7 o HERMANN VON HELMHOLTZ 



of investigation to physiologists/ Helmholtz set himself in the 

 first place to study the processes that take place in a simple 

 muscle-twitch, consequent on a stimulus of infinitesimal dura- 

 tion, when the muscle in order to do work must pass from 

 a state of rest to a state of motion, and the quantit}^ of work 

 done depends essentially upon the rapidity of this transition. 

 From these facts he went on to the question of the rate at 

 which excitation is propagated in nerve. From his first ex- 

 periments at the beginning of 1849 he had concluded, from the 

 curves obtained by plotting the height to which a weight is 

 lifted against time, that the energy of the muscle was not at 

 its maximum immediately after the excitation, but that it rose 

 for some time, and then dropped again. In order to show 

 these facts plainly, and at the same time to determine the time- 

 relations and the stages in which the mechanical activity, the 

 energy of the muscle, rises and falls after an instantaneous 

 excitation, Helmholtz, at the outset of his experiments, con- 

 trived a very ingenious piece of apparatus. He attached a 

 metal ring to the muscle, which carried a scale-pan of light 

 weight ; the upper part of the ring was supported in such a 

 way by a metal pin, that it could not drop lower when the 

 load was increased. On then closing a current, part of which 

 passed through the muscle, part through a galvanometer, 

 the pin, and the ring, the galvanometer circuit was broken 

 by the contraction of the muscle and consequent lifting of the 

 ring off the pin, and by placing weights in the pan it became 

 possible to compare the elastic force of the muscle in the 

 resting state with that after excitation. In order to follow 

 the very rapid twitch of the muscle in its successive stages, 

 and to investigate the propagation of excitation in the nerve, 

 new methods for the measurement of infinitesimal fractions 

 of time were devised by Helmholtz, who thus provided fresh 

 appliances for the delicate and complicated investigations of 

 physiological processes. 



The need of some method by which it should be as possible 

 to measure minute fractions of a second, as it is with a powerful 

 microscope to estimate fractions of length, had long been felt 

 in a variety of astronomical and physical observations. Two 

 such methods, invented mainly for the exigencies of artillery, 

 had already been devised on widely different principles. In 



