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, 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, 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, 
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, on the breathing. 
If the breathing is by voluntary effort forced for a 
time, so as to reduce the percentage of CO, in the 
alveolar air, a period of apnoea results. This effect 
depends entirely on the reduction of the percentage of 
CO, in the alveolar air, for if the inspired air con- 
tains about 5 per cent of CO, it is impossible to pro- 
duce apnoea by forced breathing, since under these 
conditions it is impossible to reduce the alveolar CO, 
percentage below normal. Careful observations by 
Douglas and myself showed that it is only necessary to 
‘ reduce the alveolar CO, 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, 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 
