268 REACTION TIME. 



acted on the retina, and thus started the nerve impulse, instead 

 of a mechanical stimulus acting 011 the nerve libers between 

 the retina and the brain. 



If we apply a stimulus of a given intensity, as of an elec- 

 tric current, whose intensity can be measured, it causes a sen- 

 sation 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 geometrically. 



" 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 shortest for touch ; longer for sight 

 than for hearing. The total reaction time is occupied by (1) 

 the time of conducting the nerve impulse to the brain, (2) the 

 time occupied in the cerebral cortex in the perception of the 

 sensation and the formation of the volition, (3) the time of 

 conducting the motor impulse and giving the signal. The 

 greater part is in the middle interval ; i.e., the central elab- 

 oration, during which the entering impression gives rise to an 

 outgoing impulse. 



In a previous diagram of reflex action, a single cell was 

 represented as receiving the afferent impulse and sending out 

 an efferent one. It is more probable that at least two cells 

 are concerned in such an act, one receiving the incoming im- 

 pulse, and influencing, by means of fine connecting branches, 

 a second cell which sends out the motor impulse, as shown in 

 Fig. 84. 



We have seen that the brain functions are more or less 



