REGULATION OF BREATHING 11 
There is no doubt that it is through the blood that 
slight changes in the CO, percentage of the alveolar 
air affect the respiratory centre. The effects of these 
changes are equally rapid and marked when all the 
nervous connections between the lungs and the respir- 
atory centre are severed. 
To most persons it must come as a surprise that the 
breathing is so exactly regulated. Common observa- 
tion shows us that the breathing is often more or less 
interrupted temporarily, and varies in frequency or 
depth at different times, as if the regulation were only 
rough. We also know that breathing is under volun- 
tary control, and there is a popular idea that by spe- 
cial forms of training in breathing we can improve 
the aeration of the blood and the supply of oxygen to 
the body. 
If samples of the alveolar air are taken it is found 
that they only give a constant percentage of CO, if 
the breathing is quite regular at the time, and they 
are taken at the same phase of the respiratory act— 
say at the end of inspiration or of expiration. Ac- 
tually the percentage is varying distinctly from 
moment to moment round the average; and it is only 
the average that is constant. If, moreover, the per- 
centage of CO, in the air inspired is suddenly 
increased, it takes some little time before the breath- 
ing increases to the new average. There is thus a 
considerable lag between changes in the alveolar CO, 
percentage and the response of the respiratory centre. 
This lag may be in either direction. If, for instance, 
the breathing is voluntarily held for a short time, 
