THE HUMAN MECHANISM 



of the one now discharges the other. Evidently some sort 

 of nervous connection has been established between them. 

 Fig. 50 gives a diagram of the new association which has 



been established 

 between the two 

 centers. 



The connec- 

 tion thus newly 

 established be- 

 tween the affer- 

 ent neurones of 

 cold (c) and the 

 efferent neu- 

 rones to the sal- 

 ivary glands (s) 

 differs in sev- 

 eral ways from 

 the connection 

 between the af- 

 ferent and ef- 

 ferent sides of 

 an inherited 

 nervous mech- 

 anism. Such ac- 

 quired reactions 

 are not evoked 

 with the same 

 certainty as the 



inherited and, once acquired, they are more readily lost by 

 disuse. Whether we get the reaction or not depends upon 

 the condition of the body at the time we apply the stimulus. 

 Hence they are spoken of as conditioned reflexes, to distinguish 

 them from the unconditioned (or inherited) reflexes. Undoubt- 

 edly many of our involuntary actions, especially acquired 

 habits in general, are conditioned reflexes acquired since birth 



FIG. 50. The acquisition of a conditioned reflex 



A, reflex mechanism for constriction of cutaneous vessels 

 when cold is applied to the skin; B, reflex mechanism of 

 the secretion of saliva when a sapid suhstance comes in 

 contact with the tongue. Above is shown the usual normal 

 condition with no connection between the two mechanisms ; 

 below, the condition after both have been repeatedly in 

 simultaneous action 



