114 COMPARATIVE ELECTRO-PHYSIOLOGY 



ment is made, the slide is pushed in, till the tissue is well in 

 the centre of the coil. When the circuit is completed, for a 

 brief period, both the sides A and B are subjected to the same 

 sudden variation of temperature, which, as we know, acts as 

 a stimulus. As the two contacts are thus in practice raised 

 to the same temperature, there will be no thermo-electrical 

 disturbance. The responsive current, therefore, will be 

 determined by any difference of excitability which may exist 

 as between A and B. The spiral also gives out heat-radia- 

 tion, which acts as a contributory stimulus. That it is the 

 thermal variation, and not the temperature, which acts as 

 the efficient external stimulus, is seen from the fact that 

 when the tissue is subjected to the higher temperature con- 

 tinuously, the galvanometric deflection obtained is opposite 

 in direction to that induced by tfie thermal shock. This is 

 because the absorption of heat, as such, increases the internal 

 energy, and thus induces an electrical effect opposite to that 

 caused by external stimulation. 



As experimental tissue, we may use the sheathing 

 petiole of Musa. The required piece is cut and mounted in 

 the apparatus, the concave surface being taken, say, as B, 

 and the convex as A. I have mentioned Musa as suitable 

 for this purpose, because I find it, when fresh, to show 

 practically no sign of fatigue in its responses. There are 

 many other sheathing petioles, which would doubtless answer 

 the same purpose more or less perfectly. 



In obtaining records with this specimen, it is found that 

 the responsive current flows across the petiole, from the inner 

 concave surface B to the outer convex surface A, showing 

 that it is the inside which is more excitable. Uniform 

 stimuli of short duration were applied at intervals of one 

 minute, and the responses obtained are seen to be fairly 

 uniform (fig. 82). The specimen was next subjected to the 

 anaesthetic action of chloroform. This, it will be seen, in- 

 duced a very great depression of the response. 



It has thus been shown that just as the greater contrac- 

 tion and concavity of a motile organ enables us to discrimi- 



