Encyclopædia Britannica, Ninth Edition/Taste, Sense of
TASTE is the sensation referred to the mouth when certain soluble substances are brought into contact with the mucous membrane of that cavity. The 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 Λ-shaped group of large papillæ, called circumvallate papillæ, supplied by the glosso-pharyngeal nerve, and the tip and margins of the tongue, covered with filiform (touch) papillæ and fungiform papillæ, 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 Lovèn in 1867. They can be most easily demonstrated in the papillæ foliatæ, large oval prominences found on each side near the base of the tongue in the rabbit. Each papilla consists of a series of laminæ 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 papillæ (see fig. 1), in the fungiform papillæ, in the papillæ 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.
Fig. 1.—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. The figures are from Landois and Stirling’s Physiology.
The taste-bulbs are minute oval bodies, somewhat like an old-fashioned Florence flask, about 1300inch in length by 1800in breadth. Each consists of two sets of cells, — an outer set, nucleated, fusiform, bent like the staves of a barrel, 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.
Fig. 2.— 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. — isolated protective cell; e, taste-cell.
The branched ends are continuous with non-medullated nerve fibres from the gustatory nerve. These taste-bodies are found in immense numbers: as many as 1760 have been counted on one circumvallate papilla in the ox. They are absent in reptiles and birds. F. E. Schultze states that they exist in the mouth of the tadpole, whilst the tongue of the frog is covered with epithelium resembling that of the gustatory bodies. Leydig has described organs having a similar structure in the skins of fresh water fishes and the tadpole: these may possibly be widely distributed taste-organs. The proofs that these are the terminal organs of taste rest on careful observations which have shown (1) 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.
Taste is no doubt closely allied to smell; hence in invertebrates organs are found that may be referred to either of the senses (see Smell). Tastes have been variously classified. One of the most useful classifications is into sweet, bitter, acid, and saline tastes. To excite the sensation, substances must be soluble in the fluid of the mouth. Insoluble substances, when brought into contact with the tongue, give rise to feelings of touch or of temperature, but excite no taste. The specific mode of action of sapid substances is quite unknown. The extent of surface acted on increases the massiveness of the sensation, whilst the intensity is affected by the degree of concentration of the solution of the sapid substance. If solutions of various substances are gradually diluted with water until no taste is experienced, Valentine 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. Von Vintschgau found that the time required to excite taste after the sapid substance was placed on the tongue varied. Thus saline matters are tasted most rapidly (·17 second), then sweet, acid, and bitter (·258 second). This is probably due to the activity of diffusion of the substance. No relation between the chemical constitution of the substance and the nature of the taste excited by it has yet been discovered, and 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 90° F. is the most favourable to the sense, water above or below this temperature either masking or temporarily paralysing it. 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, asafœtida, 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 of an onion. In these circumstances the one cannot be distinguished from the other when the nose is firmly closed. No doubt also experience aids in detecting slight differences of taste by suggesting to the mind what may be expected; it is not easy, for instance, to distinguish the tastes of red and white wine when the eyes are blindfolded. 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 tea-tasters 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 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 ( + pole) and of alkalinity at the katode ( - pole). This is in all probability due to electrolysis, the decomposition products exciting the taste-bodies. Grünhagen found that rapidly interrupted currents fail to excite the sense; Von Vintschgau, who has directed much attention to the sense of taste, says that when the tip of his tongue is traversed by a current there is only a tactile sensation. Again Hönigschmied, on the contrary, found that a current excited the metallic or acid taste at the anode placed on the tip of the tongue, whilst the alkaline taste of the katode was absent. The writer of this article has found that this is the experience of most persons examined by him.
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.
As to the comparative anatomy of the tongue, see Owen's Comparative Anatomy and Physiology of Vertebrates (London, 1868). For a full account of the physiology of taste, see Von Vintschgau's article "Geschmackssinn," in Hermann's Handbüch der Physiologie, vol. iii. part ii. (j. g. m.)