PREFACE xi 



able to show that this differential excitability is widely 

 present as a factor in determining the character of special 

 responses, and that it finds its culminating expression in the 

 electrical organs of certain well-known fishes. 



Few conclusions in Electro-physiology have been sup- 

 posed to rest on securer foundations than the generalisation 

 known as Pfltiger's Law of the polar effects of currents. I 

 have found, however, that this law is not by any means 

 of such universal application as had been supposed, since, 

 above and below a certain range of electromotive intensity, 

 the polar effects of currents are precisely opposite to those 

 enunciated by Pfluger. 



Finally, that nervous impulse, which must necessarily 

 form the basis of sensation, was supposed to lie beyond 

 any conceivable power of visual scrutiny. But it has here 

 been shown that this impulse is actually attended by change 

 of form, and is therefore capable of direct observation. This 

 wave of nerve-disturbance, moreover, instead of being single, 

 has been shown to be of two different kinds, in which fact, 

 as I have further explained, lies the significance of the two 

 different qualities or tones of sensation. 



In the concluding portion of the paper which I read 

 before the Bradford meeting of the British Association in 

 the year 1900, I said : 



' In the phenomena described above there is little 

 breach of continuity. It is difficult to draw a line and 

 say : " Here the physical process ends, and the physiological 

 process begins " ; or " That is a phenomenon of inorganic 

 matter, and this is a vital phenomenon, peculiar to living 

 organisms '' ; or " These are the lines of demarcation that 

 separate the physical, the physiological, and the beginning 



