HOW WE HEAR, SEE, SMELL, TASTE, AND FEEL.
~1. The Senses.~--We have five senses--_hearing_, _seeing_, _smelling_,
_tasting_, and _feeling_. These are called special senses because they
are very different from each other. They also differ from the general
sense of feeling by means of which we feel pain when any part is hurt.
~2. Organs of the Special Senses.~--Each of the special senses has a
special set of nerves and also special cells in the brain which have
charge of them. We say that we see with our eyes, hear with our ears,
feel with our fingers, etc.; but, really, we see, hear, taste, and smell
in the brain just as we feel in the brain. The eyes, ears, nose, and
other organs of the special senses are the instruments by means of which
the brain sees, hears, smells, etc.
~3. Sound and the Vibrations which it Causes.~--All sounds are made by
jars or vibrations of objects. Sounds cause objects to vibrate or
tremble. A loud sound sometimes jars a whole house, while other sounds
are so gentle and soft that we cannot feel them in the same way that we
feel loud sounds. But Nature has made for us an ingenious organ by means
of which we can feel these very fine vibrations as well as loud ones. We
call this organ the _ear_.
~4. The Ear.~--The part of the ear which we can see is shaped somewhat
like a trumpet. The small opening near the middle of the ear leads into
a _canal_ or tube which extends into the head about an inch. At the
inner end there is a curious little chamber. This is called the _drum_
of the ear, because between it and the canal of the ear there is
stretched a thin membrane like the head of a drum. The ear-drum is also
called the _middle ear_.
[Illustration: THE EAR.]
~5. Bones of the Ear.~--Within the drum of the ear there are three
curious little bones which are joined together so as to make a complete
chain, reaching from the drum-head to the other side of the drum. The
last bone fits into a little hole which leads into another curious
chamber. This chamber, which is called the _inner ear_, is filled with
fluid, and in this fluid the nerve of hearing is spread out. A part of
the inner ear looks very much like a snail shell.
[Illustration: THE INSIDE OF THE EAR.]
~6. How we Hear.~--Scratch with a pin upon one end of a long wooden
pole. Have some one listen with the ear placed close against the other
end of the pole. He will tell you that he hears the scratching of the
pin very plainly. This is because the scratching jars the ear and
especially the drum-head, which vibrates just as the head of a drum does
when it is beaten with a drum-stick. When the drum-head vibrates it
moves the bones of the ear, and these carry the vibration to the nerves
of hearing in the inner chamber. We hear all sounds in the same way,
only most sounds come to the ear through the air.
The snail-shell of the inner part of the ear hears musical sounds. The
rest of the inner ear hears ordinary sounds or noises.
~7. How to Keep the Ears Healthy.~--The ears are very delicate organs
and must be carefully treated. The following things about the care of
the ears should never be forgotten:
(1.) Never use a pin, toothpick, or any other sharp instrument to clean
out the ear. There is great danger that the drum-head will be torn, and
thus the hearing will be injured. Neither is it ever necessary to use an
ear-spoon to remove the wax. Working at the ear causes more wax to form.
(2.) Do not allow cold water to enter the ear or a cold wind to blow
directly into it.
(3.) If anything accidentally gets into the ear, do not work at it, but
hold the head over to one side while water is made to run in from a
syringe. If an insect has gone into the ear, pour in a little oil. This
will kill the insect or make it come out.
(4.) Never shout into another person's ear. The ear may be greatly
injured in this way.
(5.) Boxing or pulling the ears is likely to produce deafness, and ought
never to be done.
~8. The Eye.~--The eye is one of the most wonderful organs in the whole
body. It enables us to know what is going on at some distance from us,
and to enjoy many beautiful things which our sense of hearing and other
senses can tell us nothing about. It also enables us to read. Let us
learn how this wonderful organ is made.
~9. The Eyeball.~--Looking at the eye, we see first a round part which
rolls in different directions. This is the _eyeball_. We see only the
front side of the eyeball as it fits into a hollow in the skull. Being
thus in a safe place, it is not likely to get hurt.
[Illustration: THE EYE.]
The eyeball is mostly filled with a clear substance very much like
jelly. It is so clear that the light can shine through it just as easily
as it can shine through water.
~10. The Pupil.~--If you look sharply at the eyeball you will see a
small black hole just in the centre. This is a little window which lets
the light into the inside of the eyeball. We call this the _pupil_. Just
around the pupil is a colored ring which gives the eye its color. We say
a person has blue or brown or gray eyes according as this ring is blue
or brown or gray. This colored ring is a kind of curtain for the window
of the eye.
~11.~ If you observe the pupil closely, you will see that it is
sometimes larger and sometimes smaller. If you look at the light the
pupil is small; if you turn away from the light the pupil grows larger
at once. This is because the curtain closes when in a bright light and
opens in the darkness. It does this of itself without our thinking about
it. In this way the eye is protected from too strong a light, which
would do it great harm.
