CHAP. v.J THE SPINAL CORD. 591 



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 stop or inhibit 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. 



It is a remarkable fact that when the brain of a frog is re- 

 moved, reflex actions are developed to a much greater degree than 

 in the entire animal. This suggests the idea that there must be 

 in the brain some mechanism or other for preventing the normal 

 developement of the spinal reflex actions. And we learn by experi- 

 ment that stimulation of certain parts of the brain has a remark- 

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

 hemispheres only have been removed (the optic thalami, optic 

 lobes, medulla oblongata and spinal cord being left intact), be 

 suspended by the chin, 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 withdrawal 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, the stimulation of the optic lobes has 

 caused impulses to descend to the cord, which have there so inter- 

 fered with the action of the nerve-cells engaged in reflex action as 

 greatly to retard the generation of reflex impulses ; in other words, 

 the stimulation of the optic lobes 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 analogous 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 



