152 THE WORK OF THE DIGESTIVE GLANDS. 



secreted which lubricates the food bolus and facilitates its descent 

 through the oesophagus. Further, the quantity of saliva secreted is 

 closely related to the clryness of the food. The drier this is, the more 

 saliva flows a striking proof that the first of the digestive glands 

 adopts itself to the physical conditions of the food. Of special interest 

 is the peculiar relationship between the parotid secretion and acids. 

 For these, a saliva is always secreted which is particularly rich in 

 proteids. This peculiarity is still without an explanation. Perhaps it 

 concerns some antitoxic precautionary measure. 



Since ptyalin is almost absent from the saliva of the dog, we un- 

 fortunately could make no observations upon the " adaptation " of this 

 ferment. I, therefore, proceed to another aspect of the question 

 which is in the highest degree interesting. In the course of our 

 experiments it appeared that all the phenomena of adaptation which we 

 saw in the salivary glands under physiological conditions, such, for 

 instance, as the introduction of the stimulating substances into the 

 buccal cavity, reappeared in exactly the same manner under the 

 influence of psychological conditions that is to say, when we merely 

 drew the animal's attention to the substances in question. Thus, when 

 we pretended to throw pebbles into the dog's mouth, or to cast in sand, 

 or to pour in something disagreeable, or, finally, when we offered it 

 this or that kind of food, a secretion either immediately appeared, or it 

 did not appear, in accordance with the properties of the substance 

 which we had pi'eviously seen to regulate the quantity and nature of the 

 juice when physiologically excited to flow. If we pretended to throw in 

 sand, a watery saliva escaped from the mucous glands ; if food, a slimy 

 saliva. And if the food were dry for example, dry bread a large 

 quantity of saliva flowed out, even when it excited no special interest 

 on the part of the dog. When, on the other hand, a moist food was 

 presented for example, flesh much less saliva appeared than in the 

 previous case, however eagerly the dog may have desired the food. 

 This latter effect is particularly obvious in the case of the parotid 

 gland. 



Thus, in a quite unexpected way, the physiology and psychology of 

 the salivary glands have come to be associated together ; or, even more 

 than this, the psychology has in many cases displaced the physiology. 

 For example, the psychological influence, in some cases, can be unhesitat- 

 ingly accepted where formerly only pure physiological relationships were 

 thought of ; whereas the physiological influence, as such, must now 

 be proved by special experiments designed to exclude the play of 

 psychological factors. In the psychology of the salivary glands, as it 

 has displayed itself to us, we find all the elements of what we usually 

 attribute to " mental activity " namely, sensation, choice, dispassionate 



