708 INHIBITION OF REFLEX ACTIONS. [BOOK m. 



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 con- 

 tractions of which antagonize 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 ob- 

 served 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 experiment 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 pen- 

 dent 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 withdrawal of the 

 foot. If, however, the optic lobes or optic thalami be stimu- 

 lated, as by putting a crystal of sodium chloride on them, it 

 will be found on repeating the experiment while these struct- 

 ures 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, the 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, has inhibited the reflex action of the cord. And 

 similar results may be obtained in mammals by stimulating 

 certain parts of the corpora quadrigemina, which bodies are 

 homologous to the optic lobes of frogs. From this it has been 

 inferred that there is present in this part of the brain a special 

 mechanism for inhibiting the reflex actions of the spinal cord, 

 the impulses descending from this mechanism to the various 

 centres of reflex action being of a specific inhibitory nature. 



