916 INHIBITION OF REFLEX ACTIONS. [BOOK in. 



micturition set up as a reflex act by simple pressure on the 

 abdomen or by sponging the anus, is at once stopped by sharply 

 pinching the skin of the leg. And it is a matter of common ex- 

 perience that in man micturition may be suddenly checked by an 

 emotion or other cerebral event. The erection centre in the 

 lumbar cord, also in large measure a reflex centre, is similarly 

 susceptible of being inhibited by impulses reaching it from various 

 sources. And indeed many similar instances of the inhibition of 

 reflex movements might readily be quoted. 



Several apparent instances of the inhibition of reflex acts are 

 not really such : in these cases all the nervous processes of the 

 act may take place in their entirety and yet fail to produce their 

 effect on account of a failure in the muscular part of the act. 

 Thus when we ourselves by an effort of the will stop the reflex 

 movements which otherwise would be produced by tickling the 

 soles of the feet, we achieve this to a large extent by throwing 

 voluntarily into action certain muscles, the contractions of which 

 antagonise the action of the muscles engaged in carrying out the 

 reflex movements. But it may be doubted even in these cases, 

 whether inhibition is always or wholly to be explained in this 

 way ; and certainly in very many instances of reflex inhibition, 

 no such muscular antagonism is present, and the reflex act is 

 checked at its nervous centre. 



When the brain of a frog is removed, and the effects of shock 

 have passed away, reflex actions are developed much more readily 

 and to a much greater degree than in the entire animal, and in 

 mammals also reflex excitability has been observed to be increased 

 by removal of the cerebral hemispheres. This suggests the idea 

 that in the intact nervous system the brain is habitually exerting 

 some influence on the spinal cord tending to prevent the normal 

 development of the spinal reflex actions. And we learn by ex- 

 periment that stimulation of certain parts of the brain has a 

 remarkable effect on reflex action. If a frog, from which the 

 cerebral hemispheres have been removed (the optic lobes, bulb 

 and spinal cord being left intact), be suspended by the jaw, and 

 the toes of the pendent legs be from time to time dipped into very 

 dilute sulphuric acid, a certain average time will be found to 

 elapse between the dipping of the toe and the resulting with- 

 drawal of the foot. If, however, the optic lobes or optic thalami 

 be stimulated, as by putting a crystal of sodium chloride on 

 them, it will be found on repeating the experiment while these 

 structures are still under the influence of the stimulation, that 

 the time intervening between the action of the acid on the toe 

 and the withdrawal of the foot is very much prolonged. That is. 

 to say, tj^e... .stimulation of the optic lobes has caused impulses 

 to descend to the cord, which have there so interfered with the 

 nervous processes engaged in carrying out reflex actions as greatly 

 to retard the generation of efferent impulses, or in other words, 



