42 ANIMAL MECHANISM. 



Ilolmlioltz had tlie boldness to undertake tliis measurement, 

 and, by determining 1 the speed of the nervous agent, lie has 

 furnished physiologists with a method which enables them to 

 measure the duration of other phenomena connected with the 

 nervous or muscular functions. Thus the experiment described 

 above, in which we have measured the speed of the trans- 

 ference of the wave in a muscle, is only an application of the 

 method of Ilelmholtz. 



In order to make tli9 conditions of this experiment 

 thoroughly comprehensible, let us make use of a comparison. 

 Let us suppose that a letter is despatched from Paris to go to 

 Marseilles, and that, being resident in the latter town, we 

 should be informed of the precise instant at which the postal 

 train leaves Paris, while we have nothing- to warn us of its 

 arrival at Marseilles except the knowledge of the moment at 

 which the letter is delivered there. How can we, according to 

 these data, estimate the speed of the mail train ? It is clear 

 that the instant at which we receive the letter does not indi- 

 cate that of the arrival of the train ; fur between that arrival 

 and the distribution, many preliminaries take place, the sorting 

 of letters, delivery, &c., which require a certain time not 

 within our knowledge. In order to have an exact idea of the 

 speed of the train which carries the mail, we must receive 

 a signal of the passage of that train through an intermediate 

 station between Paris and Marseilles, Dijon, for instaiice ; 

 then we shall see that the distribution of letters takes place 

 fdx hours sooner after the departure from D jon than after tho 

 departure from Paris. Knowing (he distance which separates 

 these two stations, we may ascertain from the time employed 

 in traversing it, the speed of the train. By supposing this 

 speed to be uniform, we shall know the hour at which the 

 train will have arrived at Marseilles, which will give us know- 

 ledge of the time consumed in the sorting and distribution of 

 the letters. 



Ilelmholtz, in experimenting upon the nervous motive 

 agent, first excited the nerve at a point very distant from the 

 muscle, and noted the time which elapsed between the excite- 

 ment which despatched the message carried by the nerve, and 

 the appearance of motion in the muscle. Then acting on a 



