CHAP, i.] THE SPINAL COED. 709 



But, as we have already seen, impulses of an ordinary kind, 

 passing along ordinary sensory nerves, may inhibit reflex 

 action. We have quoted instances where a slight stimu- 

 lus, as in the pendulous movements of the snake, and where 

 a stronger stimulus as in the case of the micturition of the 

 dog, may produce an inhibitory result ; we may add that in 

 the frog adequately strong stimuli applied to any afferent 

 nerve will inhibit, i.e. will retard or even wholly prevent 

 reflex action. If the toes of one leg are dipped into dilute 

 sulphuric acid at a time when the sciatic of the other leg is 

 being powerfully stimulated with an interrupted current the 

 period of incubation of the reflex act will be found to be much 

 prolonged, and in some cases the reflex withdrawal of the foot 

 will not take place at all. And this holds good, not only in 

 the complete absence of the optic lobes and bulb, but also 

 when only a portion of the spinal cord, sufficient to carry out 

 the reflex action in the usual way, is left. There can be no 

 question here of any specific inhibitory centres, such as have 

 been supposed to exist in the optic lobes. But if it is clear that 

 inhibition of reflex action may be brought about by impulses 

 which are not in themselves of a specific inhibitory nature, 

 we may hesitate to accept the view that a special inhibitory 

 mechanism in the sense of one giving rise to nothing but in- 

 hibitory impulses is present in the optic lobes of frogs, and 

 after removal of the brain that the exaltation of reflex actions 

 which is manifest is due to the withdrawal of such a specific 

 inhibitory mechanism. 



467. 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 bulb, and for the latent period of the contraction of the 

 orbiscularis 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 ; 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 men- 

 tioned, that the time taken up by a reflex movement varies 

 very largely. This indeed is seen in blinking itself. When 



