310 THE NERVOUS SYSTEM 



mals also, we find the singular power of postponing, often 

 for an indefinite time, the reaction which in the lower 

 animals immediately follows stimulation. Because it is 

 impossible to trace all nervous reaction to an incoming 

 impulse, we must not thence conclude that none has been 

 received. To the higher cerebral centers alone, that is, to 

 the gray matter of the cortex, belongs this power of de- 

 ferred action. They are able to postpone the nervous 

 discharge which results from incoming impressions until 

 those impressions have been, as we may say, considered. 

 The nature and meaning of the efferent impulse to be 

 issued in consequence of the impulse received may then 

 be decided upon after long deliberation and in view of 

 very remote sensations. 



452. The cerebrum, then, is the seat of those psychic or 

 mental processes which are called consciousness, perception, 

 volition, memory, thought, imagination, and emotion. These 

 are all terms which in the present state of knowledge 

 belong rather to psychology than to physiology. They 

 have an undeniable physical basis, but to what extent they 

 depend upon material facts and physical laws, we do not 

 know. Consciousness may be said to be the knowledge 

 which the mind has of its own operations and conditions. 

 Physiologically speaking, it has been called " the final 

 phase of sensory impressions." Perception is the recogni- 

 tion by the mind of impressions received through sensory 

 nervous tracts. Volition, so far as it is physiological, is 

 the impulse in the cerebral hemispheres which originates 

 motor activity. Memory may be called the power of stor- 

 ing up nervous impressions and making them at some 

 indefinite later period the cause of efferent influences. 

 Perhaps we need not go further in the attempt to give 

 physiological definitions to metaphysical terms. 



