SEC. 7. ON THE TIME TAKEN UP BY CEREBRAL 



OPERATIONS. 



§ 515. We have already seen (§ 467) that a considerable 

 time is taken up in a purely reflex act, such as that of winking, 

 though this is perhaps the most rapid form of reflex movement. 

 When the movement which is executed in response to a stimulus 

 involves cerebral operations a still longer time is needed : and 

 the interval between the application of the stimulus and the 

 commencement of the muscular contraction varies according to 

 the nature of the mental labour involved. 



The simplest case is that in which a person makes a signal 

 immediately that he perceives a stimulus, ex. gr. closes or opens 

 a galvanic circuit the moment that he feels an induction shock 

 applied to the skin, or sees a flash of light, or hears a sound. 

 By arrangements similar to those employed in measuring the 

 velocity of nervous impulses, the moment of the application of 

 the stimulus and the moment of the making of the signal are 

 both recorded 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 trans- 

 mitted 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 transformed 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 muscular 

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

 efferent stage the events are comparatively simple, and though 

 not absolutely constant, do not vary largely ; we are able to form 

 a fairly satisfactory estimate 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 complex and the 

 estimates of its duration, being arrived at in an indirect manner, 



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