GENERAL SURVEY 353 



human tongue. The further fact that the excitation occurs 

 only at the point of kathode at make and at anode at 

 break shows that the effect transmitted is not physical 

 but physiological. 



Multiple excitation by constant current. — According to 

 the mechanical theory multiple excitation can only occur 

 under separate hydro-mechanical disturbances caused by 

 multiple blows. It is, however, found that as in the rhythmic 

 tissue of the animal, so also in that of the plant, multiple 

 excitations are induced by the action of a constant current 

 (p. 249). 



Characteristic effect of temperature on polar excitation. — 

 Excitation of animal nerve by induction shock is enhanced 

 by warmth and depressed by cold. The reverse is the case 

 when the stimulation is caused by constant current. The 

 excitatory effect here is depressed by warmth and exalted 

 by cold. These specific effects are found repeated in the 

 conducting petiole of Mimosa. The excitation caused by 

 induction shock is depressed by cold and enhanced by 

 warmth. But as in the animal nerve, so also in the petiole 

 of Mimosa, these effects are reversed in excitation under the 

 polar action of a constant current. The excitation is now 

 enhanced by cold and depressed by warmth. Minimal 

 excitation becomes maximal imder cold and ineffective 

 under warmth (p. 247). 



The crucial test of the excitatory character of the 

 transmitted impulse is furnished by the action of various 

 physiological blocks which arrest the transmission of 

 excitation. 



Paralysis of conduction hy cold. — The local application 

 of increasing cold on the conducting petiole retards and 

 finally arrests conduction of excitation. As an after-effect 

 of the application of cold the conducting power is paralysed 

 for a considerable length of time. The lost conducting 

 power may, however, be quickly restored by tetanising 

 electric shocks (p. 164). 



The electrotonic block. — The excitatory impulse may 



