TASTE-PRODUCING SUBSTANCES
_Adequate and Inadequate Stimuli_
EVER since the doctrine of the specific energy of sensory nerves was
presented by Müller, and since modified into the specific energy of
sense organs or of cerebral centers, two sorts of stimuli for a sense
organ have been spoken of. There are those called _adequate_,—for which
the sense seems to be especially adapted,—and those called _inadequate_,
to which the sensory mechanism is sensitive by virtue of its possession
of a general irritability or sensitivity. It is a matter of common
knowledge that one sees because his visual mechanism is stimulated by
light, or rather by the vibrations of the luminiferous ether, and that
this is the appropriate stimulus for visual sensations. But it is just
as well known that if one receives a blow upon the head he will see
“stars,” or if he presses his finger upon his eyeball he will see
patches of light. The sensations produced in this latter fashion are due
to the mechanical stimulation of the sensitive visual mechanism, which
responds with its specific kind of sensation. The questions to be
answered in this chapter are,—What is the kind of stimulus to which the
taste mechanism is especially adapted? and, Are there other or
inadequate stimuli which can produce taste sensations? In answering
these questions it is well to keep in mind the biological function of
the sense organ of taste. Situated as it is at the entrance of the
alimentary canal it has been called the “eye of the stomach,” whose duty
it is to prevent the entrance into the body, by way of the mouth, of
harmful substances. If this is so, the adequate stimulus for taste would
be any kind of substance which might be taken for food.
_Adequate Taste Stimuli_
All substances may be classed either as _sapid_, tastable, or _insipid_,
tasteless. And one of the main conditions for sapidity is solubility. A
substance to be tasted must enter the mouth cavity as a fluid or else
after being taken into it must be dissolved in the saliva. Thus, the
adequate stimulus for the taste organ may be said to be a fluid.
Recalling the structure and location of the sensory ends of the taste
mechanism, it is at once inferred that only fluids can enter the taste
pore and stimulate there the nerve endings of taste. It might then be
assumed that all fluids should produce taste sensations. But all soluble
substances are not sapid or tastable. Consequently, other conditions of
sapidity have been sought, among them being chemical constitution.
One of the most interesting attempts to solve the question of the
conditions of sapidity is that which makes the only condition necessary,
the contact of the substance with the nerve endings within the taste
buds. Now, Graham pointed out that all tastable substances belong to the
class of crystalloids, while tasteless substances belong to the
colloids. It is known, too, that living membranes are permeable by
certain solutions and not by others. Colloid membranes, of which all of
the mucous membranes of the body are examples, are impervious to
colloids in solution, while the passage of crystalloids in solution is
rapid. Hence, no colloids, even those in solution, could actually reach
the free nerve endings of the taste buds. As Nagel says, however, the
truth of this assumption is not easy to settle, since it is very
difficult to get pure colloids and to make sure that their chemical
constitution is not modified by the saliva before contact with the taste
nerves. Other possibilities will be discussed in the chapter dealing
with the theories of the taste function. It will be sufficient here to
say that the adequate stimulus to taste is a liquid, or a solid, or gas,
which may be dissolved in the saliva. Gases such as chloroform vapor,
carbonic acid gas, and the like, were at one time thought to act
directly upon the taste buds, but a safer view is that the gases are
first dissolved in the saliva before acting on the taste mechanism. A
few experiments have been reported in which the gases were said to have
produced taste sensations when the mucous membrane was dry, but it is
practically impossible to produce this condition, since the small glands
of the tongue open directly into the fissures containing the taste buds
and tend to keep the neighborhood of the taste pores in a moist state.
To say that the adequate stimulus for taste is a fluid is to give only
an incomplete description of adequate stimuli. In the case of vision the
adequate stimuli are ether vibrations; and the different visual
sensations, reds, yellows, blues, etc., are attributed to ether
vibrations of different rate.
