THE AUDITORY REACTIONS OF THE DOG 143 



and qualitatively by a special adaptation of the Pawlow fistula 

 method. The duct of the parotid gland is exposed by an incision 

 of the cheek and a permanent fistula or outlet to the exterior is 

 made. The saliva is collected in a tube where it can easily be 

 measured. 



Through persistent training the salivary reflex may become 

 coupled with any desired stimulus which is frequently applied 

 while the secretion of saliva is called forth by feeding the animal 

 a powder consisting of meat and sugar. After long continued 

 repetition the application of the particular stimulus alone is 

 sufficient to cause a normal flow of saliva. This indirectly- 

 produced salivary reflex is what Pawlow terms a " conditioned 

 reflex," and the success of the analysis of reactions is based upon 

 the absolute specificity of the latter. The ability of the animal 

 to differentiate between gradients of various stimuli is measured 

 by the changes registered in the fundamental salivary reflex. 



What the experimenter achieves by patiently adhering to a 

 prearranged plan happens in nature continuously. The world 

 of the individual is two-fold in its make-up. Some of its 

 elements act on the animal's receptors causing sensations by 

 directly stimulating them, others exert an influence, thanks to a 

 more or less temporary association with one of the fundamental 

 or unconditioned reflexes. In the function of the higher centers 

 Pawlow distinguishes, therefore, two mechanisms; the mechanism 

 of receptors (" analysers " in Pawlow's terminology) which is 

 for sifting out and selecting from the mass of external stimuli 

 and transforming them into nervous processes of purposeful 

 reactions; the other mechanism is that of the transitory asso- 

 ciation or interlocking of the phenomena of the outside world 

 with the organism's responses. The latter is the mechanism of 

 the conditioned reflexes in all its complexity. 



One of the most important discoveries in the investigation of 

 the conditioned reflexes is the fact that every receptor at first 

 enters into temporary association with the salivary reflex by its 

 most generalized activity, its more refined and subtle faculty of 

 differentiation being involved only gradually and by a very slow 

 process. The intensity of an illuminated area becomes the 

 cause of a conditioned reflex much sooner than the shape of that 

 area. Likewise, when the central portion of the receptor mech- 

 anism — -which may be either in the brain or in the spinal cord — 



