THE NERVOUS MECHANISM OF RESPIRATION. 375 



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. 



308. 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 afferent 

 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 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 inspira- 

 tion, 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 responsive 

 one; in normal 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, speaking generally, 

 breathing in is the first consideration we breathe out mostly because we 

 have already breathed in. 



There are many facts which support this view of the double antagonistic 

 action of afferent respiratory impulses. If the central end of the superior 

 laryngeal branch of the vagus be stimulated the effects are much more con- 

 stant than those of stimulating the main vagus trunk. Whether the main 

 trunk of the nerve be previously severed or not, the result of centripetal 

 stimulation of the superior laryngeal branch is always in the direction of a 

 slowing of the respiration (Fig. 103) ; and this may by proper stimulation 

 be carried so far that a complete standstill of respiration in the phase of 

 rest is brought about. While the main trunk of the vagus contains fibres 

 of two kinds, both augmentory and inhibitory of inspiration, the superior 

 laryngeal branch appears to contain one kind only, those which inhibit 

 inspiration. If now while this experiment is being conducted on a rabbit 

 the abdomen be watched, it will be seen that the inhibition of inspiration is 

 accompanied by a contraction of the abdominal muscles, that is by an effort 

 at expiration ; the stimulation of the nerve while inhibiting respiration pro- 

 vokes, to a certain extent, expiration. 



309. That the trunk of the vagus is the channel of these two kinds 

 of impulses, of a mutually antagonistic character, is further shown by apply- 

 ing what may be considered as natural stimuli to the endings of the nerve 

 in the lungs ; and the results so obtained have an especial value since the 

 artificial stimulation of a nerve-fibre, at a part of its course by means of an 

 electric current is at best a rough process, by which we cannot hope to do 

 more than approximate to the results actually taking place in the living 

 body when the nerve is stimulated at its endings by natural stimuli ; and 



