480 AUGMENTING AND INHIBITORY IMPULSES. [BOOK u. 



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 previously severed or not, the result of centrip- 

 etal stimulation of the superior laryngeal branch is always in 

 the direction of a slowing of the respiration (Fig. 95) ; and 



FIG. 95. SLOWING OF RESPIRATION BY STIMULATION OF SUPERIOR LARYN- 

 GEAL NERVE. 



This curve was obtained in the same way as Figs. 91, 2, 3 and the letters have 

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



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 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 con- 

 ducted 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. 



297. That the trunk of the vagus is the channel of these 

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

 further shewn by applying 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 



