THE LEAF CONSIDERED AS AN ELECTRIC ORGAN 249 



mined by the fact that the responsive current is always from the 

 more to the less excitable of the two surfaces, 



Referring once more to the definite-directioned after- 

 current which we have seen to be induced as the result of 

 polarising-currents, whether homodromous or heterodromous, 

 it is now clear that these currents act as an electrical 

 form of stimulus. The intensity of the after-current 

 here seen in the galvanometer, however, is not wholly due 

 to the excitatory electro-motive change, but in part also to 

 physical polarisation, which is added to it algebraically. Thus, 

 an exciting homodromous shock gives rise to an electrical 

 after-effect, in which the excitatory current is opposed by 

 a counter-current of negative polarisation. Under a hetero- 

 dromous shock, on the other hand, the excitatory electrical 

 change becomes summated with the negative polarisation, 

 which is now in the same direction as itself. In these cases, 

 though the preponderating nature of the excitatory effect 

 determines the definite direction of the after-effect, yet it is 

 difficult to know how much of the latter is actually due 

 to excitatory action as such, and how much to ordinary 

 polarisation helping or opposing this. 



Very much greater complexities ensue again in practice 

 rom the difference between anodic and kathodic actions 

 on the two unequally excitable surfaces. In Torpedo, for 

 instance, according to Du Bois-Reymond, the electrical organ 

 responds better to a homodromous than to a heterodromous 

 exciting current, while in Malepterurus, according to Gotch, 

 the reverse is the case, the heterodromous being more efficient 

 than the homodromous. Such diversity of results is prob- 

 ably to be accounted for by the considerations to which I 

 have referred. 



If we take, lor example, the simplest case, that in which 

 the anterior surface is more excitable than the posterior, and 

 if we suppose an induction-current of moderate intensity 

 to be sent in a homodromous direction, we may assume that 

 Pfltiger's Law the kathode excites at make, and the anode 

 at break will hold good. We shall here, for the sake of 