And in the sense of hearing, the adequate stimulus of which consists of
air vibrations, it is the different vibration rates that account for the
qualitative or pitch differences in sound sensations. What
characteristics of the stimulating fluids are responsible for the
specific sensations sweet, sour, bitter, and salt? The answer to this
question has been sought in the chemical constitution of the sapid
substances. There is found to be a certain relation between chemical
groups and the taste sensations they produce. Kiesow and others have
pointed out that acids are sour; that many chemical salts have a salt
taste; that many carbohydrates taste sweet, and that most of the
alkaloids are bitter. There are too many exceptions to these simple
relations between chemical structure and sensation quality to have them
serve as an answer to our question. There are chemical salts which taste
sweet, there are acids which do not taste sour, and there are chemical
substances whose tastes differ according to their concentration and even
according to the part of the tongue which they affect.
Sternberg, who has made a very extensive study of the relation between
chemical structure and sensation quality, has recognized the
impossibility of finding a simple relation between chemical groups and
sensation quality. His method of study consisted in cataloguing
separately all those substances which taste sweet, sour, bitter, and
salt, and then looking for similarities of structure within the same
sensation group and differences among different groups. This is just the
opposite of the customary procedure which was to take the chemical
groups as a starting point and examine into the tastes aroused by them.
He finds no difference in the molecule of a substance producing a sweet
taste and a molecule of another substance producing a bitter taste, and
finds similarities among the molecules of substances producing different
tastes. He is forced to the conclusion that the tastes are due to the
character of the intramolecular vibrations; that the taste mechanism is
capable of responding to the relations among the atoms which have
escaped the other senses, even when their keenness is increased by all
sorts of artificial devices. Nagel, in reviewing this work of Sternberg,
says that, in spite of the interesting facts which have been accumulated
by him and others, very little has been contributed to the solution of
the question of the stimulus for the different taste qualities.
Before dismissing the question of adequate stimuli one other set of
phenomena ought to be mentioned. There are cases in which sapid
substances dissolved in the blood produce taste sensations. In cases of
diabetes, where sugar is present in the blood, a sweet taste is
experienced in the absence of stimuli upon the tongue surface. Also, in
cases of jaundice, where there is an excess of bile, a bitter sensation
may be experienced. Here the stimuli affect either the taste nerves or
the taste buds directly. There is no reason to think that the
stimulation process differs in any other respect from the normal, except
in the mode of access to the taste buds.
_Inadequate Taste Stimuli_
Our second question concerns the possibility of other stimuli to taste,
or of inadequate stimuli. Among these forms of stimulation three will be
considered, namely, mechanical, thermal, and electrical. Are taste
sensations produced by mechanical stimulation of the sense organ? Some
of the older experimenters reported that tapping the tongue lightly or
putting it under slight pressure aroused taste sensations of sour and
bitter. But there does not seem to be good ground for such a conclusion.
No doubt under poorly controlled conditions such mechanical stimuli
might serve to force sapid substances already upon the tongue into the
taste pores and thus stimulate the taste nerves. This, however, would be
only an indirect effect of mechanical stimulation and not at all
analogous to the visual sensations produced by mechanical stimulation of
the eyeball.
It is rather interesting to note in this connection that mechanical
stimulation of the chorda tympani nerve, which carries the impulses from
the anterior two-thirds of the tongue, is said to produce taste
sensations. In cases in which the eardrum has been removed, stimulation
of the nerve in the middle ear is said by some authorities to produce
sweet and bitter sensations, and by others nothing but pricking
sensations.
