422 TEXT-BOOK OF PHYSIOLOGY. 



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 author 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 recoil 

 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 

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

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

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

 and hence in the center itself is more efficient in raising the irritability of the 

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

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

 than normal the respiratory movements will be increased in frequency and 

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

 be without 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, 

 the average depth was 637 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 

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

 the 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 

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

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

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

 the total ventilation to 56. 808 liters. The results of these experiments indicate 

 that an increase in the percentage of the CO 2 in the inspired air leads to an 

 increase in the percentage and pressure of the CO 2 in the arterial blood 

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

 more irritable and discharges its energy more frequently and to a greater 

 degree as shown by the increase in the rate and the depth of the inspiratory 

 movement. 



