SMELL AND TASTE
In Man the chief function of these senses is to guard the entrances
to the respiratory and digestive tracts. In this they are not
conspicuously efficient, since various poisonous gases, salts, and
powders, escape their vigilance. Merely a selection of the substances
which occur in air and in food are recognized as having odour or
flavour. Smell and taste are only partially distinguished in ordinary
parlance. No odorous substance is spoken of as tasteless when taken
into the mouth. Its volatile constituents, escaping to the chambers
of the nose, are said to afford a certain flavour. On the other
hand, it is recognized that substances which stimulate the tongue
alone—bitters, acids, sweets, and salts, unmixed with volatile
bodies—have no odour.
Biassed as we necessarily are by the paltry rôle assigned to smell in
our mental life, it seems a little unworthy of the present functions of
the great brain that it should have developed in association with the
nose. Yet smell and taste are the oldest of the senses. Their origin
goes back to the days of chemiotaxis, when the organism, having no
specialized sense-organs, was attracted to its mate or to its food, and
repelled from conditions unsuitable for its well-being, by particles in
solution acting as chemical stimulants. An amœba is chemiotactically
drawn towards its food, one spore of an alga is attracted to another,
by the particles of matter which drift across the interval between them.
In the life of many animals smell plays as important a part as that of
either of the other senses. One has but to watch a dog “looking” for
its master, already full in view, with its nose, to realize that smell
is the sense on which a dog chiefly relies. We describe it as looking,
because in ourselves the eye has so far outdistanced the other senses
as a channel of information that we speak of “looking” when we mean
seeking, and say that “we see” when we wish to imply that we understand.
The difference between smell and taste is, in fishes, a difference in
the quality of the sensation, and not in its “modality” or kind; but
in terrestrial animals the olfactory membrane of the nose has become
specialized for the recognition of particles suspended in air, the
tongue for substances dissolved in water. The olfactory membrane, which
lines the upper two of the three chambers of the nose, is covered with
elongated cells of two kinds: (_a_) Columnar cells, fairly thick; and
(_b_) fusiform cells, each carrying at its free extremity a bunch of
exceedingly minute hairs. The fusiform cells are neuro-epithelial cells
of the most primitive type. Before nerve-cells, properly so called,
appeared, certain favourably-placed epithelial cells were connected
by protoplasmic bridges with muscle-fibres, to which they delivered
the impulses which were generated in them by external forces. Later
some of the neuro-epithelial cells sank beneath the surface, where,
as ganglion-cells, they served as intermediaries between groups of
sensory cells on the surface and the nerve-net which lay more deeply
in the tissues. The olfactory membrane perpetuates the earlier stage;
in so far as it consists of elements which are combinations of
sense-cells and nerve-fibres. Each of its fusiform cells sends inwards
a nerve-filament, which, traversing the submucous tissue of the nose
and the bone (cribriform plate) on the base of the skull, between the
orbits, enters the olfactory bulb. The olfactory bulb is a part of
the local nervous mechanism of smell. It is the ganglion of the nerve
of smell _plus_ nerve-elements which in all segments behind the eye
have been withdrawn from the neighbourhood of the sense-organ into the
central nervous system (_cf._ p. 333).
The way in which odorous particles in air stimulate the fusiform
cells is unknown. The quantity which suffices as a stimulant is so
small as to put chemical stimulation out of the question. A few
grains of musk will scent a room for years. 0·00000004 milligramme of
mercaptan (sulphur-alcohol) is recognizable in a litre of air. This
is a dilution to 1 in 50,000,000,000. Probably even such figures as
these would be thrown into the shade if we could estimate the minimum
amount of human effluvium which will enable a dog to follow his
master’s trail. Explanations have been sought in alterations in the
vibrations of molecules of air caused by the presence amongst them of
relatively heavy molecules of volatile substances; but the difficulty
of accounting for the generation of nerve-impulses in the sensory cells
remains as great as ever. The hairs borne by olfactory cells are so
short that it is impossible that they should project beyond the film
of moisture on the surface of the membrane. This seems to preclude an
answering vibration. Yet an increase in the thickness of this layer
and in its density, due to the presence in it of mucus secreted during
a catarrh, renders the sense-cells incapable of responding to odorous
particles.
Smell in an animal is not a test of the quality of the air it is
breathing, but a source of information as to the direction in which it
may seek its prey; or, although far more rarely, as to the direction
from which the advance of a foe is to be feared. Hunting animals depend
for the most part on the nose. Hunted animals rely chiefly on the eye.
