154 THE HUMAN BODY 



explains why conservative tendencies usually become more pro- 

 nounced as age advances. 



The Functions of Associative Memory. It is because the cere- 

 brum is able to form associative memories that the organism can 

 adjust its responses with due regard to remote as well as to im- 

 mediate considerations. Incoming stimuli, which in a "reflex" 

 animal would produce a definite response of a certain kind, are 

 in an intact animal balanced against such related associative 

 memories as the animal possesses; if these indicate that the 

 natural reflex response is the proper one to make, the animal re- 

 sponds as does the "reflex" one; if, however, they indicate a 

 different line of action as more advantageous, the animal sub- 

 stitutes for the natural reflex response a different one, suited to 

 the situation. 



Associative memory also forms the basis for the execution of 

 complex movements from feeble, immediate stimuli, or in their 

 absence; the young puppy responds to his master's whistle only 

 by a pricking of the ears ; in the older dog the sound of the whistle 

 arouses a chain of associative memories and under their impelling 

 force he executes the complex movements which carry him to his 

 master's feet. 



In order that associative memory may influence bodily activ- 

 ities it must have access to the efferent nerve-paths of the cere- 

 brum. This access it has through rich connections from the 

 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 



