CHAP, ii.] RESPIRATION. 479 



afferent fibres of two kinds connected with the respiratory cen- 

 tre : one kind augmenting the action of the centre somewhat in 

 the same way as the augmentor cardiac fibres augment the beat 

 of the heart, and the other kind having an inhibitory effect. 

 Apparently sometimes the one and sometimes the other kind is, 

 according to circumstances, most provoked by the stimulation, 

 much in the same way as stimulation of the vagus in the frog, 

 which as we have seen, 136, is the channel for both inhibitory 

 and augmentor cardiac impulses, produces, sometimes inhibition, 

 sometimes augmentation of the heart beat. To affect the heart 

 of course the stimulation of the vagus must be centrifugal, 

 directed towards the periphery, whereas to affect the respira- 

 tion it must be centripetal, applied to the part of the nerve 

 connected with the brain ; and while the usual effect on the 

 heart of ordinary stimulation of the vagus is inhibition, augmen- 

 tation only occurring in special cases, the most common effect 

 on respiration is augmentation, though inhibition is not unfre- 

 quently seen. When the experiment is conducted on an animal 

 under the full influence of chloral stimulation of the vagus gen- 

 erally produces inhibition of respiration, probably because the 

 chloral renders the respiratory centre more susceptible to inhibi- 

 tory influence. 



296. We said just now " the action of the centre ; " but the 

 respiratory centre is a double one ; it gives rise to inspiratory 

 and to expiratory efferent impulses, and these are antagonistic 

 the one to the other. If inspiratory and expiratory impulses 

 issued from the centre at the same time and in equal potency, 

 there could be no breathing at all, they would neutralize each 

 other's effects ; and indeed any amount of inspiratory impulse 

 is antagonistic to a simultaneous expiratory impulse, and vice 

 versa. Hence for the adequate services of the respiratory cen- 

 tre we might expect to find that each kind of afferent impulse 

 ascending the vagus affected the centre in a double and oppo- 

 site way, inhibiting expiration while augmenting inspiration, or 

 inhibiting inspiration while augmenting expiration. If we allow 

 ourselves to speak of the whole respiratory centre as consisting 

 of two parts, one the inspiratory part, or inspiratory centre 

 concerned in the issue of inspiratory impulses, and the other 

 the expiratory part, or expiratory centre concerned in the issue 

 of expiratory impulses, we may suppose that these centres are 

 so related to each other that afferent impulses, reaching the 

 spinal bulb, which augment or inhibit the one, necessarily 

 inhibit or augment the other. We need perhaps hardly add 

 that of these two centres we should expect to find the inspira- 

 tory centre the dominant and the most responsive one ; in nor- 

 mal breathing it comes almost alone into obvious use, since as we 

 have seen the expiratory muscles have then a very slight task 

 only, the chest being emptied chiefly by elastic reaction ; and, 



