128 INTRODUCTION TO GENERAL PHYSIOLOGY 



And in the use of the word " association " it is to be understood 

 that we imply that an actual physiological process of connecting 

 up in some way has taken place. It is clear that in such more or 

 less complex activities of the higher centres there is a temporary 

 functional union of neurones, which are not joined up in the 

 ordinary course of affairs. 



In the actual investigation of such processes, a reflex to the 

 salivary glands in the dog was chiefly made use of. Although the 

 fact has not been specially referred to, it is scarcely necessary to 

 remark that any organ supplied with nerves which set it into 

 activity may be so activated by a reflex from sense receptors. 

 We saw in our discussion of digestion that the presence of food in 

 the mouth almost invariably results in a secretion of saliva. This 

 is the more primitive, unconditioned reflex. But it was also found 

 that many kinds of external phenomena could, by appropriate 

 means, be made to result in such a secretion, through the inter- 

 vention of the higher centres, although these stimuli had previously 

 no relation to food. Such a temporary association could be formed 

 in the following way : Food actually given to a dog produces 

 secretion of saliva. Suppose that every time that the food is given, 

 a particular bell is rung. After a number of repetitions of the com- 

 bination of bell and food, a new connection has been set up 

 between the sound of this particular bell and the presence of food, 

 so that now the sound of the bell alone, which previously had no 

 effect of the kind, excites secretion of saliva. This simple form of 

 conditioned reflex allows many experiments to be made on the 

 effect of various concurrent stimuli. The important part played 

 by inhibition becomes very obvious. If during the production, or 

 education, of the reflex to the bell some other extraneous stimulus 

 intervenes, that to the bell is for a time obliterated. It can be 

 shown also how the formation of the higher response overpowers 

 the more primitive one. The application of an electrical current, 

 strong enough to excite signs of pain, to a particular spot on the 

 skin is made the signal for food, in the same way as the sound of 

 the bell in the preceding experiment. After a time its application 

 results in secretion of saliva in the absence of food, and under these 

 conditions no signs of pain are shown. Whereas, if moved to a 

 spot of skin a short distance away, the same electrical stimulus 

 causes pain but no saliva. Such an experiment as the following 

 has several points of interest. It was noted above that an 

 extraneous influence is apt to prevent the manifestation of the 

 proper conditioned reflex. So that if we have a secretion to the 

 sound of a bell, a flash of light produced at the same time inhibits 

 the reflex. Now, in the production of the reflex to the bell, 

 suppose that the food is not presented at the same time as the 



