146 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



trunk directly, the ulnar nerve at the elbow in ourselves, for in- 

 stance, the resulting sensations are markedly different from those 

 obtained by stimulating the skin areas supplied by the same nerve ; 

 we have little or no sensations of touch or temperature, only pain 

 and a peculiar tingling in the fingers. In such an experiment the 

 stimulus applied to the trunk affects more or less equally all the 

 contained fibers, whereas in stimulation of the skin itself the effect 

 upon the cutaneous fibers of pressure, temperature, or pain pre- 

 dominates and presumably it is these fibers that normally are con- 

 nected in an efficient way with the reflex machinery in the nerve 

 centers. 



Reflex Time. Since nerve centers are involved in a reflex 

 movement, a determination of the total time between the appli- 

 cation of the stimulus and the beginning of the response gives 

 a means of ascertaining the time needed for the processes within 

 the nerve cells. Helmholtz, who first made experiments of this 

 kind, stated that the time required within the nerve centers 

 might be as much as twelve times as great as that estimated 

 for the conduction along the motor and sensory nerves 

 involved in the reflex. Most observers state that the time 

 within the center varies with the strength of the stimulus, 

 being less, the stronger the stimulus. It varies also with the 

 condition of the nerve centers, being lengthened by fatigue 

 and other conditions that depress the irritability of the nerve 

 cells. By reflex time or reduced reflex time we may designate 

 the time required for the processes in the center, that is, the total 

 time less that required for transmission of the impulse along the 

 motor and sensory fibers and the latent period of the muscle con- 

 traction. For the frog this is estimated as varying between 

 0.008 and 0.015 sec. In man the reflex time usually quoted is that 

 given by Exner for the winking of the eye. He stimulated one lid 

 electrically and recorded the reflex movement of the lid of the other 

 eye. The total time for the reflex was, on an average, from 0.0578 

 sec. to 0.0662 sec. He estimated that the time for transmission of 

 the impulse along the sensory and motor paths, together with the 

 latent period of the muscle, amounted to 0.0107 sec. So that the 

 true reflex time from his determinations varied between 0.0471 and 

 0.0555 sec. Mayhew,* using a more elaborate method, obtained 

 for the total time a mean figure equal to 0.0420 sec. If Exner 's 

 correction is applied then the true reflex time according to this de- 

 termination is equal to 0.0313 sec. In a series of experiments 

 made upon frogs, in which the efferent response to stimulation 

 of the afferent fibers of the sciatic nerve was measured by the 

 electrical variation in the muscle involved, Buchanan finds that 

 the delay in the cord, when the reflex was on the same side, was 

 * Mayhew, "Journal of Exp. Medicine," 2, 35, 1897. 



