Taste

Taste (from Lat. taxare, to touch sharply; tangere, to touch), in physiology, the sensation referred to the mouth when certain soluble substances are brought into contact with the mucous membrane of that cavity. By analogy,. the word "taste" is used also of aesthetic appreciation (see Aesthetics) and a sense of beauty - commonly with the qualifications "good taste" and "bad taste." The physiological sense is located almost entirely in the tongue. Three distinct sensations are referable to the tongue - (1) taste, (2) touch, and (3) temperature. The posterior part of its surface, where there is a A-shaped group of large papillae, called circumvallate papillae, supplied by the glosso-pharyngeal nerve, and the tip and margins of the tongue, covered with filiform (touch) papillae and fungiform papillae, are the chief localities where taste is manifested, but it also exists in the glosso-palatine arch and the lateral part of the soft palate. The middle of the tongue and the surface of the hard palate are devoid of taste. The terminal organs of taste consist of peculiar bodies named taste-bulbs or taste-goblets, discovered by Schwalbe and S. L. Lovbn in 1867. They can be most easily demonstrated in the papillae foliatae, large oval prominences found on each side near the base of the tongue in the rabbit. Each papilla consists of a series of laminae or folds, in the sides of which the taste-bodies are readily displayed in a transverse section. Taste-bodies are also found on the lateral aspects of the circumvallate papillae (see Fig. I), in the fungiform papillae, FIG. Transverse section of a circumvallate papilla: W, the papilla; v, v, the wall in section; R, R, the circular slit or fossa;. K, K, the taste-bulbs in position; N, N, the nerves.

in the papillae of the soft palate and uvula, the under surface of the epiglottis, the upper part of the posterior surface of the epiglottis, the inner sides of the arytenoid cartilages, and even in the vocal cords.

The taste-bulbs are minute oval bodies, somewhat like an old-fashioned Florence flask, about a o-s inch in length by s o a in breadth. Each consists of two sets of cells - an outer set, nucleated, fusiform, bent like the staves of a barrel, and arranged side by side so as to leave a small opening at the apex (the mouth of the barrel), called the gustatory pore; and an inner  set, five to ten in number, lying in the centre, pointed at the end next the gustatory pore, and branched at the other extremity. The branched ends are continuous with non-medullated nerve fibres from the gustatory nerve. These tastebodies are found in immense numbers: as many as 1760 have been counted on one circumvallate. papilla in the ox. The proofs that these are the terminal organs of taste rest on careful observations which have shown (r) that taste is only experienced when the sapid substance is allowed to come into contact with the taste-body, and that the sense is absent or much weakened in those areas of mucous membrane where these are deficient; (2) that they are most abundant where the sense is most acute; and (3) that section of the glosso-pharyngeal nerve which is known to be distributed to the areas of mucous membrane where taste is present is followed by degeneration of the taste-bodies. At the same time it cannot be asserted that they are absolutely essential to taste, as we can hardly suppose that those animals which have no special taste-bodies are devoid of the sense.

Evidence is accumulating that taste depends on nervous impulses excited by chemical change. Substances that have taste must be soluble. Chemical changes are in all probability set up in the taste-cells, or in the processes connected with them.. Some progress has been made in the attempt to establish a d FIG. 2. FIG. 3.

FIG. Isolated taste-bulb D, supporting or protective cells; K, under end; E, free end, open, with the projecting apices of the taste-cells.

FIG. 3. - d, Isolated protective cell; e, taste-cell.

  
connexion between the chemical composition of sapid substances and the different kinds of taste to which they may give rise. Thus acids are usually sour; alkaloids have a peculiar soapy taste; salts may be sweet, like sugar of lead, or bitter, like sulphate of magnesia; soluble alkaloids, such as quinine or strychnine, are usually bitter; and the higher alcohols are more or less sweet. Substances which taste sweet or bitter often contain definite groups in the molecule, especially in the hydroxyl (HO) and amido (NH 2) groups. By altering the chemical composition of a substance having a characteristic taste (changing the position or relations of the radicles), the substance may become tasteless or intensely bitter. The :sensation of taste may also be excited mechanically, as by smartly tapping the tongue, or by the stimulus of a continuous current. In the latter case electrolytic change may be the exciting cause; but that the sense organs may be stimulated electrically is proved by the fact that rapidly interrupted induced currents, which produce little or no electrolysis, may also excite taste. Sensations of taste are heightened by increasing the area of the tongue affected, and by mechanical stimulation, as when the tongue is pressed against the lips, cheeks or palate. A temperature of about 40 C. is most favourable, either extreme heat or cold apparently benumbing the sense for a time. Gustatory sensations affect each other: that is to say, a strong taste will affect the taste of another body taken immediately after it. Thus sweetness will modify bitterness, and sourness will modify both. Moreover, the application of a sapid substance to the tongue will affect taste in other parts. If the same taste is excited on each side of the tongue, although there are two sets of gustatory nerves, one for each lateral half, the sensations are blended into one; while if two different substances, say one sweet and the other bitter, are simultaneously applied, one to each side, the observer can distinctly differentiate the one from the other.

