386 OBSERVATIONS AND EXPERIMENTS 



a given overweight in an interval which, after an inappre- 

 ciably short excitation of the nerve, is always the same, and this 

 action could be employed to open the experimental circuit. The only 

 question was, how the time relations of the twitch corresponded to 

 those of the shock, about which nothing was then known 1 . The 

 first experiments taught me that the duration of the shock is of the 

 same order with the duration of the twitch, and this led to the 

 construction of the ' frog-interrupter,' which has also been already 

 described in detail and accurately drawn, and its properties tested 2 . 

 I need not therefore dwell upon it here. As was there stated, it is 

 a more convenient form of the same contrivance used by Helmholtz 

 in measuring the velocity of excitation in the nerve after Pouillet's 

 method, and in accordance with this more general character it has 

 acquired importance 3 in many relations. In the present experi- 

 ments it serves the following purposes: I. it allows only the first of 

 the several shocks which the fish successively makes to pass into the 

 experimental circuit ; 2. it admits a proportion of the first shock, 

 which is greater or less according to the overweight used, and 

 which, provided that the shocks have similar course but different 

 strengths, is also proportional to the strength ; 3. it may be used 

 to break a derivation circuit by which the shock had previously 

 been kept out of the experimental circuit. We shall investigate 

 later (in 10) with what degree of certainty the frog-interrupter 

 executes the second task. As, in general, it affords entrance for 

 a part only of the shock into the experimental circuit, it is evident 

 that when it is used, greater sensitiveness of the galvanometer is 

 requisite. 



Each fish was always worked with, only after intervals of several 

 days. I usually put the saddles or covers on it every ten minutes, 

 and I went on for an hour and a half or two hours 4 . Including the 



1 Herr Eckhard showed at the same time in Trieste with Torpedos, that the shock 

 must last longer than an opening induction shock. (Beitrage znr Anatomie und 

 Physiologic, vol. i. Giessen, 1858. 4. p. 168, 169 IF). Since then, Herr Marey, in 

 Naples, has investigated the duration of the shock in Torpedos, with the Pendulum- 

 myograph, and found it to be 1/14", which confirms my result (Comptes rendus, etc., 

 1871, vol. Ixxiii. p. 918). I do not need to remark, that myograph experiments on 

 living uninjured fish would meet with almost insuperable difficulties, so that I had 

 to forego them. 



3 Gesammelte Abhandl., vol. i. p. 2156. PI. III. Fig. 12. i za. Comp. the following 

 paper, 3, Fig. 44. 



3 Loc. tit. vol. i. p. 227, note, where several applications of the frog-interrupter 

 are mentioned ; Holmgren's investigation of the course of the gastrocnemius variation 

 carried out with its help, is not alluded to. 



* Geaammelte Abhandl., vol. i. p. 221. 



