The seat of the sense of taste is always at the entrance to the alimentary canal ; and its purpose is obviously to afford a means of dis crimination among the substances introduced into the mouth as food. The surface of the tongue is undoubtedly the special organ of taste in the higher animals ; but there is ade quate evidence that the sense is not entirely restricted to that organ, even in man ; and it would seem improbable, considering the ob vious purpose of the sense, that it should be wanting in that very large proportion of the animal kingdom in which no tongue exists, or in which the surface of that organ is so hard and horny as to forbid our attributing to it the possession of gustative sensibility. Without affirming (with Magendie) that the specific gustative sensibility extends over the teeth, the gums, the palate, and the pharynx, we feel justified in stating that in most persons it is distinctly present on the surface of the soft palate, especially in the neighbourhood of the uvula, and on that of the arches of the palate and of the fauces; and in a less degree on the surface of the anterior part of the soft palate. In making experiments upon this point, as well as upon many others connected with the sense of taste, it is important to bear in mind that if aromatic substances be em ployed, the impressions derived through the sense of smell may confuse the result; and also that if the sensory surface be too much exposed to cold air, its sensibility will be greatly diminished. Further, it should be borne in mind that a considerable amount of individull difference may not improbably exist, both as to the extent of the gustative surface, and the relative acuteness of the sense in different parts.
Conditions of the Sense of Taste.— In order that gustative impressions may be communi cated to the sensorium, the first requisite is an afferent nerve, endowed with the power of receiving and transmitting them. The gusta tive surface in man and the higher animals being supplied by two afferent nerves,—the glosso-pharyngeal, and the lingual branch of the fifth pair,— we shall have to inquire whether both of these arc subservient to the sense of taste, as well as to that of touch ; or whether, as in the case of the organs of smell, sight, and hearing, there is one nerve of special and another of general sense. The peripheral extremities of both these nerves are in relation with a papillary apparatus, in which they are elevated above the general surface, and come into close proximity with capillary loops ; and here, as elsewhere, it appears certain that the neighbourhood of cir culating blood is an essential condition for the reception of sensory impressions. For the gustative nerve-fibres to be impressed by the distinctive properties of sapid substances, it would further seem requisite that these sub stances should be brought into immediate relation with them, and that they should pene trate, in the state of solution, through the investments of the papilla?, into their sub stance. This would seem to be proved by the two following facts : first, that every sub stance which possesses a distinct taste is more or less soluble in the fluids of the mouth, whilst substances which are perfectly insoluble do not make their presence known in any other way than through the sense of touch ; and, second, that if the most sapid substance be applied in a dry state to the papillary sur face, and this be also dry, no sensation of taste is excited. Hence it may be inferred
that, in the reception of gustative impressions, a change is produced in the molecular condi tion of the nerve-fibres, or, to use the lan guage of Messrs. Todd and Bowman, their polarity is excited, by the direct agency of the sapid matter itself. This change may be in duced, however, both by electrical and mecha nical stimulation. If we make the tongue form part of a galvanic circuit, a peculiar sen sation is excited, which is certainly allied rather to the gustative than to the tactile, and which does not seem to be due (as at one time supposed) to the decomposition of the salts of the saliva. And, as Dr. Baly has pointed out*, "if the end of the finger be made to strike quickly, but lightly, the surface of the tongue at its tip, or its edge near the tip, so as to affect not the substance of the organ, but merely the papillae, a taste some times acid, sometimes saline, like the taste produced by electricity, will be distinctly per ceived. The sensation of taste thus induced will sometimes continue several seconds after the application of the mechanical stimulus." * Translation of Dialler's Physiology, p. 1062, note.
On the other hand, as Wagner has truly re marked, 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 produced ; and this, if the pressure be continued, passes into that of nausea, and if the pressure be increased even excites vomiting. The feeling of nausea may be excited by mechanical irritation of any part of the surface of the lances and soft palate ; and this feeling is certainly much more allied to that of taste than to that of touch. Further, it has been observed by Henle, that if a small current of air be di rected upon the tongue, it gives rise to a cool saline taste like that of saltpetre. Thus we find that the peculiar effects of sapid substances upon the nerves of taste may be imitated to a certain extent by other agencies and it also appears that the sensations excited by these vary according to the part of the gustative surface on which they operate ; me chanical or electrical stimulation of the front of the tongue giving rise to a kind of saline taste, whilst mechanical stimulation applied to the back of the tongue and fauces excites the feelings of bitterness and nausea.
One of the conditions requisite for the due exercise of the gustative sense, is a tempera ture not departing far on either side from that which is natural to the body. It appears from the recent experiments of Prof. E. H. Weber #, that if the tongue be kept immersed for nearly a minute in water of about 125°, the taste of sugar brought in contact with it, either in powder or solution, is no longer perceived ; the sense of touch, usually so delicate at the tip of the tongue, being also rendered imperfect. A`similar imperfection of taste and touch was produced by immers ing the tongue for the same length of time in a mixture of water and broken ice.
Nerves of Taste. controversy has taken place upon the question whether or not there be a special nerve of taste ; and whether the lingual branch of the fifth pair, or the glosso-pharyngeal, possesses the best claim to this title. The principal points of this con troversy have been already noticed [See