NATURE OF THE NERVE IMPULSE. Ill 



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 38.4 m. 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 have been 

 made upon man and other mammals. Helmholtz stimulated 

 the median nerve in man at two different points and recorded 

 the resulting contractions of the muscles of the thumb. By 

 this means he obtained an average velocity of 34 m. per -second, 

 but others, making use of the same method, have reported 

 varying results. Piper* has applied the string-galvanometer 

 to the investigation of this point. Using the unipolar method, 

 he stimulated the median nerve with induction shocks, the active 

 electrode being applied at the elbow and at the axilla at a distance 

 apart of from 160 to 170 mm. The muscular response was 

 recorded not by registering the contraction, but by means of its 

 action current. When the stimulus was applied at the elbow the 

 interval between the stimulation and the electrical response 

 averaged 0.00442 second; at the axilla the interval was 0.00578 

 second. The difference, namely, 0.00136 second, gave the time 

 necessary for the impulse to travel over 160 to 170 mm. of nerve, 

 and indicated a velocity of 117 to 125 m. 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 (" Miiller'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 prof>erties of the nerve fiber was the 

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



* Piper, "Archiv f. d. ges. Physiologic," 1908, 124, 591. 



