250 COMPARATIVE ELECTRO-PHYSIOLOGY 



simplicity, neglect any effects that may accrue from anode- 

 make and kathode-break. Under a homodromous induction- 

 shock, then, two different excitatory electrical changes will 

 be induced, on the lower and upper surfaces respectively, the 

 consequent currents through the tissue being in opposite 

 directions. On these, moreover, will be superposed again the 

 polarisation-current. Calling the effect induced by anode- 

 break as A,, and that of kathode-make as K WI , we shall obtain 

 a resultant consisting of A b on the more excitable anterior 

 surface, minus K m on the less excitable posterior, minus the 

 negative polarisation-effect. Under a heterodromous shock, 

 on the other hand, we shall have K m on the more excitable 

 anterior surface, minus A b on the less excitable posterior, plus 

 the negative polarisation-effect. 



Even this, however, does not exhaust the possibilities 

 of complication. For I shall show in a subsequent Chapter, 

 and have already shown elsewhere, that under a high E.M.F. 

 Pfliiger's Law does not apply. The relative excitatory 

 values of anode and kathode may indeed undergo one or 

 more reversals, according to the intensity of the acting 

 electro-motive force. Thus, under a moderately high E.M.F. 

 in what I have designated the A stage, both the anode and 

 kathode are found to excite at make, and either kathode or 

 anode at break. In the B stage, under a still higher E.M.F., 

 it is the anode which excites at make, and the kathode at 

 break. 



It will thus be seen what a number of complicating 

 factors may be present when an organ is excited by currents 

 of varying direction and intensity. If, then, we wish to study 

 the purely excitatory reaction of an organ, as dependent 

 solely upon its individual characteristics, uncomplicated by 

 defects inherent in the method of excitation, we must see 

 first that the applied stimulus is equal on both surfaces, and, 

 secondly, that such factors as are not excitatory that is to say, 

 negative or counter-polarisation are eliminated. These ends 

 may be accomplished by subjecting the responding organ 

 to symmetrical and alternating equal and opposite shocks, 



