426 PUNCTUKE OF PLEURA. [BOOK n. 



lungs is being constantly renewed through the alternate expan- 

 sion and contraction of the chest. 



256. In ordinary respiration, the expansion of the chest 

 never reaches its maximum ; by more forcible muscular con- 

 tractions, by what is called laboured inspiration, an additional 

 thoracic expansion can be brought about, leading to the inrush 

 of a certain additional quantity of air before equilibrium is 

 established. This additional quantity is often spoken of as 

 complemented air. In the same way, in ordinary respiration, 

 the contraction of the chest never reaches its maximum. By 

 calling into use additional muscles, by a laboured expiration, 

 an additional quantity of air, the so-called reserve or supple- 

 mental air, may be driven out. But even after the most forci- 

 ble expiration, a considerable quantity of air, the residual air, 

 still remains in the lungs. The natural condition of the lungs 

 in the chest is in fact one of partial distension. The elastic 

 pulmonary tissue is always to a certain extent on the stretch ; 

 it is always, so to speak, striving to pull asunder the pulmonary 

 from the parietal pleura ; but this it cannot do, because the air 

 can have no access to the pleural cavity. When, however, the 

 chest ceases to be air-tight, when by a puncture of the chest- 

 wall or diaphragm, air is freely introduced into the pleural 

 chamber, the elasticity of the lungs pulls the pulmonary away 

 from the parietal pleura, and the lungs shrink, driving out by 

 the windpipe a considerable quantity of the residual air. Even 

 then, however, the lungs are not completely emptied, some air 

 still remaining in them ; this is probably air imprisoned in the 

 infundibula by collapse of the bronchioles, the walls of which 

 are not rigid but flaccid. If in a living animal the pressure of 

 the atmosphere continue to have access to the outside of a lung 

 the air thus imprisoned is gradually absorbed and the lung 

 becomes solid. The same result may occur from the pressure 

 of fluid accumulated in the pleural cavity. 



It need hardly be added that when the pleura is punctured, 

 and air can gain free admittance from the exterior into the 

 pleural chamber, since the resistance to the entrance of the 

 air into the pleural chamber is far less than the resistance to 

 the entrance into the lungs, the effect of the respiratory move- 

 ments is simply to drive air in and out of that chamber, instead 

 of in and out of the lung. There is in consequence no renewal 

 of the air within the lungs under those circumstances. If 

 there be a sufficient obstacle to the entrance of air into the 

 pleural chamber, such as a fold of tissue blocking up the open- 

 ing, the expansion of the chest may still lead to a distension of 

 the lungs ; and in this way in some cases puncture of the chest- 

 walls has not seriously interfered with respiration. The parietal 

 and pulmonary pleura are, in normal circumstances, separated 

 by a very thin layer only of fluid, so that we may perhaps 



