458 COMPARATIVE ELECTRO-PHYSIOLOGY 



renewed depression of excitability, is necessary in order to 

 obtain renewed amplitude of response. 



The second effect due to this depression, without abolition 

 of the excitability of B, is seen in the diphasic character of 

 the responses. The positive after-effect observed in the record 

 shown in fig. 274 may thus be ascribed to the later induction 

 of negativity at the depressed point B. The electrical re- 

 sponse of the nerve is apparently liable in this way to great 

 variations, when the method of record employed is differential. 

 But it must be remembered that true characteristic variations 

 of the response as determined by physiological modification 

 can only be obtained by finding some means which shall 

 be strictly independent of this differential factor. With this 

 object, I have succeeded in devising a new mode of observing 

 and recording the direct effect of stimulus on the nerve, 

 uncomplicated by the differential factor. In a subsequent 

 chapter we shall, using this method, be able to determine 

 the conditions which induce the characteristic variations in 

 the response of nerve, from the staircase increase to the fatigue- 

 decline, or even reversal, through the intermediate phase of 

 uniform reponses. 



The method which has just been described, of exciting 

 the nerve at both contacts by equi-alternating shocks, is not 

 applicable, however, where the object of investigation is the 

 conductivity of an intervening tract of nerve between the 

 exciting and the led-off circuits. Here the employment of 

 electrical shocks as exciting stimulus gives rise to disturbing 

 unipolar effects, which persist even when the physiological 

 conductivity of the intervening tract is destroyed as, say, by 

 ligature or by crushing. Thus 



' If the nerve of a frog's leg is laid across two 

 electrodes connected with the poles of a secondary coil, 

 so as to close the induction circuit, a ligature being then 

 applied to the myopolar tract, tetanus may still be 

 observed in the isolated leg, on making the lead off from 

 it at a certain distance of coil. . . . These unipolar effects 



