The so-called higher senses are usually differentiated from the lower on the basis of their greater intellectual value. The sense of taste is one of the lower, as compared with vision and hearing, which are of the higher group. Not only in the purpose which they serve, but also in the character of their mechanism, do the senses differ. The sense of taste differs from the sense of sight in at least three respects. First, it is a mechanism not given over exclusively to the taste function, but serves other functions as well. The visual mechanism, for instance, is very highly specialized. It consists of an elaborate mechanical device,—extrinsic and intrinsic muscles, lens, iris, etc.,—which serves to prepare the physical stimulus to act upon the real receptor portion of the sense organ, the retina. None of these parts of the eye serve any other purpose than that of vision. In the case of the taste mechanism, on the other hand, only the receptor[5] portion of the mechanism is concerned exclusively in the taste function. The tongue, usually considered the most important part of the taste organ, with its great variety of movements, is a very necessary part of the speech mechanism. The salivary glands, which by their secretions put the substances entering the mouth into such condition that they may act upon the receptor mechanism, are of vital importance for the process of digestion. The small receptors imbedded within the coverings of the tongue and the linings of the mouth cavity may be considered as the only structures which perform exclusively a taste function.
5. A receptor is the part of the sense organ in which the transformation from a physical stimulus to a nerve impulse occurs.
Secondly, it is not easy to determine the limits of distribution of the receptor organs of taste, as one can do in the case of the eye, with its clearly defined retina, or the ear, with its organ of Corti, or even the nose, with its regio olfactoria (patch of mucous membrane). That is, there is no one organ with its attachments which can be called the taste organ. Consequently, one does not find any general agreement as to what structures ought to be included in the taste mechanism. For instance, it is well known that the taste function is not limited to the tongue, and that the whole mouth cavity is a more or less important part of the taste mechanism. Certain investigators are not willing to circumscribe the taste organ within such narrow limits, but extend it to the larynx, the vocal cords, and some have even gone so far as to include the membranes of the nasal cavity.
Finally, the taste mechanism is extremely simple when compared with the so-called higher senses, especially that portion of it whose function is to prepare the stimuli to act upon the receiving mechanism. The taste organ is generally believed to represent a stage in evolution very near to the original structures from which all of the sense organs have developed. That is, looked at from the evolutionary point of view, it is a more primitive and less highly developed mechanism.
Consider first those portions of the taste organ which perform the mechanical function of preparing the stimulus to act upon the receiving mechanism or the sensory ends of taste. Most important of these are the salivary glands and the tongue. The latter through its movements facilitates contact of the taste substances with the sensory ends of taste, and the former secrete saliva, which dissolves the sapid substances, reducing them to the liquid form necessary for arousing taste sensations.
The salivary glands and their secretions are of interest to the student of taste only in so far as their activity forms a necessary step in the taste process. In this connection it must be remembered that the chief function of the saliva is the part it plays in the process of digestion. As suggested above, the function of the saliva in the taste process is merely that of dissolving or transforming solids into liquid form. Consequently, the chemical constitution of the saliva and the detailed structure of the glands are aside from our interest. The action of these glands, three in number, and called the parotid, the submaxillary, and the sublingual, is of the reflex type. The stimuli for the reflex are numerous, and, it appears, are not the same for all of the glands. For instance, the submaxillary glands are said to secrete upon the presence of certain foods, acids in the mouth, the chewing of meats, etc. On the other hand, the stimulus for the parotid gland has been shown by Pavlow to be the presence of dry substances in the mouth. If no distinction is made between glands, it may be said that watery foods cause only a slight flow of saliva, while dry foods cause a large flow of saliva. One sees, then, a set of reflexes which not only promote the digestion of foods, but which also tend to make them tastable. These two functions are not entirely separate ones, for it has been shown by experiment that the tastes of various foods are determining factors in causing and regulating the flow of gastric juice in the stomach.
Although the action of the glands is reflex in character, and is due to the stimulation, by objects in the mouth, of the glossopharyngeal and lingual nerves supplying the mouth and tongue, this is not the only means by which the reflex can be excited, or the flow of saliva produced. It is a matter of common knowledge that the sight of various objects—a lemon, for instance—will cause a copious flow of saliva, and, further, that certain mental states, such as fear, anxiety, and the like, may cause reduction of the flow of saliva, with the resultant dry mouth, the cleaving of the tongue to the roof of the mouth, and other unpleasant effects of lack of saliva. These last forms of activity must be due to impulses coming to the glands or to the secretion center in the brain stem from the cortical regions of the brain, and they are called psychic reflexes or conditioned reflexes. That is, the reflexes are conditioned upon the experience of the individual somewhat as follows: If the visual experience of a lemon is followed a number of times by the taste of the lemon and the reflex excitation of the salivary glands from the presence of the acid in the mouth, it may come about that the sight of the lemon unaccompanied by its entrance into the mouth will cause the flow of saliva. The stimulus to the reflex activity has changed from one of contact of a substance with the mucous membrane of the mouth to a visual stimulus. As a result of the development of these psychic or conditioned reflexes, if one sees a certain kind of substance that he is going to taste, the flow of saliva necessary to reduce it to a tastable form is brought about even before the substance has entered the mouth. Pavlow cites a case in which a handful of clean stones placed in the mouth of a dog produces a very slight flow of saliva, while the same material in the form of fine sand causes a copious flow of saliva. Such cases as this indicate the great delicacy with which the salivary secretion is adapted to the condition of the substances entering the mouth. No sapid particles entering the mouth dry can stimulate the taste mechanism except through the mediation of the saliva.
