SEC. 11. ON THE TIME TAKEN UP BY CEREBRAL 

 OPERATIONS. 



691. We have already seen ( 594) 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 transmitted along the afferent 

 nerves to the central nervous system ; this may be called the 

 " afferent stage." (2) The time during which, through the opera- 

 tions 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 com- 

 paratively 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, and chiefly based upon calcu- 

 lations of the whole reaction time, are very uncertain. Hence all 



