REFLEX ACTION. 621 



which normally would only cause it to withdraw the limb 

 slowly. 



On the other hand, there are many poisons which seem to 

 have the effect of dulling the reflex powers of the cord centres, 

 among these are opium, chloroform, chloral, digitalin, etc. And 

 the condition of the blood which may be brought about by too 

 rapid respiratory movements (apnoea), has also the effect of 

 lowering the excitability of the spinal nerve cells, and^slowing 

 respiration. 



The great majority of reflex actions may be prevented or con- 

 trolled by the will, and the masses of cells in the basal ganglia 

 and medulla seem habitually to exert a checking or inhibitory 

 action on the reflex actions of the spinal cord. It is in this way 

 that we account for the well-known facts that in a frog which has 

 not been decapitated it is impossible to induce the ordinary regu- 

 lar reflex movements, and that a human being, when asleep, 

 shows well-marked reflex action in response to a slight stimulus 

 that would be quite ineffectual when he is awake. We know, too, 

 that for some little time after pithing a frog one cannot count on 

 constant or regular results, because the act of section of the upper 

 part of the spinal cord acts as a stimulus to those channels which 

 habitually bear impulses from the brain, and, by exciting them, 

 has the same inhibitory effect. Further, it has been said that 

 artificial stimulation of the corpora quadrigemina and medulla 

 have the effect of completely checking the reflex action of the 

 cord. 



It is not only impulses coming from the higher centres that are 

 capable of inhibiting reflex activity. If, while the cord is em- 

 ployed in reflex action, in response to gentle cutaneous stimula- 

 tion, a large sensory nerve trunk be stimulated with an inter- 

 rupted electric current, the reflex action ceases. In short, it may 

 be accepted that strong impulses arriving at the cord from any 

 direction, have the effect of inhibiting the action of its reflecting 

 cells. 



The theory of reflex action lies at the bottom of all nervous 

 activities, and it is therefore useful to attempt to work out the 

 details of the mechanisms by means of which it is carried on. 