Thermal stimuli when applied to the tongue do not cause taste
sensations, but only sensations of warmth and cold. Thermal stimuli,
however, are recognized to have considerable influence upon the effect
of taste stimuli. Two cases are to be noted, that in which the sapid
substance itself is either warm or cold and that in which the mouth has
been subjected to warm or cold stimuli before the sapid substance is
introduced into the mouth. The first condition has received the more
attention on account of the importance of temperature in the supposed
chemical reaction in the taste buds and the consequent possibility of
interpreting the facts in favor of a chemical theory of taste. Both
problems have been investigated by Kiesow and others, and the
conclusions are conflicting. Some find that there is a certain optimum
temperature for sapid bodies at which the taste sensations are best
obtained, although there is no perfect agreement as to what this optimum
temperature is. It varies from 55 to 120 degrees F. for different
authors. At the optimum temperature the least quantity of the sapid
substance is necessary to arouse a taste sensation, while a deviation
from this temperature in either direction requires a stronger stimulus
to arouse the same sort of a sensation. Kiesow, on the other hand,
believes that the temperature of the sapid substance makes no
difference, but only the temperature of the mouth previous to receiving
the sapid substance. For instance, he holds that the sensitivity is just
as acute when the temperature of the sapid substance is 32 degrees F. as
when it is 100 degrees F. But if the tongue is placed in water at 32
degrees F. for a few minutes it then becomes insensitive to sapid
substances,—a kind of anesthesia results. He believes that the
conflicting results obtained by the other investigators are merely the
results of distraction of the attention. Whenever the temperature rises
or falls beyond a certain point, then this temperature sensation becomes
more impressive than the weaker taste sensation, and the taste sensation
drops out of consciousness. It is a well-established fact, at any rate,
that thermal stimuli cannot directly arouse taste sensations.
Taste sensations aroused by passing an electric current through the
tongue were reported as early as the middle of the eighteenth century by
Sulzer. Since that time a great number of experimental researches have
accumulated in the attempt to answer the question whether an electric
stimulus can directly arouse taste sensations, as it will produce
sensations of light when it stimulates the eye. In all of this work
there has been rather general agreement as to the character of the taste
sensations present during electrical stimulation. Thus, when the anode
(positive pole) comes into contact with the tongue the taste is said to
be sour, while at the point where the cathode (negative pole) touches
the tongue a bitter taste is reported. There is less general agreement
upon the character of this cathode taste, however, than upon that of the
anode taste, some describing it as burning, some as bitter, sweet, or
alkaline.
The great differences of opinion do not concern the character of the
taste aroused by the electric current so much as the real source of the
stimulation of the taste buds. It was suggested by Humboldt about 1800
that the taste sensations were not caused directly by the electrical
stimulation of the taste buds, but rather by certain sapid substances
which were set free within the mouth by the action of the current on the
saliva. It is known that fluid salts can be broken down by electrolysis
in such a way that at the anode an acid reaction, and at the cathode an
alkaline reaction, can be detected. These facts have lent support to the
assumption that the saliva thus acted upon by the electric current
produces tastable substances. An interesting experiment attributed to
Volta about 1800 seemed for a time to refute the whole electrolysis
theory. He used an alkaline fluid for the anode, into which the tip of
the tongue was dipped. Even in this case the characteristic sour taste
was still experienced. But this experiment, like all of the others which
seem to refute the electrolysis theory of electric stimulation, neglects
the fact that the electrolysis may occur within the taste bud itself,
and hence could not be detected by any test of the tongue surface, nor
could the taste sensations thus produced be prevented by immersing the
tongue in any kind of a solution, since this need not displace the
fluids within the taste bud. The experience of both bitter and sweet
tastes at the cathode, if these experiences are genuine, offers some
difficulty for the electrolysis theory. The most that can be said is
that it is quite likely that electrolysis takes place within the taste
bud, but that, in addition to this, there may possibly be a certain
direct action of the electrical stimulus upon the taste buds or their
nerve endings.
From this review of the different ways in which the taste organ may be
affected it appears that the evidence is not conclusive that mechanical,
thermal, or electrical stimuli may arouse taste sensations. Chemical
stimulation, which is usually included among the inadequate stimuli for
the other senses, is the adequate stimulus for the taste mechanism.
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