106 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



panying figure the record of a laboratory experiment of this kind 

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

 of nerve between the points stimulated, the data are at hand to 

 calculate the velocity of the impulse. The velocity varies with the 

 temperature. According to Helmholtz, this variation lies between 

 24.6 and 38.4m. per second fora range of temperature between 11 

 and 21 C. For average room temperatures we may say that in 

 the motor nerves of the frog the impulse travels with a velocity 

 of 28 to 30 meters per second. Similar experiments upon man and 

 other mammals indicate that the velocity in the medullated motor 

 nerves does not vary greatly in different animals. Helmholtz's 

 average figure for man was 34 meters per second. 



It is interesting to recall that only six years before Helmholtz's first pub- 

 lication Johannes Miiller had stated that we should never find a means of 

 determining the velocity of the nerve impulse, since it would be impossible 

 to compare points at great distances apart, as in the case of the movement 

 of light. " The time," said he, " required for the transmission of a sensation 

 from the periphery to the brain and the return reflex movements of the mus- 

 cles is infinitely small and unmeasurable." The mode of reasoning by which 

 Helmholtz was led to doubt the validity of this assertion is interesting. He 

 says (" Muller's Archiv," 1852, 330) : " As long as physiologists thought it 

 necessary to refer nerve actions to the movement of an imponderable or 

 psychical principle, it must have appeared incredible that the velocity of this 

 movement could be measured within the short distances of the animal body. 

 At present we know from the researches of du Bois-Reymond upon the electro- 

 motive properties of nerves that those activities by means of which the con- 

 duction of an excitation is accomplished are in reality actually conditioned 

 by, or at least closely connected with an altered arrangement of their material 

 particles. Therefore conduction in nerves must belong to the series of self- 

 propagating reactions of ponderable bodies, such, for example, as the con- 

 duction of sound in the air or elastic structures, or the combustions in a tube 

 filled with an explosive mixture." One of the first fruits, therefore, of the 

 scientific investigation of the electrical properties of the nerve fiber was the 

 discovery of the important fact of the velocity of the nerve impulse. 



Numerous efforts have been made to determine the velocity 

 of the nerve impulse in medullated sensory fibers. The results 

 have not been entirely satisfactory. The end-organ in this case is 

 the cortex of the cerebrum, and its reaction consists in arousing a 

 sensation, or a reflex action. Neither end-reaction can be meas- 

 ured directly. Attempts have been made to determine it indi- 

 rectly by noting the time of a voluntary muscle response for sensory 

 stimuli applied to the skin at different distances from the spinal 

 axis. In such cases the sensory impulse travels to the cord, thence 

 to the brain, and the return motor impulse travels from brain to 

 cord and then by the motor nerves to the muscle used for the re- 

 sponse. The results of this method have been discordant, owing 

 probably to the fact that the central paths from two different points 

 on the skin are not identical. It is usually assumed without, 

 however, very convincing proof that the velocity of the impulse 



