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What Causes the Psychic Secretion of Saliva in the Dog?

It is a well known fact that a dog, when hungry, will salivate in the presence of food. Pavlov called it a natural conditioned reflex, developed in the animal as it learns to recognize certain matter as food. But is this salivation caused by the sight or the odor of the food? To answer this question I employed a number of dogs with fistulae of the submaxillary glands. These dogs were starved for a couple of days and then placed into a stand and a collecting tube attached to them in the manner described in another communication. 1 They were allowed to remain in the stand for varying lengths of time, as a control. In 30 to 60 minutes they generally secrete one or two drops of saliva. A perfectly transparent glass museum jar of prismatic shape was partly filled with cooked bread and meat, the fare that the dogs subsisted on for a year. The flat ground edges were covered with a slab of plate glass and sealed by means of vaseline. The jar was placed in front of the animals at a distance of 2 to 3 inches from its snout. In another half hour there was no secretion of saliva. If at the end of this period the cover was taken off the jar, the animal immediately changed its indifferent attitude, became agitated, began to sniff and to stretch its neck toward the food. Saliva began to flow through the fistula and drip from the mouth at once. In 15 minutes it generally secreted from 0.5 to 2.5 cc. of saliva from the fistula alone. Since all these animals were accustomed to the appearance of the food, it is clear that the dog does not recognize food at sight, and that the natural conditioned reflex or psychic flow of saliva is due to the odor of the food only.