STRUCTURE AND FUNCTIONS OF THE CEREBRUM 181 



The neurons of the nervous system generally act, in the main, 

 as conductors pure and simple. When they are stimulated nerve 

 impulses are aroused; these spread over them and escape by those 

 synapses whose resistance is not too high; thus other neurons 

 are involved and so the impulses advance to a motor termination. 



The cortical neurons of the cerebrum owe their dominant po- 

 sition in the nervous system chiefly to a peculiar ability which 

 they possess of " holding up" impulses which come to them, re- 

 taining them indefinitely, and giving them out again in the future, 

 if necessary, over and over. This storing of impulses constitutes 

 memory. The " reflex" animal, because he is deprived of this prop- 

 erty, must always respond immediately to adequate stimulation; 

 the intact animal may respond immediately 'or may retain the 

 stimulus as a memory to modify his future activities. Since the 

 intact animal has within his cerebrum a store of impulses "held in 

 leash," he may at any time become active through the liberation of 

 some of them, without immediate external stimulation. 



Association Areas. The different sensory areas and the motor 

 areas occupy only a small part of the whole cerebral cortex. Most 

 of the frontal lobes and large areas of the parietal and temporal 

 lobes are not involved in the immediate reception of impulses, 

 nor in their transmission to the Body. These areas are as richly 

 supplied with interconnecting neurons as any part of the cortex. 

 They are assumed, without very positive proof, to be the seat of a 

 function we know the cerebrum to possess, that of association. 



The Nature and Mechanism of Association. At birth the 

 brain of the infant may be compared to a clean page. It bears no 

 impressions of any sort. Such activities as the infant shows are 

 purely reflex. In course of time sense impressions begin to come 

 into the sensory areas of the cortex. These register themselves 

 more or less definitely as memories, and presently the child is in 

 possession of a considerable store of memories of various sorts. 

 He may know the sound of his mother's voice or may recognize 

 her face. As yet, however, there is no connection between these 

 independent impressions. When in the child's mind that voice is 

 associated with that face, so that he knows them as parts of a 

 single whole, he has performed an act of association. From, this 

 time throughout his life his memory is not alone of the simple 

 sound of the voice or the appearance of the face but of the mother 



