THE EVOLUTION OF MIND 



For example, when you tickle or pinch the arm 

 of a person asleep, the arm is at first withdrawn 

 by simple reflex action : the ordinary channel, 

 through the afferent nerve to the spinal centre 

 and back again through the efferent nerve to the 

 limb, suffices to carry off all the molecular dis- 

 turbance, — and there is no consciousness of 

 the irritation or of the resulting contraction. 

 But if the pinching be frequently repeated, so 

 that the disturbance is generated faster than it 

 can be thus drafted off, the surplus is sent up 

 through a centripetal fibre from the spinal gan- 

 glion to the brain ; and some dreaming ensues, 

 or perhaps a fretful sound is emitted. If the 

 impression be kept up long enough, there is full 

 consciousness of it, and the person awakes. Now 

 the rise of consciousness impUed in the dream- 

 ing and waking is due to the persistence in the 

 cerebrum of a molecular disturbance which is 

 not at once drafted off through the proper cen- 

 trifugal fibres. 



Obviously, therefore, when the number of 

 impressions sent in to the brain from moment 

 to moment exceeds the number of thoroughly 

 permeable channels which have been formed 

 there, so that there is a brief period of tension 

 during which occur the nutritive changes implied 

 in the transmission of the disturbance through 

 the appropriate channels, then there arise the 

 phenomena of conscious intelligence. For mark 

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