HERMANN VON HELMHOLTZ 



surface, as indicated by the movement of the needle 

 of the galvanometer in a certain direction. This was 

 termed the resting current. If, then, the muscle is 

 caused to contract, there occurs what DuBois Reymond 

 called 4 the negative variation,' that is to say, the needle 

 gave ; a swing towards zero, or even crossed to the other 

 side. This negative variation, now called 'the action 

 current,' is an electrical phenomenon connected with 

 the activity of the muscle, and the electric change 

 is of sufficient intensity to irritate the nerve of a 

 muscle preparation, as shown in Matteucci's famous 

 experiment, usually called the ' induced contraction.' 

 In this experiment the nerve of a muscle, a, is 

 stretched over a muscle , and if the nerve of b is 

 irritated by an electric shock, not only b but also a 

 is thrown into contraction, because the c negative 

 variation ' in muscle b irritates the nerve of muscle a. 

 Helmholtz attacked the time relations of this problem, 

 and he thought he could demonstrate that the nega- 

 tive variation occurred only in the period of latent 

 stimulation, and that it was over and gone before the 

 muscle began to contract. All this work related to 

 motor nerves and to the muscles and nerves of the 

 frog. 



It seemed a more difficult problem to determine the 

 velocity in sensory nerves. When we touch a sensory 

 nerve, the message goes to the brain, but how could 

 it be possible to estimate the time it occupies in going 

 that distance, seeing the brain can only respond by a 

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