SEC. 5. ON THE RAPIDITY OF CEREBRAL OPERATIONS. 



We have already seen (p. 593) 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 

 mental 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,' consists of three portions: (1) the passage of 

 afferent impulses from the peripheral sensory organ to the central 

 nervous system, including the possible latent period of the gene- 

 ration of the impulses in the sensory organ ; (2) the transformation, 

 by the operations of the central nervous system, of the afferent into 

 efferent impulses ; and (3) the passage of the efferent impulses to 

 the muscles, including the latent period of the muscular contractions. 

 If the time required for the first and third of these events be 

 deducted from the whole, the ' reduced reaction period,' as it may be 

 called, gives the time taken up exclusively by the operations going 

 on in the central nervous system. 



412 



