CHAP, i.] THE SPINAL CORD. 917 



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 in- 

 hibiting 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. But, as we have 

 already seen, impulses of an ordinary kind, passing along ordinary 

 sensory nerves, may inhibit reflex action. We have quoted in- 

 stances 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 in the frog adequately strong stimuli applied 

 to any afferent nerve will 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 of the reflex act will be found to be 

 much prolonged, 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 bulb, 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. But if it is clear that inhibition of reflex 

 action may be brought about by impulses which are not in 

 themselves of a specific inhibitory nature, 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, and after removal of the brain that the 

 exaltation of reflex actions which is manifest is due to the with- 

 drawal of such a specific inhibitory mechanism. 



The presence of the brain does obviously produce an effect., 

 which may be broadly spoken of as inhibitory, and a specific y 

 action of the brain, in an effort of the will, may stop or inhibit a 

 specific reflex action ; but we must not in these matters be led too 

 much away by the analogy of the special and limited cardiac 

 inhibitory mechanism. There we have apparently to deal with 

 fibres, whose exclusive duty it is to convey inhibitory impulses 

 from the bulb to the cardiac muscle, and inhibition of the heart, 

 at least through nervous influences, is exclusively carried out by 

 them. But already, in studying the nervous mechanism of respi- 

 ration, we have seen reason to think that afferent impulses passing 

 along the same nerves and probably along the same fibres may, 

 according to circumstances, now inhibit, now augment the respi- 

 ratory centre, and have thus been led to speak of inhibitory 

 impulses, that is impulses producing an inhibitory effect, apart 



