VELOCITY OF A NERVE IMPULSE 



95 



turning the rocker into the proper position. Take two simple twitches, 

 recording them one over the other, stimulating the nerve (a) at e 1 and (b) at 

 E 2 . Take a time tracing under the record. 



Fig. 80. 



-Arrangement of Apparatus for Studying the Velocity 

 of a Nervous Impulse. 



A tracing obtained in this manner is shown in fig. 81. It is seen 

 that the latent periods differ in the two cases, the second one being 

 longer than the first. This difference in time can only be accounted 

 for by supposing that the molecular changes started in the nerve by 

 the stimulation require time for their transmission along it. In the 

 figure a is the point of stimulation, b is the instant at which the 

 muscle began to contract with the electrodes in the first position, and 

 c when they were in the second position. The time tracing is at the 

 rate of 100 vibrations per second, and on measuring it is found that the 

 length between b and c corresponds to a time of 0"00369 sec. The 

 length of nerve between the two positions of the electrodes was 

 74 mm. Hence the impulse would in one second travel ™- = 20054 



Fig. 81. — Two Twitches of a Gastrocnemius when the Sciatic was Stimulated 



(a) NEAR THE MUSCLE, (b) NEAR THE VERTEBRA. TlME TRACING, 100 PER SEC. 



mm. per sec. In this experiment, then, the velocity of the nervous 

 impulse was 20"05 metres per second. In an experiment of this kind 

 it is very important that the recording surface should travel at exactly 

 the same rate when the two twitches are taken. Hence it is better 

 to take the record upon a pendulum or spring myograph, or two quite 



