340 MANUAL OF PHYSIOLOGY. 



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

 visceral 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 pneumothorax, 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 demonstrates the important fact that it is the atmospheric 

 pressure which, having access to them only through the trachea, 

 maintains the distention of 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 li 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 

 30 mm., and when the lungs are in a position of expiration, they 

 still tend to contract with a force of 6 mm. mercury. 



PRESSURE-DIFFERENCES IN THE AIR. 



The immediate effect of the increase in capacity of the chest 

 is that a pressure-difference is established between the interior 

 of the thoracic cavity and the atmosphere. 



The reduction in pressure produced in the lungs and air 

 passages by inspiratory movements, or the increase of pressure 

 accompanying expiration, is very slight during ordinary quiet 

 breathing with free air passages. But the least impediment to 

 the entrance or to the exit of the air at once makes the difference 

 very notable. 



