592 REFLEX ACTIONS. [BOOK m. 



nature. But, as we have already seen, impulses of an ordinary 

 kind, passing along ordinary sensory nerves, may inhibit reflex 

 action. We have quoted instances where a slight stimulus, as in 

 the pendulous movements of the snake, and where a stronger 

 stimulus as in the case of the micturition of the dog, may produce 

 an inhibitory result ; we may add that adequately strong stimuli 

 applied to any afferent nerve will in the frog inhibit, i.e. will retard 

 or even wholly prevent reflex action. If the toes of one leg are 

 dipped into dilute sulphuric acid at a time when the sciatic of the 

 other leg is being powerfully stimulated with an interrupted 

 current, the period of incubation will be found to be much pro- 

 longed, and in some cases the reflex withdrawal of the foot will 

 not take place at all. And this holds good, not only in the 

 complete absence of the optic lobes and medulla oblongata, but 

 also when only a portion of the spinal cord, sufficient to carry out 

 the reflex action in the usual way, is left. There can be no 

 question here of any specific inhibitory centres, such as have been 

 supposed to exist in the optic lobes. 



Hence it is clear that inhibition may be brought about by 

 impulses which are not in themselves of a specific inhibitory 

 nature, and accordingly we may hesitate to accept the view that 

 a special inhibitory mechanism in the sense of one giving rise to 

 nothing but inhibitory impulses is present in the optic lobes of frogs. 

 Nor is there adequate proof that the exaltation of reflex actions 

 which is manifest in decapitated animals is due to the withdrawal 

 of such a specific inhibitory mechanism. We shall have occasion 

 again to return to these inhibitory phenomena of the central nervous 

 system. We have seen enough to shew that the spinal cord, and 

 the same holds good, as we shall see, for the whole central nervous 

 system, may be regarded as an intricate mechanism in which the 

 ciirect effects of stimulation or automatic activity are modified and 

 governed by the checks of inhibitory influences. Seeing that in the 

 ordinary actions of life the spinal cord is to a large extent a mere 

 instrument of the cerebral hemispheres, we may readily expect that 

 among the many impulses passing from the latter to the former, 

 some under certain circumstances should result in an inhibition of 

 spinal activity, while others, or the same under different circum- 

 stances, should lead to an exaltation of the same spinal activity. 

 The experiments quoted above shew that the optic lobes when 

 stimulated are especially prone to give rise to inhibitory results; 

 but we have as yet much to learn before we can speak with 

 certainty as to the exact manner in which such an inhibition is 

 brought about. 



