NERVE-IMPULSE IN SENSORY NERVES AND REACTION TIME. 



57 



the length of the conducting nerve, but it increases with the strength of the stimulus 

 (v. Helmholtz and Baxt), although not at first (v. Vintschgau). 



Methods. 1. V. Helmholtz (1850) estimated the velocity of the nerve-impulse in a frog's 

 motor nerve after the method of Pouillet. The method depends upon the fact that, the needle 

 of a galvanometer is deflected 

 by a current of very short 

 duration, the extent of the 

 deflection being proportional 

 to the duration and strength 

 of the current. The apparatus 

 is so arranged that the " time- 

 marking current" is closed at 

 the moment the nerve is stimu- 

 lated, and opened again when 

 the muscle contracts. If the 

 nerve attached to a muscle be 

 now stimulated at the further 

 point from the muscle, and a 

 second time near its entrance 

 to the muscle, then in the 

 latter case the time between 

 the application of the stimulus 

 and the beginning of the con- 

 traction of the muscle, i.e., the 

 deflection of the galvanometer, 

 will be less than in the former 

 case, as the impulse has to 

 traverse the whole length of 

 the nerve to reach the muscle. 

 The difference between the two 

 times is the time required by 

 the impulse to traverse a given 

 distance of nerve. Fig. 411 

 shows in a diagrammatic man- 

 ner the arrangement of the ex- 

 periment. The galvanometer, G, is placed in the time-marking circuit (open at first), a, b 

 (element), c (piece of platinum on a key, W), introduced into the time-marking circuit, d, e, 



Fig. 411. 

 Helmholtz's method of estimating the velocity of a 

 nerve-pulse. 



the key, S, when 



the platinum plate of the 



f, h. The circuit is made by closing 

 key, W. At once when the current 

 is closed, the magnetic needle is 

 deflected, and its extent noted. At 

 the same moment in which the cur- 

 rent between c and d is closed, the 

 primary circuit of the induction 

 machine is opened, the circuit being 

 i, k, I (element), m, (primary 

 spiral), p. Thereby an opening shock 

 is induced in the secondary spiral, 

 R, which stimulates the nerve of the 

 frog's leg at n. Thus, the closing 

 of the galvanometer circuit exactly 

 coincides with the stimulation of the 

 nerve. The impulse is propagated 

 through the nerve to the muscle, M, 

 and the latter contracts when the im- 

 pulse reaches it, at the same time 

 opening the time-measuring circuit -. ... 



at the double contact, e and/, bv ' 



raising the lever, H, which rotates Scheme for measuring the velocity of nerve energy. /, 

 onz At the moment of opening, the clam P for femur; m, muscle; N, nerve; a, near, b, re- 

 further deflection of the magnetic moved from, C, commutator; II, secondary; I, primary 

 needle ceases. The contact at / is s P iral of induction machine ; B, battery ; 1, 2, key ; 

 made by a pointed cupola of mercury. 3 tootn on tlie smoked plate P. 



When the lever, H, falls after the contact of the muscle, so that the point, e, comes into contact 

 with the underlying solid plate, y, the contact at/ still remains open, i.e., through the gal- 

 vanometer circuit. If the nerve be stimulated with the opening shock, first at n, and then 



