NATURE OF THE NERVE IMPULSE. 107 



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 non-medullated 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 T fa 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- 

 sory 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 

 by a wave of electrical disturbance. This fact may be shown by 

 stimulating the motor areas in the cerebral cortex and testing the 

 efferent nerves, such as the sciatic, for an action current; or by 

 stimulating a posterior root on one side in the lumbar region and 

 testing the sciatic nerve on the other side with a galvanometer.f 

 Moreover, all influences that alter the velocity or strength of the 

 nerve impulse affect the electrical disturbance in the same manner. 

 It is believed generally, therefore, that the electrical alteration is 

 an invariable accompaniment of the excitatory wave, and the dem- 

 onstration of an action current in a nerve is tantamount to a proof 

 of the passage of a nerve impulse. 



Direction of Conduction in the Nerve. The fact that under 

 normal conditions the motor fibers conduct impulses only in one 



* Consult Gotch, "Journal of Physiology," 31, 1, 1904. 

 t Gotch and Horsley, "Phil. Trans., Royal Soc," London, 1891, vol. 

 182 .(B), and Boruttau, "Pfliiger's Archiv," 1901. 



