Next to the sense of touch, which is unevenly distributed over the whole cutaneous surface, taste is anatomically the least accurately localized. Though confined to the mouth, its more accurate limitations are not easily fixed. The point, sides and posterior sides of the dorsum of the tongue can appreciate tastes; and probably parts of the palate also have the power, but in a much less degree. Indeed, though "the palate" is often spoken of as if it were the seat of taste, it really enjoys an insignificant share of this function compared with the tongue.
The power of being stimulated by various tastes is not restricted to the terminals of any one nerve, but is shared by some of those of at least three trunks, which also transmit impulses arising from other forms of stimulation. The glossopharyngeal division of the 8th pair sends branches to the posterior part of the tongue, which are no doubt connected with the special taste organs. The lingual branches of the 5th - commonly called the gustatory nerve - have also terminals capable of being excited by taste, and probably some fibres of the chorda tympani are employed in this function.
In the furrows around the circumvallate papillae, and also, but more sparsely, on the sides of the fungiform papillae of the tongue, are found peculiar organs called "taste buds" or "taste goblets." They are imbedded in the stratified epithelium, with the cells of which their outer layers are intimately connected. They are flask-shaped bodies, composed of concentric series of modified epithelium cells arranged like the staves of a barrel, pinched together at the base and at the free surface, where they closely surround the projecting points of the central elements. These consist of nucleated bars, supposed to be the nerve terminals. The whole arrangement reminds one somewhat of the construction of the head of a ripe artichoke.
Nerves can be seen entering these bodies, and are in all probability directly connected with the modified epithelial cells of which they are made up. The relation of the glosso-pharyngeal nerves to these taste buds has been shown by the fact that in the rabbit (in which animal they are crowded together in a special organ so as to be easily found) they degenerate, and in a few months disappear, after one of these nerves has been cut.
Fig. 213. Drawing of upper surface of the tongue, showing the position of the papillae. 1 and 2. Circumvallate papillae. 3. Fungiform papillae. 4. Filiform papillae. (Sappey).
The genuine taste sensations are very few. Much of what we commonly call taste depends almost exclusively upon the smell of the substance, and we habitually confuse the impressions derived from these two senses.* The different tastes have been divided into four, viz., sweet, sour, bitter and salty, under some one or other of which headings all our tastes, properly so called, would naturally fall. Though this classification has no just claim to being a chemical one, it is interesting to know that each taste pretty well corresponds to a distinct group of substances chemically allied one to the other. Thus, acids are sour, alkaloids are bitter, the soluble neutral salts of the alkalies are salty, and polyatomic alcohols, as glycerine, grape sugar, etc., are sweet.
Fig. 214. Section through depression between two circumvallate papillae, showing taste buds.
(Cadiat) a, fibrous tissue of papilla; d and c, epithelial covering of papilla; b, taste buds. On the right, a, b, show the separate cells of a taste bud.
* Many of the comestibles, the taste of which we most prize, have really no taste, but only a smell which we habitually confound with taste, having mingled the experience obtained from the two senses. Thus, if the draft of air be carefully excluded from the nose, wine, onion, etc., may easily be proved to have no taste. Hence the familiar rule of holding the nose adopted in taking medicine with a nasty "taste".
These substances probably act on the nerve terminals as chemical stimuli, because they must be in solution to be appreciated. If solid particles be placed on the tongue they must be dissolved in the mouth fluid before they can excite the taste organs.
In order to explain the appreciation of the different tastes, we may imagine that there are different kinds of terminals, each of which is or is not influenced by various substances as they possess a special sweet, sour, bitter or salt energy. From these different terminals pass fibres bearing impulses to certain central cells, each of which is capable of exciting a sweet, sour, bitter or salty sensation, as the case may be.