2 ELIZABETH LOCKWOOD THOMPSON 



millimeter or two of the mouth was touched with a bit of food 

 or a clean glass rod, a chewing motion of the mouth parts began 

 which " would continue for perhaps an hour or so, even if the 

 animal were ])laced in fresh clean water." She found that the 

 same stimuli applied to other parts of the animal produced no 

 movements of the mouth parts. Familiarity with the mouth 

 movements of this snail suggested that it might be a reflex of 

 the same definite character as the salivary reflex made use of 

 by Pawlow and his pupils in their studies of association and dis- 

 crimination in the dog. 



Pawlow (1904) opened the duct of the parotid gland of a dog 

 to the outside by means of a fistula. He then measured the 

 secretion and determined its quality (viscosity), when induced, 

 first by the odor or sight of food unaccompanied by other experi- 

 mental stimuli. Pawlow called this food stimulus an " uncon- 

 ditioned " stimulus. He then measured the secretion induced 

 by food stimulation in the presence of an auxiliary (or secondary) 

 stimulus such as sound or color. This he called a " conciitioned " 

 stimulus. After using the conditioned stimuhis for a time, he 

 omitted the primary stimulus (food) and found that the secretion 

 was induced by the auxiliary stimulus, the tone or color, alone. 

 He believed that he had shown conclusive!}^ that the animal 

 had formed an association betw^een the two stimuli since at first 

 it had not reacted to the secondary stimulus, but after this 

 stimulus had been used for a time in connection with the pri- 

 mary or food stimulus it had reacted to the secondary stimulus 

 alone. Then by changing slightly the tone, color or other sec- 

 ondary stimulus used, he determined whether the animal re- 

 acted to this altered stimulus and to what extent. A change 

 in the reaction was taken as evidence that the animal discrim- 

 inated between the original and the altered stimulus. Thus 

 power to discriminate between stimuli was tested. 



Pawlow and his followers believed that the method could be 

 used on only a limited number of mammals. " Its obvious limi- 

 tation appears in the number of organisms with which it may be 

 employed. Evidently it cannot be used for the study of animals 

 which lack salivary glands, and even among those animals which 

 do possess these glands there are many which surely would not 

 lend themselves satisfactorily to the method. It seems therefore 

 as if Pawlow's method were especially important in animal psy- 



