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, pressure remains sensi- 
bly constant: for the least lowering of alveolar CO, 
pressure enables the Hering-Breuer inhibitory effect 
to become effective within narrower limits of inflation 
and deflation, while the Teast raising of of alveolar CO, 
press e can also explain 
a-very interesting phensimenot 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, plied, hardly ‘any_air “entérs_the-chest. The 
Hering-Breuer inhibition comes into play with the 
shghtest- 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,-pressure 
constant; for the depth of the drawn-out respirations 
depends on the alveolar CO, pressure. But, as might 
be expected, the regulation breaks down-easily under 
any strain, as was recently shown by Scott. The 
ee 
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