Sensory Qualities And Their End-organs
All are familiar with the five senses of our elementary physiologies,
sight, hearing, taste, smell, and touch. A more complete study of
sensation reveals nearly three times this number, however. This is to
say that the body is equipped with more than a dozen different kinds of
end-organs, each prepared to receive its own particular type of
stimulus. It must also be understood that some of the end-organs yield
more than one sense. The eye, for example, gives not only visual but
muscular sensations; the ear not only auditory, but tactual; the tongue
not only gustatory, but tactual and cold and warmth sensations.
SIGHT.--Vision is a distance sense; we can see afar off. The stimulus
is chemical in its action; this means that the ether waves, on
striking the retina, cause a chemical change which sets up the nerve
current responsible for the sensation.
The eye, whose general structure is sufficiently described in all
standard physiologies, consists of a visual apparatus designed to bring
the images of objects to a clear focus on the retina at the fovea, or
area of clearest vision, near the point of entrance of the optic nerve.
The sensation of sight coming from this retinal image unaided by other
sensations gives us but two qualities, light and color. The eye can
distinguish many different grades of light from purest white on through
the various grays to densest black. The range is greater still in color.
We speak of the seven colors of the spectrum, violet, indigo, blue,
green, yellow, orange, and red. But this is not a very serviceable
classification, since the average eye can distinguish about 35,000 color
effects. It is also somewhat bewildering to find that all these colors
seem to be produced from the four fundamental hues, red, green, yellow,
and blue, plus the various tints. These four, combined in varying
proportions and with different degrees of light (i.e., different shades
of gray), yield all the color effects known to the human eye. Herschel
estimates that the workers on the mosaics at Rome must have
distinguished 30,000 different color tones. The hue of a color refers
to its fundamental quality, as red or yellow; the chroma, to its
saturation, or the strength of the color; and the tint, to the amount
of brightness (i.e., white) it contains.
HEARING.--Hearing is also a distance sense. The action of its stimulus
is mechanical, which is to say that the vibrations produced in the air
by the sounding body are finally transmitted by the mechanism of the
middle ear to the inner ear. Here the impulse is conveyed through the
liquid of the internal ear to the nerve endings as so many tiny blows,
which produce the nerve current carried to the brain by the auditory
The sensation of hearing, like that of sight, gives us two qualities:
namely, tones with their accompanying pitch and timbre, and noises.
Tones, or musical sounds, are produced by isochronous or equal-timed
vibrations; thus C of the first octave is produced by 256 vibrations a
second, and if this tone is prolonged the vibration rate will continue
uniformly the same. Noises, on the other hand, are produced by
vibrations which have no uniformity of vibration rate. The ear's
sensibility to pitch extends over about seven octaves. The seven-octave
piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations.
Notes of nearly 50,000 vibrations can be heard by an average ear,
however, though these are too painfully shrill to be musical. Taking
into account this upper limit, the range of the ear is about eleven
octaves. The ear, having given us loudness of tones, which depends on
the amplitude of the vibrations, pitch, which depends on the rapidity
of the vibrations, and timbre, or quality, which depends on the
complexity of the vibrations, has no further qualities of sound to
TASTE.--The sense of taste is located chiefly in the tongue, over the
surface of which are scattered many minute taste-bulbs. These can be
seen as small red specks, most plentifully distributed along the edges
and at the tip of the tongue. The substance tasted must be in
solution, and come in contact with the nerve endings. The action of
the stimulus is chemical.
The sense of taste recognizes the four qualities of sour, sweet,
salt, and bitter. Many of the qualities which we improperly call
tastes are in reality a complex of taste, smell, touch, and temperature.
Smell contributes so largely to the sense of taste that many articles of
food become tasteless when we have a catarrh, and many nauseating
doses of medicine can be taken without discomfort if the nose is held.
Probably none of us, if we are careful to exclude all odors by plugging
the nostrils with cotton, can by taste distinguish between scraped
apple, potato, turnip, or beet, or can tell hot milk from tea or coffee
of the same temperature.
SMELL.--In the upper part of the nasal cavity lies a small brownish
patch of mucous membrane. It is here that the olfactory nerve endings
are located. The substance smelled must be volatile, that is, must exist
in gaseous form, and come in direct contact with the nerve endings.
Chemical action results in a nerve current.
The sensations of smell have not been classified so well as those of
taste, and we have no distinct names for them. Neither do we know how
many olfactory qualities the sense of smell is capable of revealing. The
only definite classification of smell qualities is that based on their
pleasantness or the opposite. We also borrow a few terms and speak of
sweet or fragrant odors and fresh or close smells. There is some
evidence when we observe animals, or even primitive men, that the human
race has been evolving greater sensibility to certain odors, while at
the same time there has been a loss of keenness of what we call scent.
VARIOUS SENSATIONS FROM THE SKIN.--The skin, besides being a protective
and excretory organ, affords a lodging-place for the end-organs giving
us our sense of pressure, pain, cold, warmth, tickle, and itch.
Pressure seems to have for its end-organ the hair-bulbs of the skin;
on hairless regions small bulbs called the corpuscles of Meissner
serve this purpose. Pain is thought to be mediated by free nerve
endings. Cold depends on end-organs called the bulbs of Krause; and
warmth on the Ruffinian corpuscles.
Cutaneous or skin sensation may arise from either mechanical
stimulation, such as pressure, a blow, or tickling, from thermal
stimulation from hot or cold objects, from electrical stimulation, or
from the action of certain chemicals, such as acids and the like.
Stimulated mechanically, the skin gives us but two sensation qualities,
pressure and pain. Many of the qualities which we commonly ascribe
to the skin sensations are really a complex of cutaneous and muscular
sensations. Contact is light pressure. Hardness and softness
depend on the intensity of the pressure. Roughness and smoothness
arise from interrupted and continuous pressure, respectively, and
require movement over the rough or smooth surface. Touch depends on
pressure accompanied by the muscular sensations involved in the
movements connected with the act. Pain is clearly a different sensation
from pressure; but any of the cutaneous or muscular sensations may, by
excessive stimulation, be made to pass over into pain. All parts of the
skin are sensitive to pressure and pain; but certain parts, like the
finger tips, and the tip of the tongue, are more highly sensitive than
others. The skin varies also in its sensitivity to heat and cold. If
we take a hot or a very cold pencil point and pass it rather lightly and
slowly over the skin, it is easy to discover certain spots from which a
sensation of warmth or of cold flashes out. In this way it is possible
to locate the end-organs of temperature very accurately.
THE KINAESTHETIC SENSES.--The muscles, tendons, and joints also give rise
to perfectly definite sensations, but they have not been named as have
the sensations from most of the other end-organs. Weight is the most
clearly marked of these sensations. It is through the sensations
connected with movements of muscles, tendons, and joints that we come to
judge form, size, and distance.
THE ORGANIC SENSES.--Finally, to the sensations mentioned so far must be
added those which come from the internal organs of the body. From the
alimentary canal we get the sensations of hunger, thirst, and
nausea; from the heart, lungs, and organs of sex come numerous
well-defined but unnamed sensations which play an important part in
making up the feeling-tone of our daily lives.
Thus we see that the senses may be looked upon as the sentries of the
body, standing at the outposts where nature and ourselves meet. They
discover the qualities of the various objects with which we come in
contact and hand them over to the mind in the form of sensations. And
these sensations are the raw material out of which we begin to construct
our material environment. Only as we are equipped with good organs of
sense, especially good eyes and ears, therefore, are we able to enter
fully into the wonderful world about us and receive the stimuli
necessary to our thought and action.
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