Considered as a part of the taste mechanism, the tongue is a body of irregular shape, occupying a large portion of the mouth cavity. It is composed largely of muscles, covered with a mucous membrane very similar to that which lines the whole mouth cavity. It has an upper, or superior surface, a lower, or inferior surface, two sides, and a tip, in addition to the base, or region of its fixation. The muscles are of interest in that they give to the tongue its great variety of movements, and the membrane-covered surfaces are of especial interest, because in them are found by far the greatest number of the nerve endings or the sensory ends of taste.
The tongue is made up of seventeen muscles, acting in three planes—vertical, longitudinal, and transverse. Separating these muscles one from another are layers of fatty tissue, enabling the muscles to glide easily over each other. Fifteen of these muscles are extrinsic in the sense that one end of each has its point of attachment outside of the tongue. It is these muscles especially that give the tongue its great motility. By the contraction of single muscles or contraction in various combinations the tongue is protruded or drawn back, the tip raised or lowered, the dorsal surface pressed against the roof of the mouth or withdrawn toward the floor of the mouth, the tongue protruded and turned to one side or the other. In addition to these extrinsic muscles there is a pair of intrinsic muscles, each having both points of attachment within the tongue. By their contraction the sides of the tongue are raised and drawn together.
These muscles are richly supplied with blood vessels and receive an especially large supply of nerve fibers from the hypoglossal, which is known as the motor nerve of the tongue, and some from the lingual branch of the seventh, or facial, nerve.
It is generally believed that the tongue movements serve the sense of taste only as they facilitate contact of the sapid substances with the real taste endings; for instance, by pressure of the tongue against the roof of the mouth, and by its protrusion from the mouth to receive the stimuli upon its surface. However, it has been asserted by some investigators that the movements are of more direct use in the taste process, in that tongue movements in themselves tend to increase the sensitivity of the taste mechanism. But careful experiments in which the tongue was rendered motionless during tasting show that taste sensitivity is just as great as when the tongue is free to move.
By far the most important portion of the tongue is its mucous covering. This varies considerably in character in different regions, being thickest and toughest on the superior surface where It comes into contact with objects taken into the mouth, and thinnest on the inferior surface where it is ordinarily protected from such contact. On the sides and tip it is moderately thick and tough. The tongue covering has a highly complex structure. Two distinct layers, or strata, are commonly described: the more superficial, or epithelial layer, and the deeper layer, called the chorion. The first, or epithelial, layer contains all of the sensory endings concerned in taste upon the tongue, and these will be described later. The second is more complex and consists of connective tissue, a great network of blood vessels, nerve fibers, and numerous glands and their ducts which open upon the surface of the tongue. Upon the more superficial surface of this inner layer there is an extremely large number of slight elevations. These are apparent to the unaided eye upon the tongue surface, since the epithelial layer of the membrane follows very closely the contour of this deeper layer. These elevations are called papillæ. (Fig. 2.) They vary in size and shape and are quite unevenly distributed upon the surface of the tongue. On the inferior surface there are none, while on the superior surface they are most numerous. With the aid of the papillæ on this surface the tongue can be divided into two parts, an anterior or horizontal portion and a posterior or vertical portion. The former includes about the forward two-thirds of the tongue and the latter the posterior third. These two parts are separated by a row of relatively large elevations, about ten or twelve in number, and arranged in the shape of a V, with the open portion of the V turned forward. From the apex of the V a furrow passes forward to the tip of the tongue, dividing the anterior portion laterally into two halves. The posterior portion is broken up into a series of folds taking about the same direction as the legs of the V.
Fig. 2.
The papillæ of the tongue have been described and classified more or less in detail since the middle of the seventeenth century. They can be grouped into four classes (Fig. 2.), the circumvallate, the fungiform, the filiform, and the foliate papillæ, each group having certain distinctive characters. A fifth group is sometimes added and comprises the hemispherical, or simple, papillæ.
