HERMANN VON HELMHOLTZ 



that in each case the individual who is the subject of 

 the experiment indicates the moment he feels the 

 sensation, and that the instant the stimulus is applied 

 successively to the skin on the toe and on the thigh 

 is also accurately recorded, it is clear that he will 

 signal the sensation of stimulation of the toe a little 

 later than when he signals stimulation of the skin on 

 the thigh, and that the difference will indicate the 

 time required by the change in the nerve to pass 

 along the length of the nerve from the toe to the 

 thigh. In the observation it is assumed that the time 

 required for the changes in the brain, resulting in sen- 

 sation and volition, for the transmission along the 

 motor nerve, and for the muscular contraction re- 

 quired to signal, is the same in each experiment. 

 Thus, supposing the total time between the moment 

 of stimulating and the moment when the signal that 

 the sensation has been felt and responded to be JT, it 

 is clear that this time is composed of a y the time 

 required for the passage of the nerve current in the 

 first experiment from the toe to the brain ; of , the 

 time required for the changes in the brain involved in 

 sensation and volition ; and of c y the time required for 

 the transmission along the motor nerves and for the 

 muscular contraction to move the signal that is, 

 x = a + b + c. But if the time between the moment of 

 stimulating the thigh to the moment of signalling be 

 shorter, and supposing that b and c are constant, then 

 a will vary according to the length of the nerve. 

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