338 ANNUAL OF SCIENTIFIC DISCOVERY. 



ployed. And so long as the voluntary power asserts its due predomi- 

 nance, so long can it keep in check all tendency to any other kind of 

 action save such as ministers directly to the bodily wants, as the 

 automatic movements of breathing and swallowing. The cerebrum is 

 universally admitted to be the portion of the nervous system which is 

 instrumentally concerned in the formation of ideas, the excitement of 

 the emotions, and the operations of the intellect ; and there seems no 

 reason why it should be exempted from the law of " reflex action," 

 which applies to every other part of the nervous system. And as the 

 emotions may act directly upon the muscular system through the 

 motor nerves, there is no a priori difficulty in believing that ideas may 

 become the sources of muscular movement, independantly either of 

 volitions or of emotions. Now, if the ordinary course of external 

 impressions whereby they successively produce sensations, ideas, 

 emotions, and intellectual processes, the will giving the final decision 

 upon the action to which they prompt be anywhere interrupted, the 

 impression will then exert its power in another direction, and a " reflex " 

 action will be the result. This is well seen in cases of injury to the 

 spinal cord, which disconnects its lower portion from the sensorium 

 without destroying its own power; for impressions made upon the 

 lower extremities then excite violent reflex actions, to which there 

 would have been no tendency if the current of nervous force could 



IS 



have passed upwards to the cerebrum. So, if sensations be preven- 

 ted by the state of the cerebrum from calling forth ideas through its 

 instrumentality, they may react upon the motor apparatus in a manner 

 which they would never do in its state of complete functional activity. 

 This the lecturer maintained to be the true account of the mode in 

 which the locomotive movements are maintained and guided in states 

 of profound abstraction, when the whole attention of the individual is 

 so completely concentrated upon his own train of thought, that he does 

 not perceive the objects around him, although his movements are 

 obviously guided by the impressions which they make upon his senso- 

 rium. On the same grounds, it seems reasonable to suppose that 

 when ideas do not go on to be developed into emotions, or to excite 

 intellectual operations, they, too, may act (so to speak) in the tranverse 

 direction, and may produce respondent movement, through the instru- 

 tality of the cerebrum ; and this will of course be most likely to 

 happen when the power of the will is in abeyance, as has been shown 

 to be the case in regard to the direction of the thoughts in the states of 

 electro-biology, somnambulism, and all forms of dreaming and reverie. 

 Thus the ideo-motor principle of action, as contrasted with the excito- 

 motor and sensorio-motor, finds its appropirate place in the physiolog- 

 ical scale, which would, indeed, be incomplete without it. And when 

 it is once recognized, it maybe applied to the explanation of numerous 

 phenomena which have been a source of perplexity to many who have 

 been convinced of their genuineness, and who could not see any mode 

 of reconciling them with the known laws of nervous action. The 

 phenomena in question are those which have been recently set down 

 to the action of an " od-force," such, for example, as the movements 



