A beautiful illustration of this type of memory is to be found in the results of the extremely important experiments, for psychology as well as physiology, of Pawlow and his co-workers in the reflex stimulation of saliva in dogs. These experiments show the possibility of linking a physiological process to a psychological process by education, and through the conservation of the association reproducing the physiological process as an act of unconscious memory. (The experiments, of course, were undertaken for an entirely different purpose, namely, that of studying the digestive processes only.) It should be explained that it was shown that the salivary glands are selective in their reaction to stimuli in that they do not respond at all to some (pebbles, snow), but respond to others with a thin watery fluid containing mere traces of mucin or a slimy mucin-holding fluid, according as to whether the stimulating substance is one which the dog rejects, and which therefore must be washed out or diluted (sands, acids, bitter and caustic substances), or is an eatable substance and must as a food bolus be lubricated for the facilitation of its descent. Dryness of the food, too, largely determined the quantity of the saliva. Now the experiments of the St. Petersburg laboratory brought out another fact which is of particular interest for us and which is thus described by Pawlow. “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 previously 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 was 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.”[67]