ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 725 



of the stimulus and the moment of the making of the signal are both re- 

 corded on the same travelling surface, and the interval between them is 

 carefully measured. This interval, which has been called the " reaction 

 period " or " reaction time," may be divided into three stages : 1. The time 

 during which afferent impulses are generated in the peripheral sense organs 

 and transmitted along the afferent nerves to the central nervous system ; this 

 may be called the " afferent stage." 2. The time during which, through 

 the operations of the central nervous system, the afferent impulses are trans- 

 formed into efferent impulses ; this may be called the " central stage." 

 3. The time taken up by the passage of the efferent impulses along the 

 efferent nerves and the transformation of the nervous impulses into mus- 

 cular contractions ; this may be called the " efferent stage." In the efferent 

 stage the events are comparatively simple, and though not absolutely con- 

 stant, do not vary largely ; we are able to form a fairly satisfactorily esti- 

 mate of its duration, and so of the share in the whole reaction period which 

 may be allotted to it. The events of the afferent stage are much more com- 

 plex, and the estimates of its duration, being arrived at in an indirect 

 manner and chiefly based upon calculations of the whole reaction time, are 

 very uncertain. Hence all attempts to estimate the length of the " central " 

 stage, the " reduced reaction period," as it is sometimes called, by subtract- 

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

 good deal may be learned by studying the variations under different cir- 

 cumstances 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 variations in the strength of the stimulus, especially in the case of 

 minimal stimuli, have a much more striking effect in determining the cer- 

 tainty 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 elec- 

 tric spark, for instance, if the spark be a very weak one the subject of the 

 experiment often fails to make a 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 circumstances, 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 one-seventh, 

 for hearing one-sixth, and for sight one-fifth of a second. 



Practice materially shortens the reaction period ; indeed, after long prac- 

 tice, making the signal, at first a distinct effort of the will, takes on the cha- 

 racters of a reflex act, with a correspondingly 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 



