TIME REQUIRED FOR REFLEX ACTION. 93 



skin of the back, the nerve being left attached to 

 the piece of skin and carefully protected from 

 injury, it will be found that slight stimuli applied 

 to the surface of the piece of skin easily evoke 

 reflex actions, whereas the trunk of the nerve may 

 be stimulated with even strong currents without 

 producing anything more than irregular movements. 



" In every reflex action, in fact, the central 

 mechanism may be looked upon as being thrown 

 into activity through a summation of the afferent 

 impulses reaching it." 



The change in the nerve-centre which follows 

 upon the afferent stimulus requires a certain time, 

 and this is proven as follows : 



" The Time required for Reflex Actions* When we 

 stimulate one of our eyelids with a sharp electrical 

 shock, both eyelids blink. Hence, if the length of 

 time intervening between the stimulation of the 

 right eyelid and the movement of the left eyelid be 

 carefully measured, this will give the time required 

 for the development of a reflex action. Exner 

 found this to be from '0662 to '0578 sec., being less 

 for the stronger stimulus. Deducting from these 

 figures the time required for the passage of afferent 

 and efferent impulses along the fifth and facial 

 nerves, to and from the medulla, and for the latent 

 period of the muscular contraction of the orbicularis, 

 there would remain '0555 to *047l sec. for the time 

 consumed in the central operations of the reflex 

 action. The calculations, however, necessary for 

 this reduction, it need not be said, are open to 

 * Foster, op. cit., p. 478 



