CHAP, ii.] RESPIRATION. 565 



the structures within the thorax outside the lungs, such as the 

 heart and great blood vessels and the thin layer of fluid in the 

 pleural cavities, exist at a pressure below that of the atmosphere ; 

 the enlargement of the thorax in inspiration increases an already 

 existing negative pressure. In the case of each alveolus, even 

 when the chest is at rest, while within the alveolus the pressure is 

 the same as that of the atmosphere, outside the alveolus the pressure 

 is negative; and this difference of pressure keeps the alveolus dis- 

 tended to a certain extent. But if the pleural cavity be laid open 

 to the atmosphere, as by making a hole in the chest wall, air rushes 

 in to the place where the negative pressure previously existed until 

 equilibrium is established; the pressure outside the alveolus becomes 

 like that inside the alveolus the same as that of the atmosphere, the 

 pressure is equal on both sides of the wall of the alveolus. There 

 is now no difference of pressure to distend the alveolus, the elastic 

 power of the alveolar wall comes into play, and the alveolus shrinks. 

 Hence when a free opening is made into each pleural cavity, 

 or when the whole thorax is laid open, the lungs shrink or as it is 

 said 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 sfcill 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 

 that this adhesion, having to be overcome before the two surfaces 

 can separate, assists in preventing the entrance of air into the 

 pleural cavity after puncture of the thorax ; but it has not been 

 clearly shewn that this is really of importance in the matter. 



