24 ORGANISM AND ENVIRONMENT 
has been breathed before, so that the breathing is 
naturally increased, the inspiratory or expiratory pres- 
sure mounts up very rapidly, and is soon broken by 
an inspiratory effort. If the tap is closed midway 
in inspiration, long-continued inspiratory pressure, 
gradually increasing, is shown on the gauge, just as 
if the interruption had been at the end of expiration ; 
and similarly there is long-continued expiratory pres- 
sure if expiration has been interrupted midway. 
If we put together the human observations and the 
results obtained in animals with the vagus nerves 
intact and divided, it appears that the effect of disten- 
tion of the lungs is to stop inspiratory and initiate 
expiratory discharge of the respiratory centre, while 
deflation of the lungs stops expiratory and initiates 
inspiratory discharge. Both inspiratory and expira- 
tory discharges continue until they are again stopped 
by distention or deflation. The result is that the dis- 
charges from the centre are directly co-ordinated with 
actual inflation or deflation of the lungs. This is 
brought about through the vagus nerves. The degree 
of energy of the inspiratory or expiratory discharges 
depends, however, on the action of CO, in the blood 
upon the centre. 
The degree of inflation or deflation necessary to 
inhibit inspiration or expiration and initiate expiration 
or inspiration depends quite clearly also on the chemi- 
cal stimuli acting on the centre through the blood: 
for the breathing is far deeper when the pressure of 
CO, in the alveolar air and arterial blood is higher. 
We can thus understand how it is that when the fre- 
