GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 243 



afferent stimulus arriving, let us suppose- at nerve cells controll- 

 ing the movements of the leg, may fail to cause a response of the 

 corresponding muscles because of impulses meanwhile trans- 

 mitted from higher memory centers, for the animal may have 

 learned by experience that such a movement as the local stim- 

 ulus would in itself call forth, is hurtful to its own best inter- 

 ests. This experience will have become stored away as a mem- 

 ory in the higher (memory) nerve centers, so that whenever the 

 local stimulus comes to be repeated, impulses are discharged 

 from these memory centers to the local nerve center and the re- 

 flex response does not occur, or is much modified in nature. For 

 storing away these memories and for related psychological proc- 

 esses of volition, etc., the anterior portions of the nervous system 

 in the vertebrates become very highly developed so as to consti- 

 tute the brain, and the simple chain of ganglia of the inverte- 

 brates comes to be replaced by the spinal cord. 



As we ascend the scale of the vertebrates, the brain becomes 

 more and more developed, until in the higher mammalia, such 

 as man, very few reflex actions can occur independently of the 

 higher centers which are located in it. In other words, the reflex 

 arc now involves, not one nerve center, but several, and of these 

 the most important are located in the brain. 



