FUNCTION OF THE PLEURA. 335 



The position of the inferior margin of the lung may be easily 

 recognized by percussion over the liver, and may thus be shown 

 to be moving up and down with expiration and inspiration re- 

 spectively. By percussion we also find that the space between 

 the two lungs in front is increased during expiration and dimin- 

 ished during inspiration, so that the heart is more or less covered 

 by lung, and the prsecordial dulness is altered every time we 

 draw a breath. 



By means of this free movement of the lungs in the serous cavi- 

 ties the air exerts equal force on the walls of all the air-cells, 

 whether they are situated in the apex or base of the lung, and 

 the alveoli are all equally filled with air. 



If the pleural cavity be brought into contact with the air, 

 either by puncture of the thoracic walls or by rupture of the vis- 

 ceral pleura, the lung, owing to the great elasticity of its tissue, 

 shrinks to very small dimensions, and the pleural cavity becomes 

 filled with air (pneumothorax). 



If air be admitted to both pleural cavities, so as to produce 

 double pueumothorax, death must ensue, for if the opening remain 

 free the motions of the thorax only alter the quantity of air in 

 the pleural cavity, and cannot ventilate the lungs. This demon- 

 strates the important fact that it is the atmospheric pressure which, 

 having access to them only through the trachea, distends the 

 elastic lungs, and keeps them pressed against the wall of the 

 thorax. 



The power with which the lungs can contract when the atmo- 

 spheric pressure is admitted to the pleura has been found after 

 death, without inflation, to be six millimetres of mercury, which 

 is probably below the pressure exerted during life, when the 

 smooth muscle of the bronchi is acting and the tubes are free 

 from mucus, for this rapidly collects in the minute air tubes at 

 death, and impedes the outflow of air. 



When the lungs are inflated before the pleura is opened the 

 pressure can easily be made to rise to nearly 1 inches (30 mm. 

 mercury). 



From this it would appear probable that, when the lungs are 

 stretched by inspiration, they exert a negative pressure equal to 



