REGULATION OF BREATHING 23 
The theory that the breathing is regulated merely 
by the effects of alternate distention and collapse of 
the lungs is also quite plainly absurd in view of what 
is now known about the part played by the carbon 
dioxide pressure in the alveolar air and arterial blood. 
The observations of Hering and Breuer and of others 
who have made experiments along the same lines are 
none the less significant, however. Mr. Mavrogorato 
and I have found that the main facts, apart from the 
effects of section of the vagus nerves, can best be 
observed and analysed in man. The subject breathes 
through a wide bored tap which can be opened or 
closed at any moment; the nose is clipped; and a pres- 
sure-gauge is connected between the mouth and the 
tap so as to show the inspiratory or expiratory pres- 
sure. 
When the tap is closed at the end of inspiration it 
will be noticed on the gauge that there is expiratory 
pressure, slight at first, but afterwards increasing more 
and more rapidly, till at last, after an interval occupy- 
ing the time of several normal respirations, there is a 
sudden inspiratory effort. The natural tendency of the 
respiratory centre to discharge alternate inspiratory 
and expiratory impulses thus breaks through the 
prolonged expiratory effort. Similarly, if the tap is 
closed at the end of inspiration there is a prolonged 
and increasing expiratory effort. If, now, apnoea 
is produced by forced breathing before the experiment, 
there is inspiratory or expiratory pressure as before; 
but it is a very long time before this pressure begins 
to increase. On the other hand if air containing CO, 
