CONDITIONED REFLEX 



217 



A reflex arc is traversed when the finger touches 

 a nail. The sense organ in the skin is stimulated, 

 the stimulus is carried over the afferent or sensory 

 neuron through a synapse to a motor neuron. This 

 ends in a muscle which contracts and causes the 

 finger to be pulled away. 



are motor, which result in a number of reactions. Reactions of 

 the second level are more complex than those of the first level 

 and will involve parts of the 

 bodv somewhat distant from 

 the point of stimulation. 



When the knee jerk 

 arouses thought or delibera- 

 tion, it is classified as a third 

 level response. For example, 

 if the blow is so severe 

 that one would rub the in- 

 jured spot or examine it deliberately, certain neural connec- 

 tions would be made in the brain. Such consequent thoughtful 

 activities attend the reflex so closely that they are sometimes con- 

 sidered a part of the reflex. Such reaction is called an activity of 

 the third level. 



Conditioned reflex. A baby is born with the pathways for a 

 certain number of reflexes already established. When certain sense 

 organs are stimulated, the impulses travel along these pathways 

 until they reach muscles or other mechanisms, which carry on the 

 processes essential for maintaining life ; such as, sucking, digesting 

 of food, crying, coughing, and moving. When these responses 

 are caused by other than the original stimuli, they are said to be 

 modified or conditioned. For example, Pavlov, a Russian physiol- 

 ogist, observed that the secretion of saliva in a dog is a reflex act, 

 resulting from nerve pathways established in the dog at birth. 

 The appearance of food or the taste of food in the mouth acts as the 

 stimulus, and the flow of saliva is the response. If the same person 

 always feeds the dog, the saliva of the dog will, after a certain 

 number of times, flow at the sight of that person even though no 

 food is given. The stimulus in this case is the sight of the person, 

 not the food, and the response is the flow of the saliva. The 

 response occurs in the hungry animal even when it can neither see 



WH. FITZ. AD. BIO. — 15 



