14 ANIMAL MECHANISM. 



myograpli passes over the vertical which corresponds with the 

 origin of the lines 1 and 2. 



In the experiment which regulated the shock of line 1, the 

 nerve was excited very near the muscle. In that which was 

 traced by the shock of line 2, the nerve was excited 30 centi- 

 metres farther off. As the cylinder turns with a uniform 

 motion we can estimate the time corresponding with the 

 distance which separates the two shocks. To facilitate the 

 measurement of this interval, the vertical lines indicate the 

 starting points of these shocks; in fig. 10 the interval which 

 separates them corresponds with a hundredth of a second, 

 during which the nervous agent has passed over 30 centi- 

 metres of nerve, which corresponds with a speed of 30 metres 

 per second. In order to measure this time with very great 

 exactitude, we use a method invented by Duliamel. It con- 

 sists in making the cylinder trace the vibrations of a chrono- 

 graphic tuning-fork provided for this purpose with a very 

 fine style, which scratches on the sensitive paper. We have 

 recourse to this method in all our experiments. 



Let us return to fig. 10. If the interval w r hich divides the 

 starting points of the two shocks corresponds with the time 

 which the nervous ngent lias taken to pass along 30 centi- 

 metres of nerves, there is a much more considerable time, 

 which, for each of the lines 1 and 2, is measured between the 

 signal of the excitement marked by the first of the three 

 vertical lines and the first shock. This is the lost time of 

 Helmholtz ; it represents more than a hundredth of a second 

 in this experiment. 



The greater number of authors think that the speed of tho 

 nervous agent varies under certain influences ; that heat 

 augments it, while cold and fatigue diminish it. 



It seems to us, on the contrary, that this variability of 

 duration belongs almost exclusively to those still unknown 

 phenomena which are produced in the muscle during the 

 lost time of Helmholtz. 



Just as the employes o the post, fatigued or chilled by cold, 

 cause delay in tin- di^trUiutiou of despatches, without there 

 having been any change- in tin- speed of the train which has 

 brought them, so the unix/lc-, according to whether it is rested 



