576 THE SPINAL CORD. 



seen, impulses of an ordinary kind, passing along ordinary sensory nerves, 

 may inhibit reflex action. We have quoted instances where a slight stim- 

 ulus, 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 in- 

 hibitory result; we may add that in the frog adequately strong stimuli 

 applied to an afferent nerve will inhibit, i. e., will retard or even wholly 

 prevent, reflex action. If the toes of one foot are dipped into dilute sul- 

 phuric 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 with- 

 drawal 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 ac- 

 tions which is manifest is due to the withdrawal 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 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 respiration, 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 respiratory centre, and have thus been led to speak of in- 

 hibitory impulses, that is, impulses producing an inhibitory effect, apart 

 from specific inhibitory fibres. In the complex working of the central 

 nervous system we may still more expect to come across similar instances of 

 the same channels serving as the path either of inhibition or of augmenta- 

 tion. In all probability actions or processes which we may speak of as 

 inhibitory do play, as indeed we shall see, an important part in the whole 

 work of the central nervous system ; in all probability many of the phe- 

 nomena of nervous life are the outcome of a contest between what we may 

 call inhibitory and exciting or augmenting forces ; but in all probability, 

 also, we ought rather to seek for the explanation of how vagus impulses 

 inhibit the beat of the heart by reference to the inhibitory phenomena of 

 the central nervous system, than to attempt to explain the latter by the 

 little we know of the former. At present, however, we must be content with 

 the fact that experiments on animals show that the brain not only by some 

 action or other may inhibit particular spinal reflex movements, but also 

 habitually exercises a restraining influence on the reflex activity of the 

 whole cord, though we are unable to state clearly how this inhibition is 

 carried out. 



We say " experiments on animals," because though we know, as stated 

 above, by an appeal to our own consciousness, that an action of the brain, 

 an effort of the will, may stop a particular reflex act, we have no evidence 



