4. The gums, the hard palate, and the mucous membrane of the lips are generally considered to be insensitive to taste. This absence of taste sensitiveness upon the hard palate, or roof of the mouth, is interesting when one considers its close connection with taste in popular speech. To “tickle the palate” with delicious food, to make food “palatable,” are very common, though misleading, expressions. However, two very careful investigators have reported that the lining of the whole mouth cavity, including gums, the lining of the cheeks, the hard palate, and even the teeth, are sensitive to sour stimuli. Acetic acid of full strength was used as a stimulus in these experiments.
5. The epiglottis, parts of the larynx, and the vocal cords are sensitive to taste stimuli. Not only have the taste bodies been discovered in these parts, but certain investigators, by applying solutions of bitter, sweet, etc., have been able to elicit the corresponding sensations from these parts. To these taste organs have been attributed the tastes aroused by certain vapors, such as chloroform, ether, etc.
6. The regio olfactoria, or the olfactory membrane, has been said to give rise to taste sensations, and the question has aroused much dispute. The evidence for thus attributing taste to the smell mechanism consists in the discovery in the regio olfactoria of bodies similar to taste bodies, and the further fact that breathing chloroform through the nose gives rise to a stinging sensation, followed by a sweet taste. Introspection seems to localize this taste in the nostrils. Nagel, however, denied the existence of this so-called “nasal taste” and performed a simple experiment to prove his contention. He finds that if one allows chloroform or ether vapor to be blown into his nostrils he will get a burning sensation, followed in the first case by a sweet taste and in the second by a bitter taste. But if while the vapor is entering the nostrils the person constantly utters a vowel sound, thus closing the passage between the nasal and mouth cavity, the sweet and bitter tastes will disappear, while the burning sensation remains. Nagel holds that this shows that the taste sensations must be due to the stimulation of parts other than the nasal cavity. The same conclusion has been reached by the more careful researches of Nagel’s students and others upon individuals whose nasal cavity and mouth cavity have been effectually separated either by growths or by artificial means.
One conclusion seems generally supported by the investigations of the distribution of taste, namely, that one cannot assert positively that wherever tastes have been definitely aroused there taste organs will be discoverable, or the converse of this, that wherever taste buds are found there taste sensations can always be aroused.
CHAPTER VI
Sensory Elements of the Taste Mechanism
Taste Buds and Their General Characteristics
Each sense mechanism has an arrangement more or less mechanical for modifying the stimulus, and another mechanism which transforms the stimulus into the nerve impulse, that message which is carried from sense organ to brain and there gives rise to consciousness. We know very little about the real nature of this nerve impulse transmitted from sense organ to brain, aside from its speed and a number of conditions affecting it. Some call it a chemical phenomenon, others an electrical phenomenon, and still others an electrochemical or physicochemical one. We know even less—in fact, nothing—about the change from nerve impulse to consciousness. But there are certain facts fairly well determined by microscopical examination of the sense organ about the nature of the mechanism which transforms the physical stimulus into a physiological one. In the preceding chapter the more mechanical features of the taste mechanism have been reviewed, and in this one we will describe the transforming mechanism.
About 1865 it was found that the coverings of the tongue of mammals and later the lining of the mouth cavity contained peculiar little bodies, which, on account of their apparent shape and their connection with the sense of taste, were called taste beakers. Small fissures called taste pores were discovered among the most superficial epithelial cells of the mucous membrane of the tongue, and these were thought to lead into the small, flask-like chambers. Further study showed that the beakers were not in reality hollow, but consisted of closely packed groups of modified epithelial cells. They were then given the name of taste buds, on account of their resemblance to a bud with its tightly folded petals. These have been spoken of frequently in the preceding pages as sensory ends, and the regions in which they are found were enumerated.
Each taste bud is set rather deeply in the epithelial layer of the mucous membrane covering the sides of the papillæ and communicates with the surface only by the above-mentioned taste pore. Thus the taste bud is projected from direct stimulation by all substances except those which can gain access through the narrow opening. Occasionally taste buds are found which lack this gustatory pore and communicate directly with the tongue surface. Such cases, however, are to be considered as exceptions. Likewise, twin taste buds are sometimes found, having a common base, but with peripheral ends distinct. These, too, are merely exceptions to the ordinary form and do not represent a type. The taste buds are globular in shape, measuring about 70 to 80 thousandths of a millimeter in length and about 50 thousandths of a millimeter in diameter. The taste pore averages only about 4 thousandths of a millimeter in diameter.