THE NERVOUS SYSTEM. 577 



Inhibition of Reflex Actions. Movements such as are produced by 

 irritating the skin of the lower extremities in the human subject, after 

 division or disorganization of a part of the spinal cord, do not follow 

 the same irritation when the cerebrum is active and the connection 

 between the cord and the brain is intact. This is, probably, due to 

 the fact that the mind ordinarily perceives the irritation and instantly 

 inhibits or controls the action; for, even when the cord is perfect, such 

 involuntary movements may follow an irritation, applied when the cere- 

 brum is inactive. When, for example, one is anxiously thinking, even 

 slight stimuli may produce involuntary and reflex movements. So, also, 

 during sleep, such reflex movements may be observed, when the skin is 

 touched or tickled; for example, when one touches with the finger the 

 palm of the hand of a sleeping child, the finger is grasped the im- 

 pression on the skin of the palm producing a reflex movement of the 

 muscles which close the hand. But when the child is awake, no such 

 effect is produced. 



Further, many reflex actions are capable of being more or less con- 

 trolled or even altogether prevented by the will : thus an inhibitory action 

 may be exercised by the cerebrum over reflex functions of the cord 

 and the other nerve-centres. The following may be quoted as familiar 

 examples of this action : 



To prevent the reflex action of crying out when in pain, it is often 

 sufficient firmly to clench the teeth or to grasp some object and hold it 

 tight. When the feet are tickled we can, by an effort of will, prevent 

 the reflex action of jerking them up. So, too, the involuntary closing 

 of the eyes and starting, when a blow is aimed at the head, can be 

 similarly restrained. 



Darwin has mentioned an interesting example of the way in which, 

 on the other hand, such an instinctive reflex act may override the 

 strongest effort of the will. He placed his face close against the glass 

 of the cobra's cage in the Reptile House at the Zoological Gardens, and 

 though, of course, thoroughly convinced of his perfect security, could 

 not by any effort of the will prevent himself from starting back when 

 the snake struck with fury at the glass. 



It has been found by experiment that in a frog the optic lobes and 

 optic thalami have a distinct action in inhibiting or delaying reflex ac- 

 tion, and also that more generally any afferent stimulus, if sufficiently 

 strong, may inhibit or modify any reflex action even in the absence of 

 these centres. 



On the whole, therefore, it may, from these and like facts, be con- 

 cluded that reflex acts, performed under the influence of the reflecting 

 power of the spinal cord, are essentially independent of the brain and may 

 be performed perfectly when the brain is separated from the cord : that 

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