RESPIRATION 21 



spired air was lowered to about 12 or 13 and the alveolar oxygen 

 percentage to about 8 was there any marked decrease in the CO2 

 percentage. With a greater lowering of the oxygen percentage 

 than this, however, the breathing was so much increased as to 

 lower the CO2 percentage considerably. 



When the CO2 percentage in the inspired air was increased, 

 on the other hand, the effect was strikingly different. Instead of 

 the alveolar COg rising in any direct correspondence to the rise 

 in the inspired CO2, the increase in alveolar CO2 was so slight as 

 to be hardly appreciable even with a rise of 2 or 3 per cent in the 

 CO2 of the inspired air. This is evident from the following ex- 

 periments, made in the air-tight chamber. 



The evident effect of adding CO2 to the inspired air was so to 

 increase the breathing that, if the percentage added was not too 

 high, the CO2 percentage in the alveolar air was kept nearly 

 constant. Of the delicacy of this reaction it is easy, from the fig- 

 ures, to form a fair estimate. With a moderate amount of hyperp- 

 noea, and provided that, as was actually the case, sufficient time 

 has elapsed to eliminate the influence of any temporary damming 

 back of CO2 within the body, the discharge of CO2 by the lungs 

 is about the same during hyperpnoea as during rest. Hence it 

 is possible to calculate how great a relative increase in the alve- 

 olar ventilation is brought about by a given increase in the alveolar 



