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 Pfliiger's Law of the polar effects of currents. I 

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

 f such universal application as had been supposed, since, 

 bove and below a certain range of electromotive intensity, 

 e polar effects of currents are precisely opposite to those 

 nunciated by Pfliiger. 



Finally, that nervous impulse, which must necessarily 

 rm the basis of sensation, was supposed to lie beyond 

 y conceivable power of visual scrutiny. But it has here 

 en shown that this impulse is actually attended by change 

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

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

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

 s 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 

 fore the Bradford meeting of the British Association in 

 the year 1900, I said : 



In the phenomena described above there is little 

 reach 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 



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