DETERMINATION OF DIFFERENTIAL EXCITABILITY 283 



said here that I use a D'Arsonval type of galvanometer, in 

 which, instead of a suspended needle, we have a suspended 

 coil. There is thus here not even the remote contingency 

 of disturbance which might arise from the demagnetisation 

 of the magnetic needle. Having thus tested, by null action, 

 the symmetry of the electrodes and the galvanometer, the 

 differentially excitable tissue, say the sheathing petiole of 

 Musa, is interposed, with its concave and more excitable 

 surface upwards. On now applying excitation by equi- 

 alternating shocks, the responsive current will be found to 

 flow downwards, from concave to convex, giving a deflection 

 of the galvanometer, say to the right. And this deflection 

 will continue to be to the right, even if the battery current 

 (fig. 171) be reversed by means of key K. The direction of 

 the excitatory current, moreover, depending solely, as it does, 

 on the relative excitabilities of the two surfaces of the 

 specimen, will remain constant, even if the connections with 

 the secondary coil, S, be reversed. The zinc rod, N, of the 

 non-polarisable electrode in connection with the concave 

 surface (fig. 172) has thus, up to the present, shown induced 

 galvanometric negativity, the galvanometric deflection being 

 to the right. But if we exchange the zinc rods of the non- 

 polarisable electrodes, it will then be N' which will be con- 

 nected with the more excitable concave surface, and it will 

 now be this electrode N' which will show galvanometric nega- 

 tivity. This reversal of the galvanometer deflection with the 

 reversal of the electrodes affords additional confirmation of 

 the greater excitability of the concave surface of the specimen 

 of Musa. 



In these experiments the existing current of rest may 

 be balanced previously by a potentiometer. But this is not 

 absolutely necessary. I give below a series of records 

 obtained with a specimen of the sheathing petiole of Musa 

 (fig. 173)5 in which we know the inner or concave surface 

 to be more excitable than the outer or convex. The 

 responsive current is seen under this form of electrical 

 stimulus, as we found to be the case under mechanical and 



