170 THE HUMAN BODY 



this last is the running of a dog to its master upon hearing his 

 whistle. The stimulus may be a very faint one, the motions which 

 it arouses are exceedingly vigorous and complicated. 



All these differences depend at bottom upon a single funda- 

 mental difference between the two animals which is this: in the 

 " reflex" animal the immediate stimulus dominates the situation 

 completely; in the intact animal the immediate stimulus is only 

 one factor of many which together determine what the response 

 shall be. The superior practical efficiency of the intact animal 

 as an adaptive organism depends upon this power, resident in the 

 cerebrum, of modifying immediate stimuli in accordance with the 

 demands of less obvious considerations. To illustrate: a hungry 

 man perceiving food would inevitably respond to the double 

 stimulus of hunger and the sight of food by taking the food and 

 eating it if he acted upon a purely reflex basis; his actual response 

 to these stimuli .will depend, however, upon whether they are in 

 harmony with or opposed to certain more remote factors, such as 

 the question whether the food is of a sort that will agree with him, 

 or whether he is likely to need it more urgently at some future 

 time than at present. 



Before entering upon a fuller discussion of the functions of the 

 cerebrum, its structure and its connections with lower nerve- 

 centers must be described. 



The Cerebrum Dependent on the Receptor System. If the 

 cerebrum is to introduce remote considerations as factors in de- 

 termining the nature of reflex responses it must have within it the 

 knowledge upon which these remote considerations are based. 

 That the cerebrum has little original endowment of knowledge is 

 evident from study of infants, who during the first months are 

 perfect examples of "reflex" organisms. The equipment which 

 the cerebrum finally obtains must be gotten bit by bit by ex- 

 perience or the teaching of others. Since the receptor system is 

 the organism's only means of acquiring information, the cere- 

 brum must be in communication with this system if it is to learn 

 anything whatsoever. 



Afferent Paths of the Cerebrum. We have learned in previous 

 paragraphs that all sensory neurons lead directly into the central 

 nervous system and there have numerous synaptic connections 

 with association neurons. These connections are all, however, 



