THE PHENOMENA OF CONDUCTION IN NERVE 



129 



light, is immeasurable. But this view, which was first expressed 

 by Johannes v. Miiller, in 1844, could not be maintained for any length 

 of time, because already in 1850 v. Helmholtz 1 devised a method which 

 gave fairly accurate results. In brief, it consisted in determining the 

 time elapsing between the application of an electrical stimulus to the 

 nerve of a nerve-muscle preparation and the moment when the result- 

 ing contraction of the muscle caused the circuit of a galvanic battery 

 to be broken. Very clearly, however, this interval included not only 

 the time occupied by the passage of the excitation to the muscle, but 

 also the time of contraction of the muscle itself. A few months later 

 Helmholtz devised a second method which is not only much simpler 

 but also much more accurate than the one just mentioned (Fig. 76). 



FIG. 76. SPEED OF THE NERVE IMPULSE. 



M , muscle and nerve connected with writing lever W and two pairs of electrodes 

 N and F. The wires from inductorium J are connected with the pole change P, so 

 that the nerve may be stimulated either near to or far away from the muscle. 



A nerve-muscle preparation (M) is connected with a writing lever (W) 

 in the manner described in one of the earlier chapters. The nerve 

 is then stimulated either at a point far away from the muscle (F) 

 or close to it (N). In each case, the contraction of the muscle is re- 

 corded upon a swiftly revolving kymograph, above the record of a 

 tuning fork vibrating in hundredths of seconds and the record of an 

 electromagnetic signal indicating the precise moment of stimulation. 

 If the lengths of the latent periods of these contractions are compared 

 with one another, it will be found that those obtained by stimulating 

 the nerve far away from the muscle (F), are appreciably longer than those 

 recorded by stimulating the nerve near the muscle (JV). The differ- 

 ence between these latent periods corresponds to the time consumed 

 by the wave of excitation in its passage from F to N. This distance 

 having been determined with the ruler, the time may then be calculated 



1 Monatsber. d. Berliner Akad., 1850. 