If we attempt to analyse our smell-sensations, we find that we can pick
out a number of varieties which appear so unlike as to have nothing
in common: Putrid meat, burning indiarubber, sulphuretted hydrogen,
ammonia, roses, onions, lemon verbena, methylated spirit. Everyone can
make for himself a list of typical odours which seem to have specific
qualities—odours so distinct that he never confuses one with another.
He can also class together scents about which he is often uncertain.
The type-odours he can distinguish when present in a mixture; whereas
odours which are less distinct reinforce or modify one another. It has
been found, by careful experiment, that certain type-odours even tend
to neutralize each other. Musk and bitter almonds, for example, if
present in small quantities and properly proportioned, produce a very
dim sensation, whether supplied as a mixture to both nostrils, or the
one assertive odour to one nostril and the other to the other. This
last observation is of great importance. It proves that their mutual
destruction does not occur on the olfactory membrane. It is not due to
physical interference. The sensation of musk is delivered to one side
of the brain, the sensation of bitter almonds to the other; but when
attention is directed to these two sensations there is found a quality
in the one which is irreconcilable with the quality of the other.
In certain persons and under certain pathological conditions,
sensitiveness to particular odours, or groups of odours, is absent,
while for the rest the sense is normal. Methylated spirit, prussic acid
and mignonette, constitute a group which not infrequently drops out.
Instances have also been reported of persons unable to smell vanilla
(to which some are hyper-sensitive), and of others insensitive to
violets, although normally sensitive to the scents of other flowers.
The notes sounded in consciousness extend over a long gamut; but
there are reasons for thinking that the number of keys on the clavier
which odoriferous substances strike is limited. Eleven is the number
provisionally adopted. The effect in consciousness varies according as
one key or another is struck, or several at the same time with varying
degrees of force.
Many attempts have been made to associate the sensation-qualities of
the various odours with the chemical or physical properties of their
odorants, with but little success as yet. To excite the sense of
smell, a gas must be at least a little heavier than air. No volatile
body, it is stated, is so heavy as to be odourless; on the contrary,
speaking generally, heavy molecules are more stimulating than light.
The quality of a smell-sensation would therefore appear to depend upon
the period of vibration of the molecules of the substance which evokes
it; but, as already stated, a consideration of the apparatus which
responds to stimulation by odoriferous particles does not help us to an
understanding of the way in which the particles act upon it.
[Illustration: FIG. 26.—HIGHLY MAGNIFIED SECTION THROUGH THE WALL OF A
CIRCUMVALLATE PAPILLA OF THE TONGUE, SHOWING TWO TASTE-BULBS.
These sense-organs are groups of elongated epithelial
cells, set vertically to the surface. Their cells
are of two kinds—the one fusiform, slender,
bearing each a bristle-like process which projects
through a minute pore left between the superficial
cells of the general epithelium; the other thicker
and wedge-shaped. Nerve-fibres are connected with
the fusiform cells.]
Taste is far more limited in its range of sensations than smell. The
back of the tongue is sensitive to bitters, the tip to sweets and
salts, the sides to acids. Mixtures of these qualities are distinctly
analysable by the sense of taste. Our sensations of taste do not fuse.
Slight differences in the way in which the organs on the different
parts of the tongue react to stimulation enable us to recognize that a
sapid substance is a mixture. When, with a great flourish of trumpets,
saccharin was introduced as a safe sweetener for gouty people, an
attempt was made to provide them with saccharin-sweetened jam. The
effect of the jam upon the person who consumed it was truly humorous.
First a suspicion of tartness, then its adequate suppression, followed
by nauseating sweetness. The sense-organs which subserve the sense
of taste are clusters of fusiform epithelial cells, collected in
“taste-bulbs” (Fig. 26). Each gustatory cell bears a minute bristle,
which projects through the pore left by the cells of the surrounding
epithelium which constitute a globular case for the bulb. As in the
nose, eye, and ear, a second thicker variety of epithelial cell is
also present. The nerve-fibres of the taste-bulbs are not, as in the
olfactory membrane, processes of their cells, but branches of the fifth
nerve which ramify amongst them. On the back of the tongue taste-bulbs
are much more numerous than elsewhere. They are not as sensitive as the
cells of the olfactory membrane; nevertheless, they enable us to detect
1 part of quinine in 2,000,000 parts of water.
