184 THE HUMAN BODY 



association areas to the motor areas. It must have also the power 

 to stimulate the efferent nerves. This power it exercises through 

 the function of volition. 



Volition. Although all voluntary acts result from nerve im- 

 pulses which have come from the motor areas of the cerebrum 

 by way of the pyramidal tracts, we cannot suppose that they 

 originate in the cells of the motor cortex. There is no evidence 

 that these or any cortical cells are able to originate any activities 

 whatever. All voluntary acts, as a matter of fact, are based upon 

 associative memory; the immediate stimulus to the performance 

 of the voluntary act comes, not from the motor areas, but from 

 that part of the association areas where the exciting memory is 

 stored. All memories, as we have seen, are at bottom stored sen- 

 sory impressions. What happens, then, when we perform volun- 

 tary acts is that we cause to pass on to the motor areas stimuli 

 which originally entered the nervous system by way of the re- 

 ceptors, and which have since been combined in various ways, 

 and the resulting associations stored as memories. Voluntary acts 

 are, therefore, the completion of reflexes. 



The Usefulness of Associative Memory Depends on its Order- 

 liness. It is perfectly obvious that associations to be of value 

 must be formed from related impressions or related concepts. We 

 know that our brains normally form associations in this orderly 

 way. How the brain is guided in its selection of material for mak- 

 ing associations so as to include what is relevant and exclude the 

 rest is quite beyond our knowledge or even imagination. That 

 in the highly complex associative processes which we call think- 

 ing there may be a conscious selection or rejection of memories 

 we know from our own experience. 



It is true, of course, that the brain, being an imperfect instru- 

 ment, often makes mistakes and forms associations that instead 

 of being useful give rise to harmful activities. The resulting 

 disaster, through the additional knowledge it affords, may enable 

 the brain to form correct associations next time. Thus we profit 

 by our mistakes. 



The Interaction of Associative Memories. Inhibition. The 

 human brain acquires in the course of years such a wealth of asso- 

 ciative memories, based upon so many phases of experience, that 

 the determination of the conduct to be employed in any particular 



