334 RESPIRATION. 



and thus when it leaves the chest in expiration has been the means of both 

 introducing oxygen into the chest and of removing carbonic acid from 

 it. In this way, by the ebb and flow of the tidal air, and by diffusion 

 between it and the stationary air, the whole air in the lungs is being con- 

 stantly renewed through the alternate expansions and contractions of the 

 chest. 



268. In ordinary respiration the expansion of the chest never reaches 

 its maximum ; by more forcible muscular contraction, by what is called 

 labored inspiration, an additional thoracic expansion can be brought about, 

 leading to an inrush of a certain additional quantity of air before equilib- 

 rium is established. This additional quantity is often spoken of as eomple- 

 mental 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 labored 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 distention. 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 dia- 

 phragm 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 impris- 

 oned 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 pres- 

 sure 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 in the pleural chamber, since the re- 

 sistance 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 

 movements 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 block- 

 ing up the opening, the expansion of the chest may still lead to a distention 

 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 shown that this is 

 really of importance in the matter. 



269. Before birth the lungs contain no air ; they are in the condition 

 called ateledatic. The Avails of the alveoli, the epithelial lining of which 

 is, at that time, well developed, consisting of distinctly nucleated cells with 

 granular cell-substance, are in contact, the cavity of the alveolus not having 



