22 ORGANISM AND ENVIRONMENT 
tion of the lungs during inspiration is the immediate 
cause of expiration, and the deflation on expiration is 
the immediate cause of inspiration. Subsequent inves- 
tigation by various other observers confirmed in the 
main these conclusions. The regulation of breathing 
thus appeared to be an automatic process dependent, 
so long as the vagus nerves are intact, on the effects 
of alternate distention and deflation of the lungs. 
Until recently, also, many observers concluded from 
their experiments that apnoea is the summed effect 
of frequently repeated over-distention of the lungs, 
and has nothing to do with chemical changes in the 
blood. The majority believed that there is both a 
“chemical” and a “vagus” apnoea. The continued in- 
spiratory or expiratory effort which accompanies con- 
tinuous deflation or inflation of the lungs cannot 
properly be called apnoea, however. 
I have already referred to the evidence showing that 
there is certainly no such thing as an apnoea due to the 
mere summed effects of repeated distention of the 
lungs, such as occurs in panting. The apnoea which 
follows forced breathing or excessive artificial ventila- 
tion of the lungs is due to reduction in the amount of 
CO, in the alveolar air and arterial blood, and to no 
other cause. Were it the case that repeated unusual 
distention of the lungs tends to cause apnoea we should 
have a physiological arrangement exactly suited to 
defeat the whole physiological end of increased breath- 
ing. It seems extraordinary that the extreme improba- 
bility of this should not have weighed more heavily 
with the authors of the “vagus” theory of apnoea. 
