NATURE OF THE NERVE IMPULSE. 113 



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 

 in the medullated afferent nerve fibers is the same as in the 

 efferent fibers. A large number of observations are on record 

 which show that the velocity varies greatly in the nerves of 

 different animals. In the mammal, according to Chauveau, the 

 velocity for the non-medullated fibers is only 8 meters per second; 

 in the lobster it is 6 meters per second; in the octopus, 2 meters; 

 in the olfactory (sensory) nerve of the pike, meter, and in the 

 anodon, only -j-J-g- meter per second. 



Relation of the Nerve Impulse to the Wave of Negativity. 

 A fact of great significance is that the velocity of the impulse in the 

 motor nerves of the frog corresponds exactly to the velocity of the 

 wave of negativity as measured by Bernstein. Evidently the two 

 phenomena are coincident in their progress along the fiber, and 

 physiologists generally have accepted the existence of an action cur- 

 rent as a proof of the passage of a nerve impulse. This belief is 

 strengthened by the fact that, as stated above, the negative wave ac- 

 companies the nerve impulse not only when the nerve is stimulated 

 by electrical currents, but also after mechanical, chemical, or reflex 

 stimulation. The question has been raised as to whether this elec- 

 trical phenomenon accompanies the normal nerve impulse, that is, 

 the nerve impulse that originates in the nerve centers, in the case 

 of motor nerves, or in the peripheral sense organs in the case of sen- 

 gory nerves. In regard to the latter relation we have positive evi- 

 dence that when light falls upon the living retina an electrical distur- 

 bance is produced by the visible rays of the spectrum,* and there 

 is every reason to believe that the passage of visual impulses along 

 the optic nerve is accompanied by an electrical change. With 

 regard to normal motor impulses, the evidence is also positive that 

 motor discharges from the central nervous system are accompanied 



* See Einthoven and Jolly, "Quarterly Journal of Experimental Physiol- 

 ogy," 1, 373, 1908. 



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