274 COMPARATIVE ELECTRO-PHYSIOLOGY 



that a single electrical shock of moderate intensity enters an 

 isotropic tissue at the point A, and leaves it at B ; A will then 

 be the anode, and B the kathode. Here the true excitatory 

 effect is found to take place only at the kathode, probably 

 because the anode-break excitation takes place at much 

 higher intensities of E.M.F. than the kathode-make, At these 

 higher intensities, then, the anode-break effect also will occur. 

 At an excessively high E.M.F. again, these relations, for 

 reasons already explained, may undergo reversal. The point 

 of such reversal would depend on the nature and excitability 

 of the tissue. Though, for all these reasons, the relative 

 excitations of A and B remain a matter of doubt, yet we may 

 be sure of the excitation of both points, if two, or any equal 

 number, of exactly equal shocks be sent through the tissue in 

 opposite directions, in rapid alternation. If, again, instead of 

 two alternate shocks only, we give n alternating shocks, abso- 

 lutely equal, and if, further, the natural excitability of the two 

 points A and B have been the same, then there will be nothing 

 to distinguish the excitatory effect induced at A from that at B. 

 In other words, the two excitations will be exactly equal. 

 These strictly equal and opposite alternating currents, more- 

 over, can have no resultant polarisation-effects, for the effect 

 arising from an induction shock, in either direction, will be 

 counteracted by that caused by the opposite. 



We thus see that by the employment of this method 

 the only change induced at the two electrodes will be the 

 excitatory change, the physical polarisation-factor being 

 eliminated. Thus, on subjecting two points, A and B, to 

 equal stimulation, the induced galvanometric negativity, at 

 both the points, will be equal, if the natural excitabiiities of 

 the two have been the same. But if the tissue be anisotropic, 

 and the natural excitability of one point, say B, greater than 

 that of A, then we shall obtain a resultant responsive current, 

 which will flow in the tissue from the more excited B to the 

 less excited A, the induced galvanometric negativity of B 

 being relatively the greater. We have here what is merely 

 a repetition, by electrical stimulation, of the results which 



