It is a curious circumstance, that a moderate degree of pain, when unaccompanied by fever, often tends to render the understanding more clear, lively, and active. This is confirmed by the experience of people labouring under gout. We have an account of a man who possessed very ordinary powers of understanding, but who exhibited the strongest marks of intelligence and genius in consequence of a severe blow on the head; but that he lost these powers when he recovered from the effects of the blow. Pechlin mentions a young man, who during a complaint originating from worms, possessed an astonishing memory and lively imagination, both of which he nearly lost by being cured. Haller mentions a man who was able to see in the night, while his eyes were inflamed, but lost this power as he got well. All these facts show, that a certain action or energy is necessary for the performance of any of the functions of the body or mind; and whatever increases this action will, within certain limits, increase those functions.
Feeling is by far the most useful, extensive, and important of the senses, and may be said indeed to be the basis of them all. Vision would be of very little use to us, if it were not aided by the sense of feeling; we shall afterwards see that the same observation may be applied to the other senses. In short, it is to this sense that we are indebted, either immediately or indirectly, for by far the greatest part of our knowledge; for without it we should not be able to procure any idea with respect to the magnitude, distance, shape, heat, hardness or softness, asperity or smoothness of bodies; indeed, if we were deprived of this sense, it is difficult to say whether we should have any idea of the existence of any external bodies; on the contrary, it seems probable that we should not.
LECTURE VI. TASTE AND SMELL.
From the sense of touch we proceed naturally to that of taste, for there seems to be less difference between these two senses than between any of the others. The sense of taste appears to be seated chiefly in the tongue; for any sweet substance, such as sugar, applied to any other part of the mouth, scarcely excites the least sensation of taste. The same may be observed with respect to any other sapid body, which, unless it is strongly acrid or irritating, produces no effect on any other part than the tongue; but if it is possessed of much acrimony, it then not only affects the palate, and uvula, but even the oesophagus.
The tongue is a muscular substance, placed in the mouth, connected by one end with the adjacent bones and cartilages, while the other end remains free, and easily moveable. The tongue is furnished, particularly on its upper surface, with innumerable nervous papillae, which are much larger than those I described as belonging to the skin. These papillae are of a conical figure, and extremely sensible, forming, without doubt, the true organ of taste; other papillae are found between them, which are partly conical, and partly cylindrical.
Over the papillae of the tongue is spread a single mucous, and semipellucid covering, which adheres firmly to them, and serves the purpose of a cuticle.
Under these papillae are spread the muscles which make up the fleshy part of the tongue: these are extremely numerous, and by their means the tongue possesses the power of performing a great variety of motions with surprising velocity.
The arteries leading to the tongue are extremely numerous; and, when injected with a red fluid, the whole substance appears of a beautiful red.
The tongue is likewise furnished with a large supply of nerves, some of which undoubtedly serve to supply its muscles with nervous energy, while others terminate in the papillae, and form the proper organ of taste: this office seems to be performed by the third branch of the fifth pair of nerves. The papillae, before described, are formed or composed of a number of small nerves, arteries, and veins, firmly united together by cellular substance. These papillae are excited to action by the application of any sapid body; in consequence of which they receive a greater supply of blood, become enlarged, and vastly more sensible.
The structure of the tongue differs in different animals, which likewise possess corresponding differences with respect to taste. In those quadrupeds, in which it is armed with sharp points, the sense of taste is by no means acute. The same is the case with birds and reptiles, whose tongues are very dry and rough.