REFLEX AND CONDITIONED ACTION 53 



probably directly proportioned to the ease with 

 which these personally acquired connections can be 

 made. Ramon y Cajal remarks (191 1, p. 888): 



The extension, the growth, and the multiplication of the 

 appendages of the neurons, moreover, are not arrested at birth; 

 they go on after that time; and there is nothing more striking than 

 the difference between the nerve cells of the new-born and those 

 of the adult as regards the length, number and complexity of their 

 ramifications. Doubtless exercise is not without influence upon 

 these alterations, which are probably more marked in certain 

 spheres of the brain of the cultured man. Lack of exercise, on 

 the other hand, should evoke in the inactive spheres of the 

 brains of both cultured and uncultured men those phenomena 

 of resorption which have been observed during the embryonic 

 period and which here come to expression as forgetting, etc. 



The nervous mechanism of inborn reflexes, and of 

 "conditioned" or acquired modifications or recombi- 

 nations of these reflexes, is fundamentally the same. 

 The difference between the primary reflex centers and 

 the correlation centers lies in the ease with which 

 in the latter the inborn connections can be altered by 

 personal experience and in the resulting increase in 

 the flexibility and complexity of the behavior pat- 

 terns — a difference of very profound significance. 



It is clear that in the thalamus and cerebral cortex 

 are performed essential parts of^many learning pro- 

 cesses. Pavlov's conditioned reflex (or, more appro- 

 priately, conditioned response) is a typical illustra- 

 tion of the process. In dogs these learned reactions 

 generally involve the cerebral cortex as well as the 



