From this review of the different ways in which the taste organ may be affected it appears that the evidence is not conclusive that mechanical, thermal, or electrical stimuli may arouse taste sensations. Chemical stimulation, which is usually included among the inadequate stimuli for the other senses, is the adequate stimulus for the taste mechanism. ------------------------------------------------------------------------ CHAPTER VIII FUNCTION OF THE TASTE MECHANISM _Function of Tongue and Salivary Glands_ WHEN sapid substances are taken into the mouth as solids, liquids, or gases they either become dissolved in the saliva or mixed with it. The glandular activity, with the resulting secretion of the saliva, as described in chapter V, may begin at the sight of the objects or may not begin until the substances have come into contact with the linings of the mouth cavity or tongue. The breaking down of the solid substances and their mixture with saliva is facilitated by chewing movements and by the movements of the tongue. When the substances have been transformed into the liquid state they move toward the back part of the mouth, from which the swallowing reflex movements will carry them into the gullet and stomach. In the course of this movement the fluids will come into contact with the tip, the superior surface, and sides of the tongue, and with portions of the mucous linings of the mouth. And it is just in these regions that we find that the taste organs are located. The uneven surface of the tongue, due to the presence of the papillæ, tends to retard the movement of the fluid substances and to give them time to affect the taste organs. It will be recalled that on the tip and the superior surface of the front part of the tongue there are few taste buds found, even where the papillæ of the filiform and fungiform type are numerous, but a tremendous number of free nerve endings are found close to the surface of the epithelial covering of the tongue. They can be affected by the fluids without passing through a gustatory pore into the taste bud. Now, it happens that the latent time of the sweet sense is very short compared with that for bitter. And since it is known that the bitter sensations are aroused especially by stimulating the circumvallate papillæ, which contain real taste buds, it seems quite probable that the free nerve endings in the forward part of the tongue are real sensory ends of taste and are directly affected by the fluid stimuli. It was at one time supposed that sweet tastes could not be aroused on this part of the tongue without the aid of tongue movements. Although this is no longer believed, it is, nevertheless, likely that tongue movements which would press its surfaces against neighboring parts of the mouth cavity would bring the sapid substances into contact with the free nerve endings, and that more quickly than in the absence of any movement.