6 4 



COMPARATIVE ELECTRO-PHYSIOLOGY 



tissue, stimulation was caused by application of a hot wire 

 so near as I cm. to the proximal contact, it was the hydro- 

 positive effect alone which reached it, giving rise to posi- 

 tive response. It was only, indeed, by applying the stimulus 

 very near, at a distance of 3 mm., that the true excitatory 



response of galvanometric nega- 

 tivity was in this case obtained 



(fig. 4 8). 



From what has been said, it 

 will be seen that when a given 

 point is excited by transmitted 

 stimulation, two antagonistic elec- 

 trical effects are induced one of 

 positivity, due to hydro-positive 

 action, and the other of negativity, 

 due to true excitation. When 

 the stimulator is near to, or co- 

 incides with, the responding point, 

 the tissue is subjected to rapidly 

 succeeding positive and negative 

 turgidity-variations, and the elec- 

 trical indication of the latter being 

 the more intense, it masks the 

 former, and the resulting response 

 is determined by an algebraical 

 summation of the two. In a 

 vigorous specimen, whose excit- 

 ability is great, the excitatory gal- 

 vanometric negativity masks the 



positivity. The resultant electrical response in general is 

 expressed by the formula N E P H , where N E is the galvano- 

 metric negativity due to the true excitatory effect, and P H the 

 positivity due to the hydro-positive effect, which immediately 

 precedes it. From this, it is clear that, as regards the 

 resultant galvanometric response of vegetable tissues under 

 stimulation, there may occur the two typical cases displayed 

 in the following table : 



FIG. 48. Photographic Record 

 of Electrical Responses of 

 Potato-tuber 



a, Positive response to stimulus 

 applied at distance ; b y Negative 

 response to stimulus applied 

 near. 



