CHAP. IL] RESPIRATION. 591 



extremes; the diaphragm stands still in a prolonged contraction 

 in a position which is intermediate between the height of 

 inspiration and expiration. 



These results suggest the conclusion that the vagus nerve (we 

 are dealing now with the main trunk of the nerve) contains 

 afferent fibres of two kinds connected with the respiratory centre : 

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

 which as we have seen, 158, is the channel for both inhibitory and 

 augmentor cardiac impulses, produces, sometimes inhibition, some- 

 times 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 respiration 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, augmentation only occurring 

 in special cases, the most common effect on respiration is 

 augmentation, though inhibition is not unfrequently seen. When 

 the experiment is conducted on an animal under the full influence 

 of chloral stimulation of the vagus generally, produces inhibition 

 of respiration, probably because the chloral renders the respiratory 

 centre more susceptible to inhibitory influences. 



365. 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 

 co aid be no breathing at all, they would neutralize each other's 

 effects ; and indeed any amount of inspiratory impulse is antago- 

 nistic to a simultaneous expiratory impulse, and vice versa. 

 Hence for the adequate services of the respiratory centre we 

 might expect to find that each kind of afferent impulse ascending 

 the vagus affected the centre in a double and opposite 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 medulla, 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 inspiratory centre the dominant and the most 



