RESPIRATION 19 



prisingly constant for each individual. As the depth of breathing 

 cannot be kept absolutely steady and the composition of the al- 

 veolar air varies slightly with inspiration and expiration it is 

 best to take at least two samples — one just at the end of inspiration, 

 and another just at the end of expiration. The following tables 

 give the CO2 percentages in samples of our normal resting 

 alveolar air, taken in the sitting position during rest at intervals 

 over about 20 months in 1903 to 1905. Since then we have made 

 many further determinations, but the percentages have remained 

 nearly the same. They are slightly lower or higher on some days 

 than on others, and other observers have noticed this in them- 

 selves. 



It will be seen that, as might be expected, the inspiratory 

 samples give on an average a somewhat lower result than the 

 expiratory ones. The average for one subject is 5.63 per cent and 

 for the other 6.28. The slight variations of individual results 

 from these averages are evidently not due merely to changes in 

 barometric pressure. 



When ordinary air was breathed the oxygen percentage in the 

 alveolar air was nearly as steady as the CO2 percentage. When, 

 however, the oxygen and CO2 percentages in the inspired air 

 were varied it became quite evident that the breathing is regu- 



