760 THE PHYSIOLOGY OF CONSCIOUS STATES [CH. LIT. 



in evolutional history owing to the necessity of adjustment to 

 environment. 



We may regard the physiological correlate of consciousness as 

 a state of resistance to the onward passage of the nervous impulse. 

 When the resistance is high, there is consciousness ; when it is low, 

 there is none. Thus when any new action (such as skating or 

 bicycling) is being learnt, the resistance is, as we should expect, 

 high. But the more often that act is repeated, the lower becomes 

 the resistance, until ultimately the act becomes a habit and is per- 

 formed in the complete absence of consciousness far more surely 

 and rapidly than in the earlier stages of learning. It must be borne 

 in mind, however, that this conception of lowered resistance is purely 

 hypothetical. We have no actual evidence as to which part of the 

 neuron it is that offers resistance, although we may conjecture that 

 the resistance occurs at the synapses, when the dendritic processes 

 of one neuron meet those of another. 



The hypothesis is at all events valuable in so far as it contradicts 

 an old and erroneous conception that, as an action becomes habitual 

 and no longer accompanied by consciousness, the nervous impulses 

 quit the higher parts of the brain and confine themselves to the sub- 

 cortical and spinal regions. There can be no doubt that nervous 

 impulses pursue the same course in the brain, whether at one moment 

 consciousness be present, or at another absent. 



In the spinal cord, on the other hand, there is no evidence of the 

 presence of consciousness. The acts which are executed by the 

 isolated cord are reflex. In so far as they are unaccompanied by 

 consciousness, they are comparable to habits acquired by training in 

 the higher parts of the nervous system. 



Within certain limits, reflex actions can be predicted. If we 

 apply a known stimulus to the afferent portion of a reflex system, 

 we can with fair confidence predict the result of the stimulus on the 

 efferent portions connected therewith. When, on the other hand, the 

 stimulus involves the manifestation of consciousness, prediction is 

 almost impossible ; there is so little fixity, the nervous connections 

 are so complex, and the nervous impulse may wander in such a 

 variety of directions, that one cannot forecast with certainty 

 how an individual will behave under the influence of external 

 circumstances. 



It is common to speak of the most primitive cognitive experience 

 as sensation. On the physiological side, sensation involves (1) an 

 end-organ in a sensory epithelium, adapted to receive the stimulus ; 

 (2) a sensory nerve path transmitting the nerve impulse, which 

 ultimately reaches (3) a sensory centre in the cortex of the brain. 

 But it is very doubtful whether the sensory cortical areas should be 

 regarded as the " seats " of sensation. It is quite conceivable that 



