RESPIRATION 43 i 



respond first and the excitator fibers somewhat later. Therefore when 

 they are stimulated simultaneously the primary effect is an inhibition of 

 the inspiratory center followed by an expiratory movement. The secondary 

 effect is a stimulation of the inspiratory center followed by a new inspiratory 

 movement. ^ In this view expansion of the lungs stimulates both the inhibitor 

 and the excitator^fibers, but during the expansion and for a short time after, 

 the effect of the inhibitor stimulation, viz.: cessation of inspiration and the 

 advent of expiration, alone manifests itself. With the cessation of expira- 

 tion, the inhibitor stimulation dies away and the late effect or the long after- 

 effect of the excitator stimulation, viz.: a new inspiration, manifests itself. 

 This investigator assumes the surface of the lung to be the peripheral organ 

 of the respiratory reflexes. 



When it is assumed that both inspiratory and expiratory centers cooper- 

 ate in a respiratory movement, as they do in labored respiration either 

 simultaneously or successively, the difficulties of the problem are manifestly 

 much greater. In this case it may be supposed that afferent impulses, de- 

 veloped during the expansion of the lung, inhibit the inspiratory while aug- 

 menting the expiratory center, and that impulses developed during the re- 

 coil of the lungs inhibit the expiratory while stimulating the inspiratory 

 center. 



The Effect of a Change in the Pressure of the Blood Gases on the 

 Activity of the Inspiratory Center. It has long been known that the in- 

 spiratory center is very sensitive to a change in the composition of the blood in 

 so far as its gaseous constituents are concerned. So long as the composition 

 remains normal the center retains its normal irritability and rhythm. As 

 stated in a previous paragraph it has been a subject of discussion as to whether 

 the center is more responsive to an increase in the pressure of the carbon 

 iioxid or to a decrease in the pressure of the oxygen. As the outcome of 

 i long series of experiments it is now the generally accepted opinion that an 

 ncrease in the percentage and pressure of the carbon dioxid in the blood 

 ind hence in the center itself is. more efficient in raising the irritability of the 

 :enter than a decrease in the percentage and pressure of the oxygen. Thus 

 f an animal is caused to inhale air containing but 2 per cent, of CO 2 more 

 .han normal the respiratory movements will be increased in frequency and 

 depth, while a 'corresponding diminution in the percentage of oxygen will 

 DeTwIthout effect. 



It has been shown by Haldane and Priestley that when an individual was 

 breathing normal air and the rate of the respiratory movement, 14 per minute, 

 he average depth was 63 7 c.c. and the total ventilation was 8.918 liters per 

 minute. On raising the percentage of the CO 2 in the inspired air from 

 D.CH percent, to 0.79 per cent, the average depth increased to 739 c.c. and 

 :he total ventilation to 10.346 liters per minute, the rate remaining the same 

 When the percentage of the CO 2 was raised to 2 per cent, the average depth 

 ncreased to 864 c.c., the rate to 15, and the total ventilation to 12.960 liters 

 Der minute; and when the CO 2 in the inspired air was raised to 6 per cent 

 : he average depth was increased to 2104 c.c., the rate to 27 per minute, and 

 lie total ventilation to 56.808 liters. The results of these experiments 

 ndicate that an increase in the percentage of the CO in the inspired air 

 eads to an increase in the percentage and pressure of the GO, in it 

 erial blood and hence in the inspiratory center, as a result of which the 



