carrying downwards the 

 viscera in contact with its under surface. 



Pneumothorax may be simulated on the left side by a stomach 

 much distended with gas. The stomach and colon may also 

 form the contents of a diaphragmatic hernia, and under these 

 circumstances, if filled with gas, may give rise to signs of air in 

 the left pleural sac. Eepeated examination may throw light on 

 the case by showing that the resonant area is at times replaced 

 by dulness, presumably because some fluid has been able to pass 

 into the incarcerated stomach or bowel. 



It is exceptional to find air in the pleural sac, without some 

 serous fluid or pus, for the lesion which admits the air also opens 

 up a track for infection. 



MEDIASTINAL EMPHYSEMA. 



The mediastinal connective tissue surrounds the main air 

 passages, and is prolonged into the root of the lung, where it 

 becomes continuous with the connective tissue basis of that 

 organ. Air may gain access to its spaces by perforation or 

 rupture of the trachea, or a bronchus, or by the formation of a 

 communication between the air cells of the lung and the sur- 

 rounding connective tissue planes. Such communications may 

 occur in connection with destructive pulmonary inflammations, 

 extreme emphysema, or rupture of a cell during a paroxysm of 

 whooping cough. 



The deeper connective tissue planes of the neck are continuous 



