138 PHYSIOLOGY OF MUSCLES A^'D NERVES. 



it has readied its highest point. It must, moreover, be 

 remembered that it is never merely a single cross-section 

 of a nerve which is irritated, but always a portion of 

 greater extent, and that the excitability measured by us 

 is in reality only the average excitability of the various 

 points within the irritated portion. It may further be 

 assumed that the excitability at each point, when it 

 has reached its height, is very rapidly, if not instan- 

 taneously, destroyed. As this, however, occurs sooner 

 at the higher than at the lower points, it follows 

 also that the excited portion, beginning from the top, 

 gradually becomes a powerless thread, which is, how- 

 ever, still capable of transmitting electricity. The ex- 

 citement occurs in reality only in the lower division of 

 the portion irritated, and this, as long as it retains any 

 power of action, must remain at the highest point of 

 excitability.^ 



4. In studying the law of pulsations we attended 

 only to the closing and opening of the current, entirely 

 disregarding the period during which the continuous 

 current flowed through the nerve. In reality, the 

 nerve, as a rule, remains unexcited during this period. 

 Sometimes, however, especially on the application of 

 but moderately powerful currents, an enduring excite- 

 ment expressing itself as a tetanus in the muscle is 

 observable while the current lasts. Ascending and 

 descending currents do not behave quite alike in this 

 matter. The latter are followed by tetanus, even in th6 

 case of currents of somewhat high power, while the 

 ascending currents are only followed by tetanus when 

 they are weak. In all cases this tetanus is, however, 

 but slight, and cannot be compared with that which 



' Sec Noles and Additions, No. 5. 



