952 CENTRAL NERVOUS SYSTEM 



sibility of determining which of the antagonistic elements in this com- 

 petition will prevail. Deviation from the " straight and narrow path'' 

 may well be due to the failure of the proper influences to gain control 

 of the internuncial mechanism of voluntary action to the exclusion of 

 all others. The paralyses of hysteria are perhaps also due to the con- 

 trol of the common paths from the brain by an inhibitory influence 

 which cannot be displaced by ordinary volitional impulses. 



The Relation of the Cerebrum to the Distance Receptors. The de- 

 velopment of the brain in the leading segments of the body is asso- 

 ciated in all animals with the acquirement of the distance receptors 

 of the head, i. e., the eye, ear, and olfactory organ. These sense organs 

 serve to acquaint the organism with parts of its environment with 

 which it has not yet come into immediate contact. They are suited 

 to bring about responses which are anticipatory of the consequences 

 of more direct contact with objects in the environment, that is, of the 

 seizure and consumption of food, the appropriation of a mate, or the 

 avoidance of objects which might prove harmful on closer contact. In 

 order that responses toward distant objects may be made with dis- 

 crimination, a mechanism has been evolved which apprehends the dis- 

 tant object not merely as a stimulus possessing a single quality, but 

 as a "thing" built up of a number of properties, and the response is 

 determined by these properties as a group. Since the properties of 

 such environmental objects appeal to a variety of receptors, the sensa- 

 tions aroused by each must be combined in the nervous system and built 

 up into a definite concept by a process of association. 



Moreover, since the response to the distant object is of an anticipatory 

 nature, it must be made with reference to the past experience which the ani- 

 mal has had with objects presenting a similar group of properties. Conse- 

 quently the results of contact with the object must be associated as part of 

 the concept with the various properties which have appealed to the distance 

 Receptors, in order that when such an object enters the environment at 

 a later time the results of the previous encounter may be recalled and 

 behavior modified accordingly. In other words distant objects appeal 

 to the nervous system by virtue of the meaning which is placed upon the 

 particular combination of receptors which they excite, and the sensations 

 which arise as a result. The nature of the response which is thus called 

 forth depends on the memory of past experience with similar objects. 

 For this reason the development of distance receptors is associated with 

 the development of the cerebrum in which association and memory mani- 

 fest themselves. The modification of behavior through the correlation 

 of present and past experience is the process of learning, which is one 

 of the chief characteristics by which responses influenced by the cere- 



