Popular Lectures on Zoonomia / Or The Laws of Animal Life, in Health and Disease

by Thomas Garnett

In a former lecture I took notice of a liquor which is secreted by the glands of the mouth and neighbouring parts, which is called saliva. This liquor acts an important part in the production of taste; it does not differ much from water, excepting by containing a quantity of mucilage; and nothing is sapid, or capable of affecting the sense of taste, unless it is in some degree soluble in this liquor. Hence earthy substances, which are nearly insoluble, have little or no taste.

It is not, however, sufficient that the substance be possessed of solubility alone; it is necessary likewise that it should be possessed of saline properties, or, at least, of a kind of acrimony, which renders it capable of stimulating the nervous papillae. Hence it is that those substances which are less saline, and less acrid than the saliva, have no taste.

We are capable of distinguishing various kinds of taste, but some of them with less accuracy than others. Among the different kinds of taste, the following have been considered by Haller, and some other physiologists, as primitive: sweet, sour, bitter, and saline. The others have been thought to be compounded of these; for the sense of taste, as well as sight and hearing, is capable of perceiving compound impressions. To these primitive tastes, Boerhaave added alkaline, spirituous, aromatic, and some others. Of these, in different proportions, all the varieties of tastes, which are extremely numerous, are composed.

Some tastes are pleasant and agreeable, others disagreeable, and scarcely tolerable: there is, however, a great diversity in this respect experienced by different persons; for the same taste, which is highly grateful to some, is extremely unpleasant to others.

But the most pleasant tastes, agreeably to the general laws of sensation, which I described in the last lecture, become gradually less pleasant, and at last disgusting; while, on the contrary, the most disagreeable savours, such as tobacco, opium, and assafoetida, become, by custom, not only tolerable, but highly agreeable.

Nature designed this difference of tastes that we might know and distinguish such foods as are salutary; for we may in general observe, that no kind of food which is healthy, and affords proper nutriment to the body, is disagreeable to the taste; nor are any that are ill tasted proper for our nourishment. Those substances, therefore, which possess strong or disagreeable savours, and which, in general, possess a power of producing great changes on our constitution, are to be ranked as medicines, and only to be used when the constitution is deranged; whereas, in general, those which are pleasant, or mild tasted, are proper for nourishing the body. We are therefore excited or prompted to receive nourishment by the pleasant smell or taste of the food; but the avidity with which we take it depends much on the state of the stomach, and likewise on a certain inanition or emptiness; for the coarsest food is grateful to those who are hungry, and whose digestion is good; whereas, to those who have lately eaten, or whose digestive powers are impaired, the most delicate food affords little pleasure. While we are eating, the saliva flows into the mouth more copiously, which excites a more acute sensation of taste. This flow of saliva is likewise frequently excited by the smell or sight of substances agreeable to the taste, which causes an appetite, or desire of eating, similar to that caused by an accumulation of gastric juice in the stomach.

In brute animals, who have not, like ourselves, the advantage of learning from each other by instruction, the faculty of taste is much more acute, by which they are admonished to abstain from noxious or unhealthy food. This sense, for the same reason, is more acute in savages than in those who live in civiilsed society, which, whatever perfection it gives to the reasoning faculties of man, certainly diminishes the acuteness of all our senses, partly by affording fewer inducements to exercise them, and partly by our manner of living, and by the application of substances to the organs of sense, which tend to vitiate them, and render them depraved.

Taste is modified by age, temperament, habit, and disease; and in this it obeys the general laws of sensation. Children are pleased with the taste of what is sweet, and little stimulating; as we advance in years the taste of more stimulating substances becomes agreeable to us; so that we are admonished by this sense to take into the stomach the kind of nourishment fitted to each period of life. We often, however, counteract this salutary monitor by depraving our sense of taste, by the too free use of vinous or spirituous liquors, which so far deadens the sense of taste, that sweet substances become unpleasant, and nothing but acrid and stimulating things can make an impression on our diminished and vitiated sense of taste.

This sense, as well as others, is liable to be diseased. In order that the sense may be perfect, it is necessary that the membrane which envelopes the nervous papillae of the tongue, and serves as a cuticle, should neither be too thick nor too thin, too dry nor too moist. It is necessary likewise that the qualities of the saliva be natural; for alterations in the nature of this liquor affect very much the sense of taste; if it is bitter, which sometimes happens in bilious complaints, all kinds of food have a bitter taste; if it is sweet, the food has a faint and unpleasant flavour; and if it is acid, the food too tastes sour.

This sense is seldom observed to be too acute, unless from a vitiated state of the cuticle, or membrane, which covers the tongue: if this has been abraded or ulcerated, then the substances applied to the tongue are more sensibly tasted; in many instances, however, they do not produce an increased sensation of taste, but only of pain.