248 COMPARATIVE ELECTRO-PHYSIOLOGY 



as that of Torpedo. One such leaf, already mentioned, was 

 that of Pterospermum. When induction- shocks are sent in 

 both homodromous and heterodromous directions through 

 such a leaf, between upper and lower surfaces, the leaf being, 

 it is understood, in a normal condition, a responsive current 

 is found to be evoked, always in one direction that is to say, 

 from the lower or anterior to the upper or posterior surface. 

 This is strictly parallel to the electrical reaction observed by 

 Du Bois-Reymond in Torpedo. 



That this result is really due to the excitatory effect 

 is proved by the fact that the same is found to occur 



when other forms of stimulation 

 are used. Thus, if we place the 

 leaf of Coleus aromaticus within 

 a surrounding thermal helix, suc- 

 cessive thermal shocks, acting 

 simultaneously on both surfaces, 

 give rise to responsive currents 

 which are, as in the last case, 

 FIG. 158. Photographic Records from the lower anterior to the 



upper posterior surface. Fig. 158 



Surfaces are Excited Simul- gives a series of such responses. 



taneously by Thermal Shock A , r , . , , 



trom the fact which has already 



Resultant responsive current from 



more excitable anterior to less been fully established, that On 



excitable posterior surface. ihe simultaneous excitation of 

 two points the responsive current is always from the more to 

 the less excitable, it is quite clear that in the present case it 

 is the lower or anterior surface of the leaf which is the more 

 excitable. These responsive currents, obtained under a non- 

 electrical form of stimulus, and similar to those evoked by 

 electrical shocks, completely demonstrate the fact that the 

 result is brought about, not by polarisation, either positive 

 or negative, but by the differential excitability of the tissue 

 itself. The response of electrical organs in general, then 

 may be summarised in the following law : 



The excitatory discharge is determined by the physiological 

 anisotropy of the organ, its definiteness of direction being deter- 



