THE NEW PHYSIOLOGY. 63 



to them to depend simply on the state of mechanical 

 distension of the lungs, and to have no relation to the 

 chemical regulation of breathing. Mr Mavrogordato 

 and I have quite recently re-investigated these pheno- 

 mena in man. The results showed that the amounts 

 "of inflation or deflation needed to produce the Hering- 

 Breuer effects depend entirely on the chemical stimulus 

 of carbon dioxide. When this stimulus is absent, as in 

 apnoea, a very slight inflation or deflation will suffice, 

 so that the breathing is, as it were, jammed during 

 apnoea ; while if the chemical stimulus is strong it needs 

 a great inflation or deflation to produce the Hering- 

 Breuer effect. The vagi prevent useless prolongation 

 of inspiratory or expiratory effort and consequent waste 

 of time in breathing, or damage to the lung structure. 

 They also co-ordinate the discharges of the centre with 

 actual inflations or deflations of the lungs. When the 

 vagi are cut the breathing becomes slow, and, as Scott 

 showed, can only imperfectly respond to an increased 

 chemical stimulus, since the frequency cannot be in- 

 creased. The influence of the vagi is entirely in the direc- 

 tion of keeping the alveolar air normal. Perhaps 

 nothing illustrates more clearly the dependence of 

 nervous reactions on more fundamental physiological 

 conditions than the varying response of the respiratory 

 centre to the stimulus of inflation or deflation of the 

 lungs. 



When excessive ventilation of the lungs is so arranged 

 that there is no fall in the alveolar percentage of carbon 

 dioxide, no apnoea follows. There is thus no such thing 

 as the so-called vagus apnoea. Apnoea is simply due to 

 excessive removal of carbon dioxide from the alveolar air. 



