CHARACTER OF TRANSMITTED IMPULSE 161 



There is no turgid mass of parenchyma here which by its 

 deformation might cause mechanical disturbance. It may 

 be thought that in some way exudation of sap might cause 

 the necessary hydro-mechanical disturbance. The question 

 now arises : How did excretion occur at the kathode at 

 ' make ' ? As the excitation takes place at the anode at 

 ' break ' are we to suppose that exudation also takes place 

 at the anode ? 



In the often cited instances of hydraulic transmission of 

 stimulus of a violent blow or a cut, mechanical disturb- 

 ance is necessarily present. But transmission of excitatory 

 impulse is found to take place under polar excitation, in the 

 absence of all such disturbing factors. This will be realised 

 when in Chapter XVII it is shown that in Biophytum 

 excitatory impulse is transmitted by the action of an electric 

 current which is so feeble as not to be perceived by the very 

 sensitive human tongue. This would indicate that the effect 

 transmitted here is physiological rather than physical. 

 In a nerve the protoplasmic change which is the basis of 

 excitation is initiated locally at the point of kathode at 

 ' make ' and at the anode at ' break.' The protoplasmic 

 change is then propagated from point to point giving rise 

 to the excitatory impulse. In the petiole also, excitatory 

 protoplasmic change is initiated locally at the kathode at 

 ' make ' and at the anode at ' break.' And every circum- 

 stance indicates a point-to-point propagation of excitatory 

 protoplasmic change in the conducting petiole. 



Block of Conduction by the Action of Cold 



It has been shown that the velocity of transmission is en- 

 hanced by favourable physiological changes due to warmth. 

 Conversely the conductivity is depressed by lowering of 

 temperature, and this depression may become so great as to 

 induce an actual arrest of conduction. The object of the 

 investigation being the influence of cold on conductivity, 

 special care has to be taken that the lowering of tempera- 

 ture does not in any way affect either the excitability of the 



