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.
The Determination of the Taste Areas
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.