606 COMPARATIVE ELECTRO-PHYSIOLOGY 



slow initiation of the molecular upset. After this the rate of 

 molecular distortion will be rapid, and in this second period 

 we find that the excitatory reaction also is at its maximum. 

 In any case, it is rather during the period of increasing 

 molecular distortion that we should expect to see the 

 most intense excitation, than when a static condition of 

 derangement has been attained. Thus it is at the moment 

 of K-make that we obtain the excitatory indication, and not 

 afterwards, when the molecules are being maintained in the 

 distorted position. 



Returning once more to the iron wire, we find that when 

 the distorted molecules have been set free by the break of K, 

 there is a sudden movement of recovery in the opposite 

 direction. If now the K-effect, with its particular-directioned 

 molecular movement, be termed the excitatory, then the oppo- 

 site movement must be regarded as one of depression, and it 

 is interesting to note that in a living tissue there is an after- 

 effect of depression at kathode-break. The anode-make, on 

 the other hand, with its opposite molecular distortion, is, as 

 one would expect, depressory. But at the break, the 

 direction of the rebound of the released molecules being the 

 same as that brought about by K-make, must be excitatory. 

 The close parallelism which we have thus traced out, forces 

 upon us the conclusion that the molecular actions which 

 underlie the excitation of living tissues may be polar in their 

 character. 



The fact that magnetic excitation undergoes diminution 

 during transmission, can be shown by moving the receiving 

 coil R further and further away from S, when the responsive 

 sound in the telephone, or deflection in the galvanometer, will 

 be found to undergo a graduated diminution, till, with a given 

 stimulus, the effect, from being considerable, is reduced at a 

 certain distance to nil. Keeping this distance the same, 

 however, a stronger stimulus will be found efficient to evoke 

 response, and the responding coil will now have to be moved 

 further, in order again to reduce the response to zero. 



We shall next study the variation of conductivity induced 



