140 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 in a reflex movement the nerve centers 

 are involved 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 element concerned in the central 

 processes. 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. 



Inhibition of Reflexes. One of the most fundamental facts 

 regarding spinal reflexes is the demonstration that they can be 

 depressed or suppressed entirely that is, inhibited by other im- 

 pulses reaching the same part of the spinal cord. The most sig- 

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



