Chap. XII.] 



THE FROG-INTERRUPTER. 



120 





has begun to move. The needle will then move just 

 as if it had received a single blow, as it were, and 

 will be deflected till the influence of the earth's 

 magnetism neutralises the shock, and brings the 

 needle back to zero. Under these circumstances the 

 amount of observed deflection of the needle will be pro- 

 portional to the intensity of the current, and the time 

 during which it acted. If, however, the intensity of 

 the current be always constant, but the duration of 

 the current varied, then the deflection will be pro- 

 portional to the length of time during which the 

 current has acted, that is, the extent of deflection will 

 measure the time. Thus -by means of the galvano- 

 meter small intervals of time may be measured. 



This principle is made use of in estimating the latent 

 period of stimulation of 

 the muscle, that is, the 

 time that elapses between 

 the moment of the muscle 

 receiving a shock, and the 

 moment of its response by 

 contraction, and in esti- 

 mating the rapidity of 

 the nerve current. For 

 this purpose the instru- 

 ment shown in Fig. 66, 

 devised by Helmholtz, and 

 modified by Reymond, has 

 been employed. It is 

 called the frog-interrupter 

 (frosch unterbrecher). It 

 consists of a brass plate 



supported by two pillars Fig< 6 6.-The Frog-Interrupter. 



on a block of mahogany 



w, levelled by screws. From the brass plate rises 

 a pillar, up and down which a forceps can slide, 

 and be fixed at any point by a screw. On the 

 J 7 



