THE NEW PHYSIOLOGY. 61 



is absent. The exact cause of this apnoea was till recently 

 obscure. In 1868 Hering and Breuer showed that the 

 inflation of the lungs in inspiration gives rise to impulses 

 passing up the vagus nerves, and inhibiting further 

 inspiratory impulses from the respiratory centre, at the 

 same time starting expiration. Deflation of the lungs 

 in expiration has a converse effect. So long as the 

 vagi are intact they are constantly playing this game 

 of battledore and shuttlecock with the respiratory centre, 

 and Hering called this the " self-regulation " (Selbst 

 steuerung) of breathing. The apncea following excessive 

 ventilation of the lungs was interpreted by subsequent 

 physiologists as the summed inhibitory effect of repeated 

 distensions. Fredericq showed, however, that apnoea is 

 produced when the respiratory centre of one animal is 

 supplied with blood from another animal the lungs of 

 which are excessively ventilated. This, therefore, is a 

 true " chemical " apnoea, due to over-aeration of the 

 arterial blood, and was distinguished from " vagus " 

 apnoea. Nevertheless the correlation of the various 

 " factors " apparently involved in the regulation of 

 breathing remained extremely obscure. 



I observed that when air breathed is gradually and 

 increasingly vitiated by re-breathing it, or by what is 

 known to miners as " black damp," the breathing is 

 also increased, but not in any simple relation to the 

 extent of the vitiation. With a steady increase in the 

 vitiation the breathing at first increases only a little, 

 but as the vitiation increases further the effect on the 

 breathing is greater and greater. Thus an increase 

 from 4 per cent, to 5 per cent, in the percentage of 

 C0 2 in the inspired air produces about twenty times as 



