6 14 COMPARATIVE ELECTRO-PHYSIOLOGY 



excitation applied at one end, with a responding arrange- 

 ment, to give motile indications, at the other. This latter 

 consists of a secondary coil, which may be slipped over the 

 responding point, being in series with some sensitive metallic 

 powder in circuit with a galvanic recorder, and a voltaic 

 cell as source of energy. The excitatory molecular dis- 

 turbance transmitted through the conducting iron rod gives 

 rise, on reaching the responding-point, to an electrical 

 disturbance in the secondary coil connected with the motile 

 indicator. This electrical disturbance causes secondary 

 excitation of the sensitive substance, in consequence of 

 which the electric conductivity of the particles becomes 

 suddenly enhanced. By this * relay ' action the stored-up 

 energy of the cell is suddenly released, with a consequent 

 induction of motile response in the galvanic recorder. It 

 is thus seen that this motile response, initiated by the 

 transmitted stimulus, need not be proportionate to its 

 primary exciting cause, since it may possibly be much en- 

 hanced by the amount of energy set free in the responding 

 circuit itself. This transmission of excitation is liable, 

 moreover, as in nerve, to be modified by subtle molecular 

 changes induced in the conducting tract through which it 

 takes place. Excitation may be arrested in the one case 

 by an electrical block ; and in the other, similarly, we are 

 able to stop the transmission of a message, by means of a 

 magnetic block. It is by no gross physical restraint that 

 the impulse is so arrested, but by invisible molecular 

 distortion within the rod. Molecular freedom is next re- 

 stored by the removal of the magnetic block, and we find 

 that the message, which, though constantly reiterated, was 

 hitherto inhibited, is suddenly allowed to rush onwards 

 and bring about the signal. 



