HELMHOLTZ IN KONIGSBERG 



muscular act ? Helmholtz devised a method by which 

 what is called the ' reaction period ' may be computed, 

 that is the time between the moment of stimulating, 

 say the skin of the foot and the moment the indi- 

 vidual makes a signal that he has felt the sensation. 

 The reasoning is as follows : Suppose a sensory nerve 

 to be excited in the hand, the theory of nervous con- 

 duction is that a change is propagated along the nerve 

 to the brain, and that in the brain the molecular 

 changes occur which result in a sensation. The 

 individual having the sensation may feel it and make 

 no sign by which anyone else might be made aware 

 that he has felt it, or the subject of the sensation 

 might, by a muscular movement, such as the motion 

 of an arm, let anyone else see that he has felt the 

 sensation. We have no means of knowing whether 

 or not an individual has felt a sensation except by the 

 individual making some kind of gesture or muscular 

 movement. Now, it is clear that, if we regard the 

 brain as the seat of the changes resulting in sensation, 

 the nearer any stimulated portion of skin is to the 

 brain the sooner will the brain feel and respond to 

 the stimulus. Thus, if the skin on the big toe of 

 the right foot be stimulated, the effect of the stimulus 

 will pass to the brain and there call forth a sensation ; 

 but if the stimulus be applied to the skin at the top 

 of the thigh, it is evident the effect will have to pass 

 along a shorter length of nerve, and that the sensation 

 in the brain will be aroused sooner. If we suppose 

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