112 THE PHYSIOIOGY OF MUSCLE AND NERVE. 



nerve between the points stimulated, the data are at hand to cal- 

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

 temperature. According to Helmholtz, this variation hes between 

 24.6 and 38.4 m. per second for a 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. Piper* has applied the string-galvanom- 

 eter 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 (" Midler's Archiv," 1852, 3.30) : " 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 thia 

 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. Tlierefore 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 nex-ve 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 directty. 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 

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



