There has been much discussion regarding the precise seat of the sense of taste and the true nerves of taste. Although the surface of the tongue is the special seat of gus tative sensibility in man, the sense of taste is by no means restricted to that organ, being diffused, in a less degree, over the soft palate, the arches of the palate, and the fauces. Moreover, the gustative sensibility varies on different parts of the surface of the tongue. It is generally allowed that acute taste "resides at the base of the tongue, over a region of which the circumvallate papillm may be taken as the center, and also on the sides near the base. These parts are supplied solely by the glossal twigs of the glosso-pharyn Beal nerves. Some writers, among whom are Valentine and Wagner, believe the mid file and anterior parts of the dorsum of the tongue to be usually incapable of appreciat ing flavor; while numerous others hold the contrary opinion, with which our own care ful and repeated experiments, on other persons as well as ourselves, quite accord. Sour, sweet, and bitter substances applied to the sides, and especially to the tip of the pro truded tongue, we find to be at once distinguished; though, when placed on the middle of the dorsal region, they make little or no impression till pressed against the roof of the mouth. This region of the tongue is supplied almost solely by the lingual branch of the fifth nerve. We conclude generally, with regard to the tongue, that the whole dorsal sur face possesses taste, but especially the circumferential parts, viz., the base, sides, and apex." —Op. cit., pp. 442, 443. The investigations of Messrs. Todd and Bowman further show that the soft palate and its arches are endowed with taste in some persons, but not uni versally, while they got no evidence in any case of gustative sensibility on the pharynx, gums, or elsewhere. The soft palate and its arches are supplied by palatine branches from Meckel's ganglion, and sparingly by the glosso-pharyngeal nerves. From (1) the evidence afforded by the anatomical distribution of the nerves to parts enjoying the sense of taste, (2) the evidence of experiments, in which the various nerves of the tongue were divided, and (3) the evidence afforded by disease, it may be safely inferred that the glosso-pharyngeal and the lingual branches of the fifth pair of nerves respectively par ticipate in the sense of taste; and there is also reason to attribute a share to the palatine branches of the fifth.
Impressions of taste may be produced by a mechanical or chemical excitement of the gustatory nerves. A quick light tap of the finger on the tip of the tongue causes a taste, sometimes acid, sometimes saline, which lasts for several seconds; and galvanism acts. similarly. If the surface of the tongue, near the root, be touched with a clean dry glass rod, or a drop of distilled water be placed upon it, a slightly bitterish sensation is pro duced; and if the pressure be continued, a feeling of nausea ensues. If a small current. of cold air be directed against the tongue, it excites a cool saline taste like that of saltpeter. From the experiments of E. II. Weber, it appears that one of the conditions requisite for the due exercise of the sense of taste is a temperature not departing far on either side from the natural standard. Thus, if the tongue be immersed for a minute in water at a temperature of 125°, or in iced water, the taste of sugar, etc., is no longer perceived.. In order that sapid bodies should cause taste, it is necessary that they should be dis solved, and made to permeate the tissue of the papillm, so as to come in contact with. their nerves. This is proved by the two following facts: 1, that every substance, whether solid, fluid, or gaseous, which possesses a distinct taste, is more or less soluble in the fluids of the mouth, while substances which are perfectly insoluble are only recognized by the sense of touch; and 2, that if the most sapid substance be applied in a dry state to a dried part of the surface of the tongue, no sensation of taste is excited.
Bitters and acids appear to be the most sapid bodies, since they may be diluted to a greater extent than any other known substances without ceasing to excite sensations of Thus, according to Valentin, 1 part of extract of aloes, or of sulphuric acid, in 900,000 of water, and even 1 part of sulphate of quinia in 1,000,000 parts of water; may,, with ease, be distinguished from perfectly pure water. " The contact of a sapid sub stance," says Dr. Carpenter, " much more readily excites a gustative sensation when it. is made to press upon the papillm, or is moved over them. Thus there are some sub stances whose taste is not perceived when they are simply applied to the central part of the dorsum of the tongue, but of whose presence we are at once cognizant by pressing, the tongue against the roof of the mouth. The full flavor of a sapid substance, again, is more readily perceived when it is rubbed on any part of the tongue, than when it is simply brought in contact with it, or pressed against it. Even when liquids are received into the mouth, their taste is most completely discriminated by causing them to move over the gustative surface: thus, the "wine-taster" takes a small quantity of the liquor in his mouth, carries it rapidly over every part of its lining membrane, and then ejects. it."—Principles of Human Physiology, 6th ed. p. 621. Most sapid substances affect the nerves of smell to a greater or less degree, as they pass down the throat; and it is this. compound of taste and smell that constitutes flavor. It is a common habit to hold a child's nose when he is taking a nauseous draught, with the view, as is supposed, of deadening the taste. The efficacy of the process depends upon the exclusion of smell, and the reduction of the flavor of the medicine to its mere taste. The agreeable sensa tion produced by sipping good wine is due to what is termed its bouquet, or in other words, to its flavor, or combined taste and smell. Some substances leave a taste in the mouth very different from that which they first produced. This after-taste is usually bitter; but in the case of one of the most bitter substances known, namely, tannin, it is sweet. This connection seems, in a degree, to correspond to the complementary colors in vision.
There can be no doubt that the sense of taste has for its primary object to direct us in the choice of food, to make the act of eating agreeable, and to excite the flow of mucus and saliva which aid the digestive process; and among the lower animals, the instinctive perceptions connected with this taste are much more remarkable than in man, As a general rule, it is found that those substances whose taste is agreeable are useful) articles of food, and rice vend; although there are some well-known exceptional cases Sir Henry Holland, in his Medical Notes and Reflections, observes that in the majority of instances of actual illness, the desires of the patient as to food and drink may be safely complied with, even when some seeming extravagance of diet is suggested; and that in the early stage of recovery from gastric fevers, he has seen ninny curious instances of such contrariety to all rule acquiesced in with manifest good to the patient. " Diet etics," he adds, "must become a much more exact branch of knowledge, before we can be justified in opposing its maxims to the natural and repeated suggestions of the stomach, in the state either of health or disease."