STRUCTURE AND FUNCTIONS OF THE CEREBRUM 183 



suit of any increase in the number of nerve-cells, for the child 

 is born with his full number. It is, however, based upon their 

 continuous development; this development consisting chiefly of 

 greater and greater branching with correspondingly richer synaptic 

 connections. At birth scarcely any cortical cells are sufficiently 

 developed to be functional. The sensory areas first become so. 

 The association areas reach their highest point of development at 

 about the thirty-fifth year. At this age the anatomical progress of 

 the brain comes to an end; all possible paths of association have 

 been laid down. This does not mean, however, that all possible 

 associations have been formed. These continue to be formed so 

 long as the brain continues active. It is probably true, however, 

 that with advancing years there is a diminution in the freedom 

 of associative activity; the brain no longer accomplishes daring 

 feats of thought, such as constitute creative genius, but plods 

 along in the ruts established by its earlier activities. This fact 

 explains why conservative tendencies usually become more pro- 

 nounced as age advances. . 



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

 ebrum 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 responds as 

 does the "reflex" one; if, however, they indicate a different line of 

 action as more advantageous, the animal substitutes 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 



