Although the action of the glands is reflex in character, and is due to the stimulation, by objects in the mouth, of the glossopharyngeal and lingual nerves supplying the mouth and tongue, this is not the only means by which the reflex can be excited, or the flow of saliva produced. It is a matter of common knowledge that the _sight_ of various objects—a lemon, for instance—will cause a copious flow of saliva, and, further, that certain mental states, such as fear, anxiety, and the like, may cause reduction of the flow of saliva, with the resultant dry mouth, the cleaving of the tongue to the roof of the mouth, and other unpleasant effects of lack of saliva. These last forms of activity must be due to impulses coming to the glands or to the secretion center in the brain stem from the cortical regions of the brain, and they are called psychic reflexes or conditioned reflexes. That is, the reflexes are conditioned upon the experience of the individual somewhat as follows: If the visual experience of a lemon is followed a number of times by the taste of the lemon and the reflex excitation of the salivary glands from the presence of the acid in the mouth, it may come about that the sight of the lemon unaccompanied by its entrance into the mouth will cause the flow of saliva. The stimulus to the reflex activity has changed from one of contact of a substance with the mucous membrane of the mouth to a visual stimulus. As a result of the development of these psychic or conditioned reflexes, if one sees a certain kind of substance that he is going to taste, the flow of saliva necessary to reduce it to a tastable form is brought about even before the substance has entered the mouth. Pavlow cites a case in which a handful of clean stones placed in the mouth of a dog produces a very slight flow of saliva, while the same material in the form of fine sand causes a copious flow of saliva. Such cases as this indicate the great delicacy with which the salivary secretion is adapted to the condition of the substances entering the mouth. No sapid particles entering the mouth dry can stimulate the taste mechanism except through the mediation of the saliva. _The Tongue: Its Muscles and Covering Membranes_ Considered as a part of the taste mechanism, the tongue is a body of irregular shape, occupying a large portion of the mouth cavity. It is composed largely of muscles, covered with a mucous membrane very similar to that which lines the whole mouth cavity. It has an upper, or superior surface, a lower, or inferior surface, two sides, and a tip, in addition to the base, or region of its fixation. The muscles are of interest in that they give to the tongue its great variety of movements, and the membrane-covered surfaces are of especial interest, because in them are found by far the greatest number of the nerve endings or the sensory ends of taste.