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 2 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 2 in the alveolar air and arterial blood is higher. 

 We can thus understand how it is that when the fre- 



