RESPIRATION 25 



excessive lung ventilation is due to lowering of the alveolar CO 2 

 percentage, and not to either of the other causes to which the 

 apnoea has also been attributed. The vagus theory of the apnoea 

 caused by increased lung ventilation involved the very great 

 improbability that a special arrangement exists in the body for 

 bringing increased breathing to an end, regardless of whether a 

 continuance of the increased breathing is physiologically required 

 or not. It seemed almost incredible that such a theory could be 

 correct. 



The ease with which apnoea due to reduction of CO 2 in the 

 alveolar air might be taken for an apnoea due to the after effect 

 of mere distention of the lungs is clearly shown by the stetho- 

 graphic tracings of human breathings reproduced in Figures 2 

 to 7. 10 Figure 2 shows apnoea as an after effect of inflation of the 

 lungs, while Figure 3 shows that when the inflation is made with 

 air containing 4.6 per cent of CO 2 , so as to prevent reduction of 

 the alveolar CO 2 percentage, no apnoea succeeds the period of 

 inflation. The apnoea appearing as an after effect in Figure 2 is 

 therefore due to reduction of the alveolar CO 2 in consequence of 

 the distention with pure air. 



Figure 2. 



Figure 3. 



Effects of distention for 8 sees. Crosses show beginning 

 and end of distention. To read from left to right. In Fig. 2 

 pure air is used for distention ; in Fig. 3 air containing 

 4.62 per cent COz. 



Figures 4, 5, and 6 illustrate the same point. In Figures 4 and 

 5 there is apnoea succeeding a short distention, but not immedi- 



10 Christiansen and Haldane, Journ. of Physwl., XLVIII, p. 274, 1914. 