Sensations of taste and smell endure for a long time after stimulation,
because the odorous or sapid substance remains in contact with the
sense-organs. This accounts for the confusion into which a man is
thrown if he sip alternately port and sherry. After a short time he
cannot tell the one from the other. The organs are quickly fatigued,
using the term loosely. How intolerable patchouli would be to the
ladies who use it were it otherwise! If for some time one sniffs the
odour of mignonette, it ceases to be recognizable; whereas, turning to
a rose, the olfactory membrane is found to be as sensitive as usual.
When the sense is fatigued for a particular smell, it is dull for
others of the same group, thus affording an opportunity of classifying
smell-sensations according to their qualities; but the method is
difficult to apply. Taste-organs are greatly affected by temperature.
Quinine is not tasted just after drinking ice-cold water. Alcohol,
ether, or chloroform paralyses the organs much in the same way.
Castor-oil slips down the throat unnoticed if the mouth, just before
swallowing it, has been rinsed with brandy or with a strong solution of
tincture of chloroform.
Englishmen make but little use of their sense of smell. It might
teach them much regarding the various emanations from putrid matter
which are produced by bacterial action; but, dreading drains, they
decline to cultivate proficiency in the exercise of this sense. The
nose is valued for the warning it gives of “nasty smells,” but is not
allowed to analyse them. Burnt milk, soap-boilings, rancid oils, are
taboo, because they are associated with bungling in the kitchen. With
moderated ardour, we allow our sense of smell to distinguish foods and
beverages, but we are not a race of epicures. The perfumes of flowers
are classed as “nice smells.” The idea of greediness is not associated
with their enjoyment; besides, they remind us of gardens, sunshine,
pretty forms and colours. When bottled, musk, orange-blossom, violets,
lavender, are valued not so much for their own sweetness, as for their
singular efficiency in obscuring nasty smells. Few persons practise
the recognition and distinction of even pleasant odours. Very few, on
first coming across a scented herb or shrub, pay sufficient attention
to its perfume to impress it on their memories. They note the shape
of its leaves and the colour of its flowers, but they are unable to
identify it by its odour when they meet with it again. It is not much
to be wondered at, therefore, that this slighted sense tends to leave
us after middle life. It has been asserted—and probably the statement
is justified—that rarely is the olfactory bulb of a man over forty
free from signs of atrophy. We have no statistics concerning the brains
of Japanese, who regard the sense of smell as one of the chief avenues
of pleasure; but it may be that in this respect their brains present
a contrast to our own. Yet the deadening of the sense is scarcely
noticed, since its results are of little consequence as compared with
those which follow loss of sight or loss of hearing. Many a man, as he
grows older, declares that the cook of his club has lost his cunning,
or frankly asserts that he “no longer cares for kickshaws. Cold beef,
beer, and pickles, are good enough for him.” He little suspects that
his palate has lost its power of distinguishing the flavours of
dainty meats and wines. Others continue to be exacting, because their
imaginations still endow food with the qualities which they remember,
just as people eat preserved asparagus or tinned peas because they
look—however little they taste—like the gifts of Spring.
Taste accompanies the reception of food in the mouth. We have no
knowledge of the situation of our own olfactory membranes, and
therefore we suppose that a flavour, whether it be due to stimulation
of taste-bulbs or olfactory membrane, is in the mouth. The odour of
a flower we mentally project to a distance, because we associate the
sight of a flower with its perfume. A dog, able to judge the freshness
or staleness of a scent, must project its sensations of smell in the
same way in which we project our sensations of sight. It forms an
estimate, of a sort, of the time that it will take in reaching the
source of the scent. Its excitement increases as the trail grows
fresher.
Taste and smell are heavily laden with affective tone. When
disagreeable, the feeling which they evoke is near akin to pain.
It may gather head until, like hunger, it causes the discharge of
motor neurones; but under its influence food is ejected, instead of
preparation being made for its reception.
Taste and smell are senses which afford us no information with
regard to time or space. They give rise to massive sensations. Such
sensations, devoid of detail, produce a frame of mind rather than
thought. The smell of tobacco does not distract attention. On the
contrary, the steady flow of impulses to which it gives rise helps
to inhibit, to subdue, the yapping of more exigent sensations. And
since sensations of smell have no features of their own, they form
a background to sensations of other kinds, entering with them into
memory. No two scenes are exactly alike. One cannot recall another. But
the scent of syringa is always the same. Wherever smelled, it opens the
pathways in the brain in which were first associated a June evening and
syringa, with a scene and a situation upon which memory loves to dwell.