66o 



COMPARATIVE ELECTRO-PHYSIOLOGY 



In fig. 404 we have a series of magnetic responses to arith- 

 metically increasing magnetic stimulation. The magnetising 

 current by which this was accomplished increased from *2 

 ampere to 2 amperes by steps of '2 ampere at a time. The 

 responses at first, as will be seen, increased at a very rapid 

 rate, and afterwards the rate declined. The remarkable 

 similarity between this record and that obtained with the 

 optic nerve of Ophiocephalus is at once apparent. 



In the chapter on the modification of Response under Cyclic 

 Molecular Variation, I have already shown how the responsive 

 change differs in intensity at different 

 parts of the characteristic molecular 

 curve. We there saw that just beyond 

 the point of transition, in the B state, 

 the rate of change was very rapid, and 

 that it became slower in the higher 

 part of the curve. The induced trans- 

 formations in the condition of the 

 nerve, by which it passes from the 

 phase B to C, and so on, are of them- 

 selves sufficient to elucidate not only 

 these, but also other obscure charac- 

 teristics of the phenomenon of sensa- 

 tion. They will, for example, explain 

 the curious case in which an identical 

 stimulus given at regular intervals 

 proves at first not unpleasant, and at 

 the end of the series actually painful. Here sensation 'has 

 become intensified, though the exciting stimulus remains the 

 same. I may quote here an account of an experiment by 

 Professor Sherrington, which exhibits this fact in a very 

 interesting manner : 



* I found that by focussing the heat rays from a lamp 

 upon a skin area of about 25 sq. mm. (on the back of the 

 hand), and allowing a perforated screen to intermittently 

 intercept the radiation, the heat-pain begins to be perceptibly 

 intermittent when, the times of play and of interception 



FIG. 404. Photographic 

 Record of Magnetic 

 Responses in Steel to 

 Arithmetically increas- 

 ing Stimulus 



