536 PUNCTURE OF PLEURA. [BOOK n. 



constantly renewed through the alternate expansion and con- 

 traction of the chest. 



325. In ordinary respiration, the expansion of the chest 

 ^never reaches its maximum ; by more forcible muscular contrac- 

 tions, 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 complemental 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 supplemental air, may be driven out. But even after the 

 most forcible 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 

 collapse, 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, which 

 as we have seen have flaccid and not rigid walls. 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 opening, 

 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 speak of 

 them as being in a state of * adhesion,' such as obtains between 

 two wet membranes superimposed. And it has been suggested 



