218 



NERVOUS REACTIONS 



V cerebrum. 



The three levels of the nervous system are shown 

 here The first level is in the spinal cord, the second 

 level is in the cerebellum, and the third level is in the 



cerebrum. 



nor smell any food. The animal has evidently acquired a new re- 

 flex path. The acquired reaction is conditioned upon a memory 



association which con- 

 nects the presence of a 

 certain person with the 

 eating of food. 



In another experiment, 

 a bell was rung, and ex- 

 actly two minutes later 

 food was given a dog. 

 Ultimately, after the ex- 

 periment had been con- 

 tinued a long time, the 

 saliva flowed in the dog's 

 mouth exactly two min- 

 utes after the ringing of 

 the bell even though no 

 food was given. This could be carefully measured because of 

 a little tube that was injected into the duct leading from the 

 salivary gland of the dog. Saliva could be seen flowing from the 

 duct exactly two minutes after the ringing of the bell. Sparrows 

 usually build nests in the gutters and holes in barns and houses. 

 Building the nest is inborn, but buildings have not existed as long 

 as nest-building, so nesting in houses must have become conditioned 

 by buildings. Conditioned -receptor 

 responses depend upon 

 associations formed in the 

 cerebrum. Injury to a 

 certain part of the cere- 

 brum of a dog with a con- 

 ditioned response has caused the ability of the animal to respond to 

 the stimulus to be entirely destroyed. 



The original reflexes that are a part of the organism at birth are 



sensory 



irve 

 orcl 



mctor neuron 



In a reflex of the first level, the center of control is 

 the spinal cord. This is the simplest type of reflex. 

 The response is usually very simple. 



