664 COMPARATIVE ELECTRO-PHYSIOLOGY 



identical nerve will be rendered acceleratory or inhibitory 

 by the opposite effects of the inducing tonus. 



If an external force be thus capable, according as it is 

 positive or negative, of inducing opposite molecular disposi- 

 tions, by which the conducting power of nerve is so pro- 

 foundly modified as to render it for the time being either an 

 accelerator or an inhibitor, then it is not difficult to under- 

 stand that these molecular dispositions may also be varied 

 in a similar manner by impulses from an internal source. 

 The molecular dispositions themselves, by which these effects 

 are brought about, are no doubt curious, but they are neither 

 mysterious nor unique ; for we have seen that by molecular 

 distortions of one sign or another, artificially induced in 

 magnetic substances, a given magnetic impulse may be either 

 accelerated or retarded. 



We have seen that the transmission of excitatory changes 

 is facilitated, if the nerve be subjected to incipient molecular 

 distortion in a favourable direction. Now in considering the 

 attitude of attention, we can see at once, if only from the 

 muscular indications which it induces, that the nervous 

 channel is probably thrown by it into a state of moderate 

 contraction. Inattention, on the other hand, must generally 

 be attended by an attitude of relaxation. We have again 

 found that while incipient contraction or K-tonus enhances 

 conduction, an intense contraction, or very strong K-tonus, 

 will inhibit, because the molecular distortion thus induced 

 is already maximum, and external stimulus can produce 

 little further effect (p. 610). It is here interesting to note 

 that the expression * steeling the nerves to pain ' is not 

 altogether fanciful or metaphorical. The attitude of pre- 

 paredness thus denoted is one of rigid contraction. 



In the course of the present chapter, then, it has been 

 shown that there are two distinct nervous impulses, positive 

 and negative. The former, induced by feeble stimulus, gives 

 rise to a positive tone of sensation. The negative, on the 

 other hand, due to stronger stimulus, gives rise to a sensation 



