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 CO 2 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 CO 2 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 CO 2 

 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 CO 2 on the breathing. 



If the breathing is by voluntary effort forced for a 

 time, so as to reduce the percentage of CO 2 in the 

 alveolar air, a period of apnoea results. This effect 

 depends entirely on the reduction of the percentage of 

 CO 2 in the alveolar air, for if the inspired air con- 

 tains about 5 per cent of CO 2 it is impossible to pro- 

 duce apnoea by forced breathing, since under these 

 conditions it is impossible to reduce the alveolar CO 2 

 percentage below normal. Careful observations by 

 Douglas and myself showed that it is only necessary to 

 reduce the alveolar CO 2 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 CO 2 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 



