154 



COMPARATIVE ELECTRO-PHYSIOLOGY 



I shall next describe the more sensitive galvanometric 

 method of investigation. The resistance is here maintained 

 constant by having the permanent contacts at A and the 

 indifferent point B (fig. 107). The specimen is a stem of 

 Calotropis gigantea. A thermal section is made at first, say, 

 at a distance of 3 cm. from A. The persistent galvanometric 

 negativity of A will now be due to the after-effect of stimula- 

 tion by section. The thermal injury is now repeated at 



FIG. 107. Experimental Arrange- 

 ment for determining Electrical 

 Effect due to Section 



J 



FIG. 108. Records showing in- 

 creasing Persistent Galvano- 

 metric Negativity, according 

 as injury is caused nearer to 

 proximal contact A, i.e. moved 

 from 3 to -5 cm. distance 



decreasing distances from A. I give a series of records 

 (fig. 108), from which it will be seen that when the stimulus 

 of thermal section occurs at some distance, there is no 

 persistent after-effect, recovery being complete. But as the 

 injury is made nearer and nearer A, the permanent after- 

 effect becomes greater and greater. From observations made 

 in the course of a similar experiment, I obtained the following 

 results, given in tabular form, which show the increasing 

 value, with lessening distance, of this persistent galvano- 

 metric negativity. 



