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 CO 2 

 percentage. With a greater lowering of the oxygen percentage 

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

 lower the CO 2 percentage considerably. 



When the CO 2 percentage in the inspired air was increased, 

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

 the alveolar CO 2 rising in any direct correspondence to the rise 

 in the inspired CO 2 , the increase in alveolar CO 2 was so slight as 

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

 CO 2 of the inspired air. This is evident from the following ex- 

 periments, made in the air-tight chamber. 



The evident effect of adding CO 2 to the inspired air was so to 

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

 high, the CO 2 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 CO 2 within the body, the discharge of CO 2 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 



