REFLEX ACTION. 621 



the respiratory function (asphyxia). These toxic conditions of 

 the blood bring about a peculiar excitable condition of the cells 

 or conducting fibres of thfe spinal cord, in which impulses seem to 

 pass with unwonted facility from one part to another, and give 

 rise to an excessive degree of action even in response to normal 

 stimulations. A frog poisoned with strychnia is thrown into gen- 

 eral spasm of the entire body by even the least touch, which nor- 

 mally 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 (apno3a), 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 interrupted 

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

 accepted that strong impulses arriving at the cord from any 



