mouth "waters" when meat is offered him. But the mouth does not always 

 water: a dog that has just finished a good meal, for example, behaves dif- 

 ferently. The chemical condition of the body's juices seems to make a 

 difference. At any rate, Pavlov arranged a tube inside the dog's cheek to 

 collect the secreted saliva; and then he took the amount of saliva delivered 

 to indicate, or measure, the dog's response to a particular stimulus. Since it 

 was found that the dog's response — saliva secretion — varied with the condi- 

 tion of the animal's nutrition, the experimenters then used dogs in a 

 "hungry" state. 



Now in one series of experiments some special stimulus was combined 

 with the feeding. For example, just before the meat was presented, a bell 

 would be sounded, or a light would be flashed, or the dog's name would be 

 called. After a number of such experiences — more with some dogs than 

 with others — the animal's mouth would water as soon as the stimulus acted, 

 before he saw or smelled the meat. Later, dogs were taught to discriminate 

 between different lights or colors or sounds. For example, the note G on a 

 piano or tuning-fork was sounded every time the animal was fed. At other 

 times, a different note — say G sharp — was sounded, but unaccompanied by 

 food. After a period of training the dog would secrete saliva when he heard 

 G, but not when he heard G sharp. In these cases a new stimulus — the note 

 G, or a flash of light, or a particular color — acted as a substitute stimulus 

 for setting up saliva-secretion. 



We can see some resemblance between the modified behavior of animals 

 and the tricks which dogs and horses and other animals "learn". But Pavlov 



Pavlov started his famous researches that 

 developed the Idea of "conditioned reflex" 

 with experiments on secretions of the di- 

 gestive system. Such secretions are some- 

 times started by stimuli that are not directly 

 related to food. Your mouth waters when 

 you see food through a window, or even 

 when you read about food. How does 

 that happen? Do other animals secrete 

 juices without relation to immediate food 

 conditions? Pavlov tried to measure the 

 reflexes by measuring the amount of saliva 

 secreted by a dog under different condi- 

 tions. His work started with a dog that 

 had been wounded in the stomach, and 

 has had a great influence in furthering 

 research, and in interpreting human be- 

 havior, as well as animal behavior 



© Bacliracli 



IVAN PETROVICH PAVLOV (1849-1936) 

 267 



