CHAP, ii.] 



RESPIRATION. 



623 



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 constant than those of stimulating the 

 main vagus trunk. Whether the main trunk of the nerve be pre- 

 viously 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. 81); and this may by proper stimulation 



FIG. 81. SLOWING OF RESPIRATION BY STIMULATION OF SUPERIOR LARYNGEAL 



NERVE. 



This curve was obtained in the same way as Figs. 77, 78, 79 and the letters have 

 the same meaning as in those figures. Stimulation begins at x, and ends at y. 



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 augmentor 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 provokes, to a certain extent, expiration. 



366. That the trunk of the vagus is the channel of these two 

 kinds of impulses, of a mutually antagonistic character, is further 



