With respect to taste, this sense resides in a branch of the fifth cerebral nerve, called, from its function, gustatory. For the sensation of taste, moisture must be present; an utterly insoluble body, as crystal, is tasteless; we reduce to a pulp, by means of the teeth and the saliva, the food which we eat, excepting, indeed, such articles as come under the denomination of drink, or, in other words, viands in a fluid state, such as wine, ale, coffee, etc. Perfectly pure water is tasteless.
The contact of two metals in the mouth, producing a galvanic discharge, excites a sensation or flavour which cannot be well described; but the moisture of the saliva is requisite for the effect. It is said, that the superior flavour of porter drunk out of a pewter vessel results from a slight galvanic action going on. Mr. Mayo says, "various substances, after exciting the sense of touch on the fauces, and that of taste on the tongue, are capable of producing a third impression, which is popularly referred to the palate, but which is really felt upon the sentient membrane of the nostrils." In the case of metals taken into the mouth, this mode of the reception of flavour would appear to take place.
Loss of taste, like that of smell, may arise from various causes; when the mouth is parched up by fever or intense thirst, the sense of taste is greatly impaired, or even altogether suspended; but this condition may be regarded as temporary. A few instances have come under our own observation, in which, though the senses of smell and taste were not obliterated, they were extremely obtuse, insomuch that the most nauseous medicines or offensive odours created no feeling of repugnance or disgust, and the most delicate viands, or the fragrance of sweetest flowers, no pleasurable sensation. In idiots, this loss of taste and smell is far from being uncommon; but it is not to such that we here allude, nor is it to those who have suffered from repeated partial strokes of paralysis or apoplexy, but to persons of sane mind and sound health. In such cases, the deficiency is either in the sensitive and conducting power of the nerves, or in some a loss of tone in the portion of the brain to which they immediately lead--at all events, a barrier is interposed between organic structure and the mind.