in ELECTRICAL EXCITATION OF MUSCLE 277 



physiological changes in the entire area traversed, by which, on 

 the one hand, the excitability, and on the other, the conductivity, 

 of the intrapolar tract were fundamentally affected. Since 

 changes in the excitability, or conductivity, of any section of 

 the muscle can only be inferred indirectly from corresponding 

 changes in the magnitude of contraction, observed at the same 

 spot with uniform excitation, the main point in the case before 

 us is to apply uniform stimuli to any point of the intrapolar 

 tract, before, during, and after the passage of current, and then 

 to measure the height of twitch by graphic methods. It is 

 obvious that only the electrical stimulus is applicable in this 

 case, since it alone permits of exact graduation of the strength, 

 and does no immediate injury beyond the spot excited. But the 

 application of the electrical current as a test stimulus of the 

 excitability of a tract of muscle already traversed by current, has 

 to contend with considerable practical difficulties on account of 

 the hardly avoidable interference of the two currents. If the 

 battery current, the effect of which is to alter the excitability 

 and conductivity of the tract of the muscle traversed, is termed 

 the " polarising," while the induction current, on the other hand, 

 used as the test stimulus, is called the " exciting/' current, it is 

 clear that if the electrodes of the latter are applied directly to 

 the muscle traversed by the constant current, current must 

 necessarily flow from the one circuit into the other in proportion 

 with the resistance in both circuits. But if the polarising (con- 

 stant) current is partly diverted into the circuit of the exciting 

 (test) current, a physiological kathode will necessarily be formed 

 at one of the exciting electrodes, thereby producing a continuous 

 state of excitation, which, on its side, complicates the effects of 

 the test stimulus, so that temporary changes in the height of the 

 twitch produced by the test stimulus before and after closure of 

 the polarising current, cannot well be referred to alteration of 

 excitability in the parts in question, which would be inde- 

 pendent of direct excitation by the polarising current. The 

 following point must, moreover, be taken into consideration. 



According to the law of polar excitation, stimulation takes place 



following the direction of the current now at one, and 



now at the other, end of the tract traversed, on closure of the 



test current, from which it necessarily follows, owing to the 



oblique direction of the lines of current (from the lateral 



