THE KEFLEX ACTIONS OF THE SPINAL CORD. 577 



that in man separation of the cord from the brain leads, as in animals, to 

 heightened reflex activity. In diseases, or injuries to the cord, reflex actions 

 are, as we have said, sometimes exaggerated, but it is possible, and indeed 

 probable, that the increase is due to the morbid processes producing a 

 greater irritability of the cord itself, and not to the withdrawal of any in- 

 hibitory influences. In many cases, in perhaps the greater number, no 

 exaggeration but a diminution or even absence of reflex activity is observed ; 

 so much so that could we trust explicitly to clinical experience, we should 

 be inclined to conclude that the scantiness of spinal reflex action in man 

 was due not to any preoccupation of the cord by influences proceeding from 

 a dominant brain, but to an inherent paucity of spinal reflex mechanisms. 

 But we have already said all we have at present to say on this point. 



507. 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 move- 

 ment 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 0.0662 to 0.0578 second. 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 bulb, and for the latent period of the con- 

 traction of the orbicularis muscle, there would remain 0.0555 to 0.0471 

 second 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 ; 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 pre- 

 ceding 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 experiments in which light serves as the 

 stimulus are not nearly so uniform as when the blinking is caused by stimu- 

 lation of the eyelid. 



In general it maybe 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 nat- 

 urally 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 prop- 

 agated 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. 

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