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 2 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. 