~12.~ If you look a little sidewise at the eyeball, you will see that
the curtain has something in front of it which is clear as glass. It is
about the shape of a watch crystal, only very much smaller. This is to
the eye what the glass is to the windows of a house. It closes the
opening in the front of the eyeball and yet lets the light shine in.
~13. The White of the Eye.~--The white of the eye is a tough, firm
membrane which encloses the eyeball and keeps it in a round shape.
~14. The Lens.~--Do you know what a lens is? Perhaps you do not know it
by this name, but you are familiar with the spectacles which people
sometimes wear to help their eyes. The glasses in the spectacle frames
are called lenses. Well, there is something in the eye almost exactly
like one of these lenses, only smaller. It is also called a _lens_. If
some one will get the eye of an ox for you, you can cut it open and find
this part. The lens is placed in the eyeball just behind the pupil. (See
picture.)
[Illustration: THE INSIDE OF THE EYE.]
~15. The Nerves of Sight.~--But a person might have an eyeball with all
the parts we have learned about and yet not be able to see. Can you tell
what more is needed? There must be a nerve. This nerve comes from some
little nerve cells in the brain and enters the eyeball at the back of
the eye; there it is spread out on the inside of the black lining of the
white of the eye.
~16. The Eyelids.~--Now we know all that it is necessary for us to learn
about the eyeball, so let us notice some other parts about the eye.
First there are the eyelids. They are little folds of skin fringed with
hairs, which we can shut up so as to cover the eyeball and keep out the
light when we want to sleep or when we are in danger of getting dust or
smoke into the eye. The hairs placed along the edge of the lids help to
keep the dust out when the eyes are open.
~17. The Eyebrows.~--The row of hairs placed above the eye is called the
eyebrow. Like the eyelids, the eyebrows catch some substances which
might fall into the eye, and they also serve to turn off the
perspiration and keep it out of the eyes.
~18. The Tear Gland.~--Do you know where the tears come from? There is a
little gland snugly placed away in the socket of the eye just above the
eyeball, which makes tears in the same way that the salivary glands make
saliva. It is called the _tear gland_. The gland usually makes just
enough tears to keep the eye moist. There are times when it makes more
than enough, as when something gets into the eye, or when we suffer pain
or feel unhappy. Then the tears are carried off by means of a little
tube which runs down into the nose from the inner corner of the eye.
When the tears are formed so fast that they cannot all get away through
this tube, they pass over the edge of the lower eyelid and flow down the
cheek.
~19. Muscles of the Eyes.~--By means of little muscles which are
fastened to the eyeball, we are able to turn the eye in almost every
direction.
~20. How we See.~--Now we want to know how we see with the eye. This is
not very easy to understand, but we can learn something about it. Let us
make a little experiment. Here is a glass lens. If we hold it before a
window and place a piece of smooth white paper behind it, we can see a
picture of the houses and trees and fences, and other things
out-of-doors. The picture made by the lens looks exactly like the view
out-of-doors, except that it is upside down. This is one of the curious
things that a lens does. The lens of the eye acts just like a glass
lens. It makes a picture of everything we see, upon the ends of the
nerves of sight which are spread out at the back of the eyeball. The
nerves of sight tell their nerves in the brain about the picture, just
as the nerves of feeling tell their cells when they are touched with a
pin; and this is how we see.
~21.~ Did you ever look through a spyglass or an opera-glass? If so, you
know you must make the tube longer or shorter according as you look at
things near by or far away. The eye also has to be changed a little
when we look from near to distant objects. Look out of the window at a
tree a long way off. Now place a lead pencil between the eyes and the
tree. You can scarcely see the pencil while you look sharply at the
tree, and if you look at the pencil you cannot see the tree distinctly.
~22.~ There is a little muscle in the eye which makes the change needed
to enable us to see objects close by as well as those which are farther
away. When people grow old the little muscles cannot do this so well,
and hence old people have to put on glasses to see objects near by, as
in reading. Children should not try to wear old persons' glasses, as
this is likely to injure their eyes.
~23. How to Keep the Eyes Healthy.~--(1.) Never continue the use of the
eyes at fine work, such as reading or fancy-work, after they have become
very tired.
(2.) Do not try to read or to use the eyes with a poor light--in the
twilight, for instance, before the gas or lamps are lighted.
(3.) In reading or studying, do not sit with the light from either a
lamp or a window shining directly upon the face. Have the light come
from behind and shine over the left shoulder if possible.
(4.) Never expose the eyes to a sudden, bright light by looking at the
sun or at a lamp on first awaking in the morning, or by passing quickly
from a dark room into a lighted one.
