RESPIRATION 435 



the vagus excitator fibers, the central terminations of which are in relation 

 with the inspiratory center also (Fig. 195); and just as the inhibitor fibers 

 are stimulated by the expansion of the lungs, so the excitator fibers are stimu- 

 lated in turn by the recoil of the lungs. The nerve impulses developed by 

 the recoil of the walls of the alveoli ascend the vagus nerve to the inspiratory 

 center, excite it to activity, and thus call forth a new inspiration sooner 

 than it would otherwise take place. The nerve impulses developed by 

 the expansion of the walls of the alveoli in turn ascend the vagus nerve to the 

 inspiratory center and inhibit its activity and thus lead to an expiratory 

 movement, sooner than it would otherwise take place. According to this 

 view the respiratory mechanism is self-regulative and maintained by the 

 alternate expansion and recoil of the lungs. 



3. Another explanation which is satisfactory in many respects has been 

 presented by Meltzer. This investigator asserts also the existence in the 

 trunk of the vagus the two classes of nerve-fibers, the inhibitor and the 

 excitator; but that for some reason they do not respond to stimulation at 

 the same time as shown by the effects which follow; the inhibitor fibers 

 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 

 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 



