ANIMAL MECHANICS. 



15 



the occurrence of that phenomenon. It is well known that dif- 

 ferent observers, equally competent, allow different but always 

 constant intervals to elapse between the moment of the oc- 

 currence of the phenomenon and the moment of recording it, 

 and the differences between different observers is called the 

 " personal equation." In other words, the time occupied by 

 the sensitive nerves in conveying the impression to the optic 

 thalamus, and by the motor nerve in reconveying the order of 

 the brain from the corpus striatum, is different in different per- 

 sons — or, the velocity of nerve action differs in different per- 

 sons, and a fortiori in different animals. * 



Although the velocity of the wave of sensation, or of volition, 

 may thus vary, according to original differences in individuals, 

 or according to the state of health in the same individual, 

 there is good reason to believe that the time occupied by each 

 particle of the muscle or nerve fibre in going through all its 

 changes of contraction and relaxation remains constant. 



According to the well-known formula for wave motion — 



\=vt, (5) 



where X denotes the wave length, v the velocity of the wave, 

 and t the time of molecular vibration or change of state. 



From this formula it follows that, if the time of vibration be 

 constant, the length of the wave will vary as the wave velo- 

 city ; so that waves of different degrees of velocity may pass 

 along the same nerve or muscle, without any change having 

 taken place in the molecular conditions of contraction and re- 

 laxation of each portion of the muscle or nerve. 



Nothing proves so conclusively as the observation of per- 

 sons afflicted with softening of the brain, the fact that the 



* The recent experiments of Prof. Donders show that the time occupied in the 

 transmission of a sensation from the eye to the brain, the formation of a judgment, 

 and the transmission of volition from the brain to the hand, is 0.15 second. When 

 the ear is employed instead of the eye, the time h reduced to 0.09 second. 



