ix ELECTRICAL EXCITATION OF NERVE 215 



be accomplished by lengthening the intrapolar region. In order 

 to obtain the necessary interval between kathodic and anodic 

 stimulation, the intrapolar tract must, however, be at least 150 

 mm., conductivity being reckoned at 30 m., and the latent 

 period at O'OOS sec. It is not possible to produce summation 

 of muscular contraction by this means with induction currents in 

 frog-preparations (Mares, 57). On the other hand, the method of 

 time-measurements still further supports the assumed bipolar 

 excitation from strong induction currents. If, i.e., the excitation 

 occurs at one pole only the kathode the latent period of the 

 muscle-twitch must (as is indeed well established) be longer with 

 ascending than with descending direction of current, and that 

 proportionately with the time occupied by rate of transmission 

 in the intrapolar region. If, on the contrary, excitation takes 

 place at both poles, the latency with both directions of current is 

 equal, and corresponds with the excitation from the pole proximal 

 to the muscle. This presumption was experimentally verified by 

 Mares' (I.e.). 



VIII. EFFECT OF REPETITION OF STIMULUS 



No matter what conception we adopt of the nature of the 

 excitatory process, it is always interesting to see in any appropriate 

 terminal organ the effect of several simultaneous or successive 

 stimuli at different points of the nerve. We have already referred 

 to the case of bipolar excitation by induced or constant currents, 

 but still greater interest attaches to the action of simultaneous 

 stimuli. A priori it is, as Hermann points out, most probable 

 that the two independent processes of excitation travel undisturbed 

 over the nerve, at an interval corresponding with the distance 

 between the two points of stimulation, and arrive successively at 

 the terminal organ. The resulting effects must depend solely 

 upon the nature of the end-organ. In muscle, e.g., the second 

 stimulus would, according to the interval, be ineffective, or would 

 cause a superposed twitch, or a second independent twitch. 

 Even when two excitations meet in the same fibre, an undisturbed 

 passage in either direction is conceivable ; and such an encounter 

 must, in fact, take place in every simultaneous stimulation of two 

 points of a nerve, since the upper excitation cannot reach the 

 muscle without crossing the lower impulse, which, of course, 



