EVER since the doctrine of the specific energy of sensory nerves was presented by Müller, and since modified into the specific energy of sense organs or of cerebral centers, two sorts of stimuli for a sense organ have been spoken of. There are those called _adequate_,—for which the sense seems to be especially adapted,—and those called _inadequate_, to which the sensory mechanism is sensitive by virtue of its possession of a general irritability or sensitivity. It is a matter of common knowledge that one sees because his visual mechanism is stimulated by light, or rather by the vibrations of the luminiferous ether, and that this is the appropriate stimulus for visual sensations. But it is just as well known that if one receives a blow upon the head he will see “stars,” or if he presses his finger upon his eyeball he will see patches of light. The sensations produced in this latter fashion are due to the mechanical stimulation of the sensitive visual mechanism, which responds with its specific kind of sensation. The questions to be answered in this chapter are,—What is the kind of stimulus to which the taste mechanism is especially adapted? and, Are there other or inadequate stimuli which can produce taste sensations? In answering these questions it is well to keep in mind the biological function of the sense organ of taste. Situated as it is at the entrance of the alimentary canal it has been called the “eye of the stomach,” whose duty it is to prevent the entrance into the body, by way of the mouth, of harmful substances. If this is so, the adequate stimulus for taste would be any kind of substance which might be taken for food. _Adequate Taste Stimuli_ All substances may be classed either as _sapid_, tastable, or _insipid_, tasteless. And one of the main conditions for sapidity is solubility. A substance to be tasted must enter the mouth cavity as a fluid or else after being taken into it must be dissolved in the saliva. Thus, the adequate stimulus for the taste organ may be said to be a fluid. Recalling the structure and location of the sensory ends of the taste mechanism, it is at once inferred that only fluids can enter the taste pore and stimulate there the nerve endings of taste. It might then be assumed that all fluids should produce taste sensations. But all soluble substances are not sapid or tastable. Consequently, other conditions of sapidity have been sought, among them being chemical constitution.