NATURE OF THE NERVE IMPULSE. Ill 



impulse. As the latter term is less specific regarding the nature 

 of the movement, and emphasizes the fact of the conduction of an 

 isolated disturbance or pulse, it seems preferable to employ it 

 until a more satisfactory solution of its nature has been reached. 

 The Velocity of the Nerve Impulse. — The determination of 

 the velocity of the nerve impulse was first made by Helmholtz* 

 upon the motor nerves of frogs. His experiment consisted in 

 stimulating the sciatic nerve, first, near its ending in the muscle 



Fig. 51.— Record to show the method f estimating the velocity of the nerve impulse 

 in a motor nerve, 'ihe e.vperiment was made upon a nerve-muscle preparation from the 

 frog, the contractions bring recorded upon the rapidly moving plate of a pendulum myo- 

 graph. Two contractions were obtained, the first (a) when the nerve wa,s stimulated 

 near the muscle, the second (6) when the nerve was stimulated as far as possible imva the 

 muscle. The latent period of the second contraction was longer, as shown by the distance 

 between the curves measured on the line x. The value of this distance in time is obtained 

 by reference to the record of a tuning fork vibrating 100 times per second, which is given 

 on the lower line. In the experiment the length of a tuning fork wave (O.OI sec.) was 21 

 mms., the distance between the two muscular contractions was 3.35 mms., and the dis- 

 tance between the points stimulated upon the nerve was 49 mms. Hence the velocity of the 

 nerve impulse in this experiment was 49 divided by (/j-%% X j^o) or30716mms. (30.716 m.) 

 per second. 



and. second, near its origin from the cord, and measuring the time 

 that elap.sed in each case between the moment of stimulation and 

 the moment of the muscular response. It was found that when 

 the nerve was stimulated at its far end this time interval was 

 longer, and since all other conditions remained the same this dif- 

 ference in time could only be due to the interval required for the 

 aerve impulse to travel the longer stretch of nerve. In the accom- 

 panjdng figure the record of a laboratory experiment of this kind is 

 reproduced. Knowing the difference in time and also the length of 

 * Helmholtz, "Mtiller's Archiv f. Anat. u. Physiol.," 1852, p. 199. 



