954 CENTRAL NERVOUS SYSTEM 



cision toward the light. The flight of the moth into a candle is deter- 

 mined in a similar way. The nervous system of the insects is singularly 

 incapable of modifying its responses as a result of past experience; it is 

 weak in the display of association and memory. Consequently the 

 moth does not learn that contact with a caudle flame is attended by dis- 

 aster and repeats its flights into the flame until it has achieved its self- 

 destruction. 



It is because of the development of the associative powers of the 

 cerebrum parallel with the perfection of the ocular mechanism that the 

 behavior of the mammals and man is adjusted to past experience with 

 greater success than is that of the insects. The human infant devel- 

 ops in the fifth month a reaction which leads it to reach for any ob- 

 ject brought sufficiently close within its field of vision. The response 

 is elicited by a lighted candle as well as by a harmless object such as 

 a piece of candy. When reaching for the candle is first developed, the 

 movement is not checked until the heat of the candle actually reaches 

 the fingers and sets up a protective flexion deflex. By repeated trials 

 the baby develops within two months a modified response to the candle, 

 the reaching reaction being completely inhibited. Reaching for candy 

 still persists without diminution. The infant has developed an asso- 

 ciation between the particular configuration of stimuli which the candle 

 produces upon the retina and the harmful consequences of reaching for 

 this object, and responds to the stimulus by a response suitable to the 

 painful effects which have previously attended its experiences with the 

 candle. 



Conditioned Reflexes 



The nature of the responses of animals with the cerebrum intact is less 

 predictable than that of the spinal animal in which the cord is removed 

 from cerebral control, because these responses are conditioned by 

 the previous experience of the animal and the associations which it has 

 formed between various stimulating objects and the consequences which 

 result from its hereditary types of response. The altered responses 

 which develop by virtue of the modifying influence which the cerebrum 

 exerts over reflex action are consequently called conditioned reflexes, 

 in contrast to the unconditioned reflexes of the spinal cord. 



It must be considered as one of the greatest advances of modern 

 physiology that Pavlov and others should have succeeded in evolving 

 methods by which we may arrive at conclusions regarding the nature of 

 certain of the integrations which occur when such conditioned reflexes 

 are formed, since they show us the elementary nature of the processes 

 by which the association of the results of sensory stimuli leads to modi- 

 fied forms of behavior. 