Tastes have been variously classified. One of the most useful classifications is into sweet, bitter, acid and saline tastes. Insoluble substances, when brought into contact with the tongue, give rise to feelings of touch or of temperature, but excite no taste. If solutions of various substances are gradually diluted with water until no taste is experienced, G. G. Valentin found that the sensations of taste disappeared in the following order - syrup, sugar, common salt, aloes, quinine, sulphuric acid; and Camerer found that the taste of quinine still continued although diluted with twenty times more water than common salt. The time required to excite taste after the sapid substance was placed on the tongue varies. Thus saline matters are tasted most rapidly (. 17 second), then sweet, acid and bitter (-258 second). There are many curious examples of substances of very different chemical constitutions having similar tastes. For example, sugar, acetate of lead and the vapour of chloroform have all a sweetish taste. A temperature of from 50 to 9Q F. is the most favourable to the sense, water above or below this temperature either masking or temporarily paralysing it.

As a general rule, bitter tastes are most acute at the back of the tongue, near the circumvallate papillae, and sweet tastes at the tip, but there are considerable individual variations. Some persons taste both bitter and sweet substances best at the back, while others taste bitter things at the tip. Many experience :salt tastes best at the tip, and acid tastes at the sides of the tongue. When we consider that there are three kinds of papillae, on the surface of the tongue, one would expect to meet with different degrees of sensitiveness to different tastes, even while we admit that the papillae may also have to do with sensations .of touch and of temperature. By experimenting with fine capillary tubes containing sapid substances, observations have been made with individual papillae. Some are found to be sensitive to many tastes, others to two or three, others to only ,one, while others are insensitive to taste altogether. Again, it has been found that a mixture of sapid substances, say of quinine and sugar, may taste sweet when applied to one papilla and bitter when applied to another. The inference must be that there are special terminal organs for different tastes. Assuming that there are different kinds of taste-cells, it might be possible to paralyse some without affecting others, and thus different sensations of taste might be discriminated. This has been done by the use of the leaves of a common Indian plant, Gymnema sylvestre. If some of these be chewed, it has been found that bitters and sweets are paralysed (neither quinine nor sugar giving rise to sensation), while acids and salines are unaffected. Again, certain strengths of decoctions of the leaves appear to paralyse sweets sooner than bitters. These observations show the existence of different taste-cells for sweets, bitters, acids and salines; and it is clear that the region of the tongue most richly supplied with taste-cells sensitive to sweets will respond best to sweet substances, while another region, supplied by taste-cells sensitive to bitters, will respond best to bitter substances. In like manner the argument may be applied to other tastes. Suppose, again, a set of taste-cells sensitive to bitter substances: it is conceivable that in whatever way these were irritated, a bitter taste would result. If so, a substance which, applied to one part of the tongue, would cause a sweet sensation, might cause a bitter if applied to a part of the tongue richly supplied with taste-cells sensitive to bitters. This may explain why sulphate of magnesia excites at the root of the tongue a bitter taste, while applied to the tip it causes a sweet or an acid taste. Saccharine, a peculiar toluene derivative, in like manner is sweet to the tip and bitter to the back of the tongue. It has also been found that if the sweet and bitter taste-cells are paralysed by Gymnema, electrical irritation of the tip by a weak interrupted current does not give rise to an acid taste mixed with sweet, as it usually does, but to sensations somewhat different, which may be described as metallic or salt or acid. This experiment indicates that the action of the interrupted current on the terminal organ is analogous to the action of sweet or bitter substances (Shore). No direct observations of importance have yet been made on single circumvallate papillae. Further experiments with capillary tubes show that fungiform papillae destitute of taste buds, and areas of the surface of the tongue having neither papillae nor taste buds, may still, when stimulated by sapid substances, give rise to tastes. Taste is often associated with smell, giving rise to a sensation of flavour, and we are frequently in the habit of confounding the one sensation with the other. Chloroform excites taste alone, whilst garlic, asafoetida and vanilla excite only smell. This is illustrated by the familiar experiment of blindfolding a person and touching the tongue successively with slices of an apple and off an onion. In these circumstances the one cannot be distinguished from the other when the nose is firmly closed. Taste may be educated to a remarkable extent; and careful observation - along with the practice of avoiding all substances having a very pronounced taste or having an irritating effect - enables teatasters and wine-tasters to detect slight differences of taste, more especially when combined with odour so as to produce flavour, which would be quite inappreciable to an ordinary palate. As to the action of electrical currents on taste, observers have arrived at uncertain results. So long ago as 1752 J. G. Sulzer stated that a constant current caused, more especially at the moments of opening and of closing the current, a sensation of acidity at the anode (-Fpole) and of alkalinity at the katode (- pole). This is in all probability due to electrolysis, the decomposition products exciting the taste-bodies. Rapidly interrupted currents fail to excite the sense.

Disease of the tongue causing unnatural dryness may interfere with taste. Substances circulating in the blood may give rise to subjective sensations of taste. Thus santonine, morphia and biliary products (as in jaundice) usually cause a bitter sensation, whilst the sufferer from diabetes is distressed by a persistent sweetish taste. The insane frequently have subjective tastes, which are real to the patient, and frequently cause much distress. In such cases, the sensation is excited by changes in the taste-centres of the brain. Increase in the sense of taste is called hypergeusia, diminution of it hypogeusia, and its entire loss ageusia. Rare cases occur where there is a subjective taste not associated with insanity nor with the circulation of any known sweetish matters in the blood, possibly caused by irritation of the gustatory nerves or by changes in the nerve centres.

For the anatomy of the organs of taste, see the articles MOUTH and TONGUE. (J. G. M.)