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

nerve.



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

reveal.



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.