The circumvallate group comprise the largest and the most important papillæ. There are only from eight to twelve or sixteen of this type, and by their arrangement they form the V-shaped figure on the superior surface of the tongue, mentioned above, and which divides the anterior two-thirds from the posterior third of the tongue. These papillæ are found nowhere else. They rise only slightly above the surface of the tongue from the bottom of a pit or cup-shaped depression, giving the impression of a small mound surrounded by a ditch, whence the name, circumvallate. In a few cases more than one papilla rises from a single pit, but in most cases there is but one. The papilla itself has an average height of 2 millimeters, with a diameter at the top of 1.0 to 1.5 millimeters, and slightly less at the base. The cup-shaped depression averages about 1 to 1.5 millimeters in depth. The largest of these papillæ is that one forming the apex of the V-shaped figure and is called the foramen cæcum. The smallest are those found at the ends of the V.
The fungiform papillæ are so called on account of their resemblance to a toadstool. Each consists of a rather slender stalk capped by a relatively large, rounded head, about .8 to 1.0 millimeter in diameter. The whole papilla has an average height ranging from 1.0 to 1.5 millimeters. These are scattered very irregularly over the superior surface of the tongue, most of them being found on its anterior two-thirds. However, a few are found back of the circumvallate papillæ, but always very close to them. They are most numerous on the sides and the tip of the tongue, where they appear as bright red points upon the paler background of the tongue covering. The total number of this type has been estimated at from 150 to 200.
The third type, the filiform papillæ, cover the whole superior surface of the tongue and are so numerous that no estimation of their total number has been made. They are arranged in fairly regular lines running to each side from the middle line of the tongue and parallel to the lines formed by the circumvallate papillæ. They are believed to contain no sensory ends of taste and have only a mechanical function, if any, in connection with the taste mechanism. They are conical or cylindrical in form and vary in height from 1.5 to 2.5 millimeters. Sometimes these papillæ are covered with tiny secondary papillæ, which, however, are not apparent to the unaided eye.
The foliate papillæ consist of numerous small folds of the membranous covering of the tongue upon its sides and just in front of the line formed by the circumvallate papillæ. In these folds a large number of the taste bodies are imbedded. In certain animals, especially the rodents, these foliate papillæ form a very prominent part of the tongue, while in man they are no more prominent than the other forms.
The hemispherical, or simple, papillæ are found distributed over the whole tongue surface and form in many cases secondary papillæ upon the larger fungiform and circumvallate type.
The papillæ of these different types contain the largest number of the sensory ends of taste, or the taste bodies. These structures will be discussed in detail later. They are most numerous in the circumvallate papillæ, where one may find hundreds imbedded in the side walls of one papilla. They are also found in the side walls of the depression from which the papilla rises. The fungiform type also contain these taste bodies imbedded in their side walls, although there are some in which none have been discovered. According to Nagel, the taste bodies have never been found in the filiform type. He ventures the opinion that the taste bodies are not necessary to produce taste sensations, with the result that the filiform papillæ may, after all, have something to do with taste sensations. There is always the possibility, first suggested about 1870, that the free nerve endings in the tongue surface may give rise to taste sensations.
How shall the limits of the taste mechanism be determined? Two methods have been employed, and usually in conjunction. The first, or the anatomical method, consists in searching for the taste bodies (to be described later), and when they are found, to assume the possibility of taste sensations from the stimulation of that region. The second, or the physiological method, consists in applying stimuli of various sorts to different regions and finding whether taste sensations result. The limits of the taste mechanism, when determined by these two methods taken separately, do not always agree. But there are cases in which only one or the other method taken alone can be applied. For instance, in the case of the embryo it is clear that only the anatomical method can be used, and in the case of the living human being only the physiological, or stimulation, method can be used.
The discrepancy between structure and function is possibly due, in part, to the presence of functionless taste bodies, useless landmarks in regions where in earlier stages of evolution the taste function may have been of vital importance. Such an explanation has been offered for the presence of taste buds upon the upper surface of the soft palate and upon the walls of the larynx and the vocal cords. But there are at least two other causes for the differences in limits of the taste sense as determined by the different experimenters,—careless and imperfect technic of experimentation and the great individual differences, according to age, race, and other conditions. Just on account of these individual differences it is impossible to define the limits of the taste sense which shall hold for all persons; one can only give averages and avoid the exceptions. Taking the average adult as tested by the two methods, one can say that the taste mechanism includes the following:
1. The superior, or upper, surface of the tongue, with the exception of a patch just back of the tip. The size of this patch, which is insensitive to all taste stimuli, varies considerably with the kind of stimulus used and its intensity.
2. The sides and tip of the tongue. The under surface of the tongue of human beings is said to be insensitive to taste.
3. The soft palate, the uvula, and the tonsils, although the extent of these parts that is sensitive is subject to considerable variation. Kiesow and others fail to find the uvula sensitive at all.
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.