262 PHYSIOLOGY. 



ing the nerve with a hot wire, or holding a hot wne near 

 the nerve, may be used as a stimulus ; chemical stimuli, as 

 acids, strong salt solution, etc., may also be used. 



Essential Similarity of All Nerve Fibers. It is to be 



noted that while special stimuli act on specially modified 

 nerve endings, all nerve fibers are essentially alike, and 

 the nerve impulse, however started, is probably the same 

 kind of force. For instance, cutting the optic nerve, or 

 severe shock, as a blow on the head, causes a sensation of 

 light not quite so definite, but essentially the same as 

 though light had acted on the retina, and thus started the 

 nerve impulse, instead of a mechanical stimulus acting on 

 the nerve fibers between the retina and the brain. 



Relation of Stimulus and Sensation. If we apply a 

 stimulus of a given intensity, as of an electric current, 

 whose intensity can be measured, it causes a sensation of 

 a certain degree. Doubling the stimulus, or increasing it 

 by a definite amount, does not increase the intensity of the 

 sensation to the same degree. The sensations do not 

 increase at the same rate as the stimuli. To increase the 

 sensations arithmetically, the stimuli must increase geo- 

 metrically. 



Reaction Time. " Reaction time " is the time between 

 the application of a stimulus and the signal given as a 

 response to show that the stimulus has been "felt." Thus 

 a blindfolded person gives a signal as soon as he is touched. 

 This interval between the stimulus and response varies 

 with the individual, mode of stimulation, health, attention, 

 etc. It is from one tenth to one fifth of a second ; is short- 

 est for touch ; longer for sight than for hearing. The total 

 reaction time is occupied by (i) the time of conducting 

 the nerve impulse to the brain, (2) the time occupied in 



