REGULATION OF BREATHING 25 



quency of breathing is varied voluntarily or involun- 

 tarily, the depth naturally adjusts itself in such a way 

 that the average alveolar CO 2 pressure remains sensi- 

 bly constant: for the least lowering of alveolar CO 2 

 pressure enables the Hering-Breuer inhibitory effect 

 to become effective within narrower limits of inflation 

 and deflation, while the least raising of alveolar CO 2 

 pressure has the opposite effect. We can also explain 

 a very interesting phenomenon recently discovered 

 independently by Yandell Henderson in America and 

 Liljestrand, Wollin and Nilsson in Sweden. When 

 artificial respiration is performed on a conscious sub- 

 ject by Schafer's or any of the other usual methods, 

 air enters and leaves the chest in just about the normal 

 amount, although the subject carefully refrains from 

 himself making any breathing efforts. If the rate 

 of artificial respiration is increased there is no increase 

 in the air entering the chest per minute : for the breaths 

 become shallower. If, finally, apnoea is produced by 

 previous forced breathing, and artificial respiration 

 is then applied, hardly any air enters the chest. The 

 Hering-Breuer inhibition comes into play with the 

 slightest inflation or deflation of the lungs, and the 

 breathing is, as it were, jammed. 



When the vagi are cut, an animal can still regulate 

 its breathing so as to keep the alveolar CO 2 pressure 

 constant ; for the depth of the drawn-out respirations 

 depends on the alveolar CO 2 pressure. But, as might 

 be expected, the regulation breaks down easily under 

 any strain, as was recently shown by Scott. The 



