244 HERMANN VON HELMHOLTZ 



to resume his nerve work, and if possible bring it to a con- 

 clusion, as his mind was busying itself over problems of quite 

 another kind. On February 6, 1867, ne wrote to Wittich : 



' In regard to rate of transmission in nerve, I have been 

 making some experiments myself this winter with one of my 

 Russian laboratory assistants, which are not yet fully worked 

 out, and which give about 34 m. ; these, however, refer to the 

 motor nerves of man, as I have recorded upon the myograph the 

 muscular contractions of the ball of the thumb excited from 

 the wrist and axilla respectively. We spent a long time on 

 the improvement of our method, but eventually obtained very 

 good and concordant results, which are infinitely superior in 

 regularity of effect to my old method. I think one might apply 

 it to many other questions, e. g. the supposed difference of 

 velocity in different parts of the nerve.' 



These experiments were complicated by the difficulty that an 

 instantaneous excitation of the motor nerves of man is not 

 transmitted in an absolutely unaltered form through any con- 

 siderable length of nerve. It is accordingly necessary to take 

 precautions that the electric shock shall be so far weakened for 

 the upper portions of the nerve that the contraction which it 

 excites shall be of the same height and strength as the maximum 

 of contraction excited from the lower point : the two instan- 

 taneous excitations of the nerve will then produce equal external 

 mechanical effects, the delayed response on exciting the upper 

 portion being referred solely to conductivity within the nerve. 

 The curves recorded by the myograph indicate that weaker 

 stimuli are propagated in nerve more slowly than stronger 

 shocks, and three long series of experiments gave rates of trans- 

 mission of about 31, 33, and 37 meters per second. 



Owing to external circumstances the experiments were inter- 

 rupted, and Helmholtz only took them up again three years 

 later. He showed in a paper published in 1870, from experi- 

 ments undertaken with Pick's pendulum myograph, that the 

 rate of transmission of the nervous impulse was more than 

 twice as great in nerves at higher temperatures, e. g. in the arm, 

 as at lower temperatures. 



' This is a most extraordinary thing/ writes du Bois on April 

 4, 1870. ' Such a dependence on temperature is unheard of; 

 one would suppose then that the velocities would be enormously 



