CHAP, v.] THE SPINAL CORD. 593 



The Time required for Reflex Actions. 



When one eyelid is stimulated 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 measured, this will give the total time required for 

 the various processes which make up a reflex action. It has been 

 found to be from '0662 to '0578 sec. 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 contraction of the orbicularis muscle, 

 there would remain *0555 to '0471 sec. for the time consumed in 

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

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

 of error. 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 the 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 exceedingly prolonged ; moreover the results 

 in different experiments of such a kind 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 effective 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 



F. 38 



