—The conical or filiforzn, which are the most numerous of the compound papillm, are dif fused over the whole surface of the tongue, though most largely and numerously de veloped in its central part. They consist of clusters of simple papillm of a peculiarly elongated form, containing tubular nerve fibres, which may frequently be seen to form distinct loops in their interior ; but their chief peculiarity consists in their epithelial invest ment, which forms about two-thirds of their length, and gives to them their whitish tint. This investment consists of a tuft of long pointed processes, some of which present a near approach in their dense texture to hair ; whilst others may be regarded as soft or un condensed hairs. These processes are sent off from the sides and summits of the secondary papillm, and are usually inclined backwards, lying buried in the recesses of the mouth. The foregoing description applies, however, only to the conical papillm of the human tongue ; and there is a considerable diversity in their structure in other animals. Thus, in the Ruminants, each conical papilla is ter minated by a long, slender, flexible, horny filament, curving backwards ; and in the Fe lines some of them are furnished with a brush like tuft of slender horny filaments, like those of man*, whilst others are encased by firm horny sheaths, which are prolonged backwards as stiff spines. It is to this arrangement that the peculiar roughness of the tongue of the cat is due ; the organ being thus enabled to act as a flexible rasp, whereby the bones which they lick may be effectually cleaned of the smallest particles of flesh that may adhere to them ; and a single stroke of the tongue of a lion is said to be capable of abrading the whole thickness of the human skin.
After what has been already said of the conditions essential to the exercise of the sense of taste, there is no occasion to der more than point out the evident fact, that, if some of the papillm be covered with an epi thelial investment so dense as to resemble horn, and offering an effectual barrier to the penetration of fluids, these cannot be sup posed to have much participation in the sense, if they possess any. It is obvious that in the Felines the function of the spiny pa pillm must be purely mechanical ; and it seems probable that the brush-like papillm which lie among them, in common with those of man, possess simple tactile sensibility, serving to direct those muscular actions of the organ, which so remarkably adapt it to deal with minute particles of food. On the other hand, the thinness of the epithelial investment on the simple papillae which are scattered over the surface of the tongue, and which form the circumvallate and fungiform papillm by their aggregation, indicates their special adaptation to receive gustative impressions ; and this must be admitted to be more especially the case with the fungiform papilla., which often undergo a kind of erection when sapid sub stances are brought into contact with them. This inference is in complete harmony with the relative acuteness of the senses of taste and touch, in the portions of the surface on which the one or the other class of papillae is most abundant ; thus the sides and back part of the tongue are unquestionably the parts where the gustativc sensibility is the greatest, and it is there that the fungiform, circumvallate, and simple papillm are most thickly set; whilst the apex and central part of the dorsum, over which the tactile sensi bility is predominant, is that on which we meet with the largest number of filiform papillm. Whether there be any difference among the simple isolated papillae, and among those which are aggregated into the com posite bodies that are termed fungiform and circumvallate, we are scarcely in a con dition even to form a guess, until it shall have been determined whether the gustative and tactile nerve-fibres are identical or diverse in their endowments. If the former, it will be reasonable to suppose that every papilla through which a gustative impression can be made in virtue of the penetration of sapid matter into its tissue, may also be subservient to the reception of tactile impressions from mechanical stimulation. On the other hand,
if it should be proved that the gustative sense depends on a special set of fibres, we should still have to inquire whether the same papilla may not contain fibres of both classes, so as to minister to both functions ; or whether some of the papillae are purely gustative, whilst others are purely tactile.
At present there is no adequate reason to suppose that there is any essential difference of function among the papillm covered with a soft thin epithelium, whether these be solitary or aggregate. An attempt was made by Horn * to establish some such diversity ; but the results of his experiments would rather lead to the supposition that there is a dif ference in the gustative sensations excited by the same substance, according as it is applied to different regions of the tongue, than to different papillm. Thus he found that more than three-fourths of the substances which he applied to the circumvallate papillm excited a bitter taste, or one in which a bitter was associated with some other flavour, especi ally an alkaline or saline ; whilst the majority of the substances applied to the filiform papillm tasted acid, or acid with a mixture of bitter and sweet. But since, as we have seen, mere mechanical stimulation produces different gustative sensations according to the part of the tongue to which it is applied, it is probable that the difference in Horn's results is not to be set down to the account of the _ papillm but rather to that of the nerves by ;hey are respectively supplied. _Exercise of the Sense.—The simple appli ation of a sapid substance to the gustative urface is usually sufficient to excite the sen sation ; and if this application be restricted to one particular spot, we are able to recog nise its place more or less distinctly. In this respect, then, the gustative impression resem bles the tactile ; for whilst we cannot, by our own consciousness, distinguish the parts of the retina or of the auditory apparatus on which visual or auditory, impressions are made, we can make this distinction in regard to the surface which is supplied by the nerves of general sense. This determination is most precise when the impression is made on the parts of the tongue of which the gus tative sensibility is most acute ; namely, the apex, sides, and posterior part of the dorsum; being probably aided, however, near the tip, by the acuteness of its tactile sensibility. The impressibility of the middle portion of the dorsum is greatly inferior ; but still, when the gustative sensation has been excited there, it is referred to the spot on which the sapid substance was laid. The contact of sapid substances much more readily excites a gustative sensation, when it is made to press upon the papillm, or to move over them. Thus there are some substances 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 rendered cognizant by pressing the tongue against the roof of the mouth. The full flavour 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 taken into the mouth, their taste is most completely discri minated by causing them to move over the gustative surface : thus the " wine-taster" takes a small quantity of the liquor into his mouth, carries it rapidly over every part of its lining membrane, and then ejects it. It is not improbable that this exaltation of the usual effects is simply due to mechanics causes ; the sapid particles being brought by the pressure or movement into more rapid and complete operation on the nerve-fibres, than they would be if simply placed in con tact with the papillm.