PHYSIOLOGICAL 241 



This association may become so strong that the dog's mouth will 

 water when it hears the whistle, although there is no flesh in sight. 

 The pseudo-stimulus need not be a whistle: sometimes a colour will 

 serve. The flow of saliva in the dog's mouth illustrates a "condi- 

 tioned reflex"; and this particular experiment, much elaborated by 

 Pavlov, has the advantage that the strength of the reflex reaction 

 can be measured by the amount of saliva secreted. Measurement 

 goes to the core of science. 



If a water-snail that has become accustomed to the biologist's 

 ways has its mouth touched with a piece of lettuce it makes three 

 or four munching movements. If the touch of lettuce is always 

 accompanied by the gentle pressure of a glass rod on the snail's foot, 

 an association is established; and after a hundred lessons the snail 

 usually munches — at nothing — when its foot is pressed by the glass 

 rod. It will answer back hour after hour — perhaps a hundred times — 

 to what we may call the faked stimulus. The shadow works as well 

 as the substance; and we need not smile too broadly at the snail, 

 for we are ourselves frequent victims of the same trick, responding 

 to a fictitious stimulus just as to the real one. But the snail forgets 

 in ninety-six hours, whereas our enregistration lasts tyrannously. 

 Think of our automatised reactions to the merely associative 

 stimuli connected with rank, status, wealth — but we must not 

 pursue this painful subject. 



Let us think of ourselves more prosaically. To walk past some 

 corners of Soho induces in many a salivary secretion; the savoury 

 smell has become a trigger-pulling associative stimulus. It is sound 

 physiology to put a menu outside the restaurant door; and on the 

 Continent we have seen illustrative vignettes. To some susceptible 

 people the sound of the dinner-gong at once brings about a condi- 

 tioned reflex. It does not in the case of a stranger or a young child, 

 since no individual association has been formed. The "conditioned" 

 reflex, though it is usually based on some much older "uncondi- 

 tioned" reflex, is always built up in the experience of the individual. 

 In mankind there is usually implied some appreciation of the 

 meaning of the association, and that is probably true in many cases 

 among the higher animals. A particular sound in the wood is followed 

 by life-saving reactions, partly because of an intelligent reminiscent 

 appreciation of the previous sequels of that sound. The higher wild 

 animals are not fools; but we need not be surprised by the recent 

 evidence that lions are not perturbed by the arrival of a motor-car 

 against the wind. They have had no opportunity of forming associa- 

 tions with motor-cars. Among many of the lower animals the 

 psychological aspect of the conditioned reflex is probably very dull. 

 That is to say, there is very little awareness of any meaning in the 

 secondary stimulus. In experiments, indeed, the connection between 

 the secondary stimulus and the original stimulus may be quite 



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