710 INHIBITION OF REFLEX ACTIONS. [BOOK m. 



the blinking is caused not by an electric shock applied to the 

 eyelid, but by a flash of light falling on the retina, in which 

 case complex visual processes are involved, the time is dis- 

 tinctly prolonged ; moreover the results in different experi- 

 ments in which light serves as the stimulus are not nearly so 

 uniform as when the blinking is caused by stimulation of the 

 eyelid. In the " knee-jerk" the time is very short, it may be 

 not more than -03 sec.; this is one of the reasons which have 

 led some to regard the act as not truly a reflex one. 



In general it may be said that the time required for any 

 reflex act varies very considerably with the strength of the 

 stimulus employed, being less for the stronger stimuli ; this we 

 should expect, seeing that the efferent impulses of the reflex act 

 are not simply afferent impulses transmitted through the central 

 organ, but result from internal changes in the central organ 

 started by the afferent impulse or impulses ; and these internal 

 changes will naturally be more intense and more rapidly effected 

 when the afferent impulses are strong. It is stated that when 

 the movement induced is on the same side of the body as the 

 surface stimulation of which starts the act, the time taken up 

 is less than when the movement is on the other side of the body, 

 allowance being made for the length of central nervous matter 

 involved in the two cases ; that is to say the central operations 

 of a reflex act are propagated more rapidly along the cord than 

 across the cord. The rapidity of the act varies of course with 

 the condition of the spinal cord, the act being greatly prolonged 

 when the cord becomes exhausted ; and a similar delay has been 

 observed in cases of disease. The time thus occupied by purely 

 reflex actions must not be confounded with the interval required 

 when the changes taking place in the central nervous system 

 are of a more complicated nature, and more or less distinctly 

 involve mental operations ; of the latter we shall speak later 

 on. 



