50 RESPIRATION 



creased (Figure 15). The explanation of this is obvious from the 

 foregoing account of the physiology of the Hering-Breuer reflex. 

 When a resistance is thrown in deflation or inflation of the lungs 

 is slowed, but continues till the point is reached at which the 

 phase of respiration is reversed by the reflex. Meanwhile, how- 

 ever, CO 2 has begun to accumulate, so that the next respiration 

 is not only more vigorous but deeper ; and the final result is deeper 

 and less frequent respiration. 



When there is no resistance to breathing the compensation of 

 diminished frequency by increased depth is almost perfect, as 

 shown by the experiments already quoted of Priestley and my- 

 self; but when the slowing is due to resistance the compensation 

 is less perfect, since the extra work performed by the respiratory 

 muscles implies a more powerful stimulus of CO 2 to the respira- 

 tory center. Accordingly the alveolar CO 2 percentage rises quite 

 considerably with resistance to breathing. The following table 

 shows the rises observed by Davies, Priestley, and myself with 

 varying resistances. 



Just as, in the absence of resistance a very slight increase in the 

 alveolar CO 2 percentage, and consequent slight increase in the 

 chemical stimulus to the respiratory center, increases the depth 

 of breathing, so a slight diminution in alveolar CO 2 percentage 

 diminishes the depth. It was recently discovered independently by 

 Yandell Henderson in America and by Liljestrand, Wollin, and 

 Nilsson in Sweden that if apnoea is first produced and artificial 

 respiration then carried out by Schafer's or one of the other usual 

 methods the quantity of air which enters the chest at each artificial 

 inspiration is only about a third or less of what enters during 

 artificial respiration when the subject has simply suspended vol- 

 untarily his own breathing. With voluntary suspension of the 

 natural breathing, moreover, the volume of air which enters at 

 each artificial inspiration varies (roughly speaking) inversely 

 as the frequency of the artificial breathing, so that it is impossible 

 to produce a condition of true apnoea by increasing the frequency 

 of the artificial breathing. If, finally, the air artificially inspired 

 contains an excess of CO 2 , the volume introduced by the artificial 

 respiration increases just as it would with natural breathing. It is, 

 in fact, just as if the subject were himself breathing naturally all 

 the time, in spite of the undoubted fact that he has suspended his 

 natural breathing. 



These phenomena are completely intelligible on the theory that 

 the limits within which inflation or deflation of the lungs inhibits 



