RESPIRATION 215 



similar effect. But chemical stimulation (eg., with a strong 

 solution of potassium chloride) or long-continued mechanical 

 excitation like that produced by stretching or compression 

 of the nerve, or certain kinds of electrical stimulation for 

 instance, the closure of an ascending voltaic current* 

 cause slowing of the respiratory movements or expiratory 

 standstill. This is also the usual, though not the invariable, 

 result of stimulating the superior laryngeal, even when in- 

 duction shocks are employed. These facts undoubtedly 

 suggest the existence in the vagus of two kinds of afferent 

 nerve-fibres that affect the respiratory centre in opposite 

 ways inspiratory fibres, which stimulate it to greater 

 activity of discharge, and expiratory fibres, which inhibit 

 its action. The latter variety we may suppose to be more 

 numerous in the superior laryngeal, the former in the pul- 

 monary branches of the vagus. And there is nothing forced 

 in the hypothesis that certain kinds of stimuli act par- 

 ticularly on the one set of fibres, and certain kinds on the 

 other, for we have already seen an instance of this in 

 studying the differences between the vaso-constrictor and 

 the vaso-dilator nerves (p. 150). It is possible, however 

 (although this view has less inherent probability, in spite 

 of the fact that it has been maintained by some of the most 

 recent writers on the subject), that, at any rate in the vagus 

 trunk, only one set of fibres exists, and that these are 

 affected differently by different kinds of stimulation 

 momentary stimuli, for example, setting up in them im- 

 pulses which we may call inspiratory, and long-lasting 

 stimuli impulses which we may call expiratory. 



However this may be, the facts we have been discussing 

 have an importance of their own, apart from any hypo- 

 thetical explanations of them ; and they may be readily 

 demonstrated by means of such a graphic method as is 

 described in the Practical Exercises (p. 273), or by merely 

 opening the abdomen in a rabbit, and observing the lungs 

 through the thin diaphragm (Gad). Some of them have 

 been more than once unintentionally illustrated en man. In 

 one case the left vagus trunk was included in a ligature 

 * I.e., a current passing towards the head in the nerve. 



