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.
The Salivary Glands and Their Activity
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.
The Tongue: Its Muscles and Covering Membranes
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.