152 ELECTRO-PHYSIOLOGY CHAP. 



opening tetanus may even be reinstated by closure in the oppo- 

 site direction, if closure and rapid reopening of the homodromous 

 current fail to do so. If the tetanus which derives from the 

 opening of an ascending or descending current is strengthened 

 by closure in the opposite direction, and if this last is persistent, 

 the tetanus which was at first reinforced will disappear gradu- 

 ally. But if the current remains closed after its disappearance, 

 tetanus reappears when it is opened, and the preparation reacts 

 to this current, as previously to its opposite, i.e. as if this current 

 had acted from the first upon the nerve. The new current, 

 therefore, in the first instance abolishes the original modification, 

 and then begins to reinstate it. 



All these facts find a direct explanation in the polar altera- 

 tions of excitability, or excitatory and inhibitory phenomena in 

 nerve, as described above, and might indeed have been predicted 

 from this standpoint. In this sense Eosenthal's law is no more 

 than a consequence of the polar law of excitation as set forth 

 above a necessary effect of the simultaneous and antagonistic 

 changes produced at the two poles by current, and the suc- 

 cessive changes at one and the same pole during closure and after 

 opening of the current. It thus appears almost self-explanatory 

 that renewed closure of the homodromous current should at once 

 break off a persistent opening excitation, since at that moment 

 anelectrotonus would again prevail at every point of the nerve 

 at which the excitability had been heightened. Closure of 

 current in the opposite direction would of course have the con- 

 trary effect, since excitation resulting from the vanishing anelec- 

 trotonus would be supported by the katelectrotonus appearing at 

 the same point. 



Seeing the importance which has accrued to the application 

 of the electrical current in practical medicine, the many attempts 

 to determine electrotonic alterations of excitability, and the law 

 of contraction on man, become intelligible. Yet it is a priori 

 obvious that the difficulties of investigation are incomparably 

 greater, since the complicated and in part intangible relations of 

 current distribution and conduction render any direct comparison 

 of results with those obtained from the stimulation of isolated 

 nerve difficult, and often quite impossible. Under any circum- 

 stances the greatest caution is required in accepting the conclu- 

 sions obtained from experiments on man. 



