918 INHIBITION OF REFLEX ACTIONS. [BOOK in. 



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 augmentation. 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 phenomena 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 shew 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 that in man separation of the cord 

 from the brain leads, as in animals, to heightened reflex activity. 

 In diseases, or injuries to the cord, reflex actions are, as we have 

 said, sometimes exaggerated, but it is possible and indeed probable 

 that the increase is due to the morbid processes producing a 

 greater irritability of the cord itself, and not to the withdrawal of 

 any inhibitory influences. In many cases, in perhaps the greater 

 number, no exaggeration but a diminution or even absence of 

 reflex activity is observed ; so much so that could we trust expli- 

 citly to clinical experience, we should be inclined to conclude that 

 the scantiness of spinal reflex action in man was due not to any 

 preoccupation of the cord by influences proceeding from a dominant 

 brain, but to an inherent paucity of spinal reflex mechanisms. 

 But we have already said all we have at present to say on this 

 point. 



594. The Time required for Reflex Actions. When one 

 eyelid is stimulated with a sharp electrical shock, both eyelids 

 blink. Hence, if the length of time intervening between the 

 stimulation of the right eyelid and the movement of the left 

 eyelid be measured, this will give the total time required for the 

 various processes which make up a reflex action. It has been 

 found to be from '0662 to '0578 sec. Deducting from these figures 

 the time required for the passage of afferent and efferent impulses 

 along the fifth and facial nerves to and from the bulb, and for the 

 latent period of the contraction of the orbicularis muscle, there 

 would remain '0555 to *047l sec. for the time consumed in the 



