REGULATION OF BREATHING 9 



of 100 per cent in the ventilation of the lungs over 

 the normal resting ventilation there was an increase 

 of about 0.2 per cent in the CO2 percentage in the 

 alveolar air. Very accurate methods of sampling and 

 gas analysis were of course needed in order to detect 

 these differences. When the percentage of CO2 in the 

 inspired air reaches about the normal percentage in the 

 alveolar air there is extreme panting. With higher 

 percentages a point is soon reached where the CO2 

 begins to produce abnormal effects, culminating in 

 loss of consciousness. The breathing then quiets down 

 to a large extent, and this quieting down of the breath- 

 ing, as observed in animals, led formerly to a misinter- 

 pretation of the effects of COg on the breathing. 



If the breathing is by voluntary effort forced for a 

 time, so as to reduce the percentage of CO2 in the 

 alveolar air, a period of apnoea results. This effect 

 depends entirely on the reduction of the percentage of 

 CO2 in the alveolar air, for if the inspired air con- 

 tains about 5 per cent of CO2 it is impossible to pro- 

 duce apnoea by forced breathing, since under these 

 conditions it is impossible to reduce the alveolar CO2 

 percentage below normal. Careful observations by 

 Douglas and myself showed that it is only necessary to 

 reduce the alveolar CO2 percentage by 0.2 per cent in 

 order to produce apnoea. It thus appears that a rise 

 of about 0.2 per cent in the alveolar CO2 percentage 

 is sufficient to double the breathing, while a fall of 

 0.2 per cent produces cessation of breathing. 



We are now in a position to understand, up to a 

 certain point, how the breathing is regulated. The 



