CHAP, i.] THE SPINAL CORD. 919 



central operations of the reflex act. The calculations, however, 

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

 sources of error ; moreover the reflex act in question is carried out 

 by the bulb and not by the spinal cord proper. Blinking thus 

 produced is a reflex act of the very simplest kind ; but as we have 

 seen in the preceding pages, reflex acts differ very widely in nature 

 and character; and we accordingly find, as indeed we have 

 incidentally mentioned, that the time taken up by a reflex 

 movement varies very largely. This indeed is seen in blinking 

 itself. When 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 distinctly prolonged ; moreover the results in different ex- 

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

 uniform as when the blinking is caused by stimulation of the 

 eyelid. 



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 compli- 

 cated nature, and more or less distinctly involve mental operations ; 

 of the latter we shall speak later on. 



