604 COMPARATIVE ELECTRO-PHYSIOLOGY 



propagation of molecular disturbance from point to point. 

 The characteristics of such propagation must be (i) that the 

 transmitted molecular disturbance becomes enfeebled with 

 distance, so that at a certain point the transmitted excitation 

 would be reduced to zero ; (2) that while a moderate 

 stimulus is transmitted to a short distance, a stronger 

 stimulus would be carried further ; and (3) that the intensity 

 of excitation transmitted would depend on the conducting 

 power of the intervening tract, this conductivity being 

 capable of enhancement by certain agencies, and depressible 

 by others. 



I shall now proceed to show that in an iron wire a 

 transmission of molecular disturbance takes place which is 



FIG. 371. Experimental Arrangement for obtaining Response in Iron by 

 Induction Current 



similar to that at the basis of the transmission of excitatory 

 changes, both, as I shall show, being modifiable by similar 

 circumstances in a similar manner. For these investigations 

 I have employed the Induction or Electro-motive Method of 

 observation. In the experimental arrangement a diagram- 

 matic representation of which is shown in fig. 371 S is 

 the stimulating or exciting coil applied at the point to be 

 excited. The conducting region intervenes between S and R, 

 which is the responding point, over which is wound the 

 receiving coil, placed in series with either a telephone or a 

 galvanometer. When the excitatory molecular disturbance 

 reaches R, it gives rise to an induction current in the coil, 

 which in turn causes a sound in the telephone, or a 

 responsive deflection in the galvanometer. For the purpose 

 of simplicity, we shall take north polar or K-excitation as 



