THE SENSE ORGAN OF TASTE
_Comparison With Other Sense Organs_
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.
Footnote 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.
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.
[Illustration: Fig. 2.]
_Classification of Papillæ_
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.
_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.
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.
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