CHAP. IL] THE BRAIN. 1121 



attempts to estimate the length of the " central " stage, the 

 "reduced reaction period" as it is sometimes called, by subtracting 

 the efferent and afferent stages, must be subject to much error. 

 But a good deal may be learnt by studying the variations under 

 different circumstances of the reaction period as a whole. 



Taking first of all the cases in which the events of the central 

 stage are simple, such as those where the subject has merely to 

 make a signal upon feeling a sensation, we find that the length 

 of the reaction period is dependent on the intensity of the 

 stimulus, being shorter with the stronger stimulus. But varia- 

 tions in the strength of the stimulus, especially in the case of 

 minimal stimuli, have a much more striking effect in determining 

 the certainty of the reaction than in affecting the length of the 

 period. Thus when the signal is made in response to some visual 

 sensation, upon seeing an electric spark for instance, if the spark 

 be a very weak one the subject of the experiment often fails to 

 make the signal at all, though he may rarely fail if the spark be a 

 strong one. 



Some of the most marked variations in the length of the 

 reaction period are determined by the individuality of the subject. 

 Thus with the same stimulus applied under the same circum- 

 stances the reaction period of one person will be found very 

 different from that of another. 



The length of the reaction period varies also according to the 

 nature and disposition of the peripheral organs stimulated. In 

 general it may be said that cutaneous sensations produced by 

 the stimulus of an electric shock applied to the skin (the signal 

 for instance being made by the right hand when the shock is felt 

 by the left hand) are followed by a shorter reaction period than 

 are auditory sensations, while the period of these is in turn 

 shorter than that of visual sensations produced by luminous 

 objects; on the other hand, the shortest period of all is said 

 to be that of visual sensations produced by direct electrical 

 stimulation of the retina. Roughly speaking we may say that 

 the reaction period is for cutaneous sensations fth, for hearing 

 th, and for sight -^th of a second. 



Practice materially shortens the reaction period ; indeed, after 

 long practice, making the signal, at first a distinct effort of the 

 will, takes on the characters of a reflex act, with a correspond- 

 ingly shortened interval. Lastly, we may add that in the same 

 individual and with the same stimulus, the length of the period 

 will vary according to circumstances, such as the time of year, 

 the weather, and the like, as well as according to the condition of 

 the individual, whether fresh or fatigued, fasting or replete, having 

 taken more or less alcohol, and the like. 



The reaction period of vision has long been known to astrono- 

 mers. It was early found that when two observers were watching 

 the appearance of the same star, a considerable discrepancy existed 



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