858 THE CENTRAL NERVOUS SYSTEM 



there may be a fine discrimination between those stimuli which shall 

 and those which shall not serve to call forth the conditioned reflex, an- 

 other mechanism becomes involved that of analysis. This is performed 

 by a sense organ the function of which is to separate and distinguish 

 the complicated phenomena of the outer world. For example, it has 

 been proved that small differences in the pitch of a musical note may 

 determine whether or not a conditioned reflex will be excited or in- 

 hibited, as in the case of one animal that was trained to respond by 

 the secretion of saliva to a tuning fork vibrating at 100 per second. It 

 was found that no secretion was produced by a tuning fork vibrating 

 at 104 or at 96. Much work has also been done with the skin receptors. 

 Thus, when a given spot of skin is stimulated every time that food is 

 presented, this becomes an active spot for the conditioned reflex. At 

 the same time another spot may be stimulated so as to be associated by 

 the animal with the nonpresentation of food; it is a conditioned reflex 

 for no food, and is associated with the absence of salivary secretion. 



By comparing the responses from active and inactive spots when both 

 are stimulated either simultaneously or at close intervals, much can 

 be learned concerning the delicacy of appreciation for external stimuli 

 and the influence of the inhibitory on the excitatory process. Bayliss 

 cites the following experiment. Along a series of spots on the skin 

 of the leg five devices are arranged for producing equal mechanical 

 stimulations of the skin. The four uppermost of these are made active 

 spots for the salivary reflex, and the lowest one inactive that is, when- 

 ever it is stimulated no food is presented. Let us suppose that upon 

 administering mechanical stimuli of equal intensity to each of the active 

 four spots, a certain amount of saliva is produced in a certain time; if 

 now the inactive spot is stimulated and then thirty seconds later one 

 of the uppermost spots, there will be no secretion. The previous stimu- 

 lation of the inactive spot must evidently have caused an inhibition to be 

 set up in the nerve centers concerned in the reflex. This inhibition only 

 gradually passes away, disappearing first in the spot farthest removed 

 from that made inactive, but it may take several minutes before all the 

 active spots have reacquired their original sensitivity. 



The persistence of the inhibition produced by stimulating the inac- 

 tive spot in the above experiment indicates an important factor in con- 

 nection with the production of conditioned reflexes. For example, an 

 animal can be trained to know that in a certain number of minutes after 

 the sound of a given bell food will be presented to him ; the condi- 

 tioned reflex will become established so that he salivates at exactly 

 the same time after the bell is sounded. Something must be going on 

 in the centers during this time something inhibiting the reflex. If 



