EXCITATION AND INHIBITION 



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Fig. 123). In the case of the vasomotor reflexes I stated the same conclusion 

 thus: the results "de- 

 pend entirely on the 

 relative strengths of the 

 stimuli ; whichever nerve 

 is under the stronger 

 excitation shows its own 

 effect, and when the 

 excitation of this one is 

 stopped, the effect of the 

 other manifests itself. 

 In fact, there appears to 

 be a perfect antagonism " 

 (1893, p. 318). 



" Inhibition of Inhibi- 

 tion." Suppose that we 

 have in tonic activity the 

 centre from which nerves 

 proceed which cause in- 

 hibition or relaxation of 

 the smooth muscle which 

 they supply. Such a case 

 is that of the vaso-dilator 

 centre under certain con- 

 ditions, as will be seen 

 later. Now there are 

 afferent nerves which 

 inhibit this centre, just 

 as the depressor nerve 

 inhibits the vaso-con- 

 strictor centre, so that 

 we have, as Sherrington 

 has called it, an " inhibi- 

 tion of inhibition." In 

 Fig. 124 we have, in fact, 

 a case which is incapable 

 of explanation on any 

 other view. This is from 

 an experiment in which 

 I was engaged in investi- 

 gating the blood flow 

 through the submaxillary 

 gland of the rabbit. It 

 will be seen that, when 

 the central end of the 

 vagus nerve of the oppo- 

 site side was stimulated, 

 there was a longer in- 

 terval between the drops 

 of blood from the vein, 

 that is, the arterioles 

 were narrowed. Now 

 the vagus nerve in the 

 rabbit is a pressor nerve, 

 that is, it causes a rise of 

 arterial pressure by reflex 

 constriction of arterioles. 

 This rise of pressure is not shown on the tracing, because an arrangement was used 

 which kept the general blood pressure at a constant level by allowing blood to 



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