(5.) Do not read when lying down, or when riding on a street car or
railway train.
(6.) If any object gets into the eye have it removed as soon as
possible.
(7.) A great many persons hurt their eyes by using various kinds of
eye-washes. Never use anything of this kind unless told to do so by a
good physician.
~24. How we Smell.~--If we wish to smell anything very strongly, we
sniff or suddenly draw the air up through the nose. We do this to bring
more air to the nerves of smell, which are placed at the upper part of
the inside of the nose.
[Illustration: INSIDE OF THE NOSE.]
~25.~ Smelling is a sort of feeling. The nerves of smell are so
sensitive that they can discover things in the air which we cannot taste
or see. An Indian uses his sense of smell to tell him whether things
are good to eat or not. He knows that things which have a pleasant smell
are likely to be good for him and not likely to make him sick.
We do not make so much use of the sense of smell as do the savages and
many lower animals, and hence we are not able to smell so acutely. Many
persons lose the sense of smell altogether, from neglecting colds in the
head.
~26. How we Taste.~--The tongue and the palate have very delicate nerves
by means of which we taste. We cannot taste with the whole of the
tongue. The very tip of the tongue has only nerves of touch or feeling.
~27.~ The use of the sense of taste is to give us pleasure and to tell
us whether different substances are healthful or injurious. Things which
are poisonous and likely to make us sick almost always have an
unpleasant taste as well as an unpleasant odor. Things which have a
pleasant taste are usually harmless.
~28. Bad Tastes.~--People sometimes learn to like things which have a
very unpleasant taste. Pepper, mustard, pepper-sauce, and other hot
sauces, alcohol, and tobacco are harmful substances of this sort. When
used freely they injure the sense of taste so that it cannot detect and
enjoy fine and delicate flavors. These substances, as we have elsewhere
learned, also do the stomach harm and injure the nerves and other parts
of the body.
~29. The Sense of Touch.~--If you put your hand upon an object you can
tell whether it is hard or soft, smooth or rough, and can learn whether
it is round or square, or of some other shape. You are able to do this
by means of the nerves of touch, which are found in the skin in all
parts of the body. If you wished to know how an object feels, would you
touch it with the elbow, or the knee, or the cheek? You will say, No.
You would feel of it with the hand, and would touch it with the ends of
the fingers. You can feel objects better with the ends of the fingers
because there are more nerves of touch in the part of the skin covering
the ends of the fingers than in most other parts of the body.
~30.~ The sense of touch is more delicate in the tip of the tongue than
in any other part. This is because it is necessary to use the sense of
touch in the tongue to assist the sense of taste in finding out whether
things are good to eat or not. The sense of touch is also very useful to
us in many other ways. We hardly know how useful it really is until we
are deprived of some of our other senses, as sight or hearing. In a
blind man the sense of touch often becomes surprisingly acute.
~31. Effects of Alcohol and Tobacco on the Special Senses.~--All the
special senses--hearing, seeing, smelling, tasting, feeling--depend upon
the brain and nerves. Whatever does harm to the brain and nerves must
injure the special senses also. We have learned how alcohol and tobacco,
and all other narcotics and stimulants, injure and sometimes destroy the
brain cells and their nerve branches, and so we can understand that a
person who uses these poisonous substances will, by so doing, injure the
delicate organs with which he hears, sees, smells, etc.
~32.~ Persons who use tobacco and strong drink sometimes become blind,
because these poisons injure the nerves of sight. The ears are
frequently injured by the use of tobacco. Smoking cigarettes and
snuff-taking destroy the sense of smell. The poison of the tobacco
paralyzes the nerves of taste so that they cannot detect flavors.
Tea-tasters and other persons who need to have a delicate sense of taste
do not use either alcohol or tobacco.
SUMMARY.
1. We have five special senses--hearing, seeing, smelling, tasting, and
feeling.
2. The ear is the organ of hearing, and has three parts, called the
external ear, the middle ear, and the inner ear. The inner ear contains
the nerve of hearing.
3. The middle ear is separated from the external ear by the drum-head.
The drum-head is connected with the inner ear by a chain of bones.
4. Sounds cause the drum-head to vibrate. The ear-bones convey the
vibration from the drum-head to the nerve of hearing.
5. To keep the ear healthy we must avoid meddling with it or putting
things into it.
6. The eye is the organ of sight. The chief parts of the eye are the
eyeball, the socket, and the eyelids.
7. In the eyeball are the pupil, the lens, and the nerve of sight.
8. The eyeball is moved in various directions by six small muscles.
9. The eye is moistened by tears from the tear-gland.
10. When we look at an object the lens of the eye makes a picture on the
nerve of sight, at the back part of the eyeball.
11. To keep the eyes healthy we should be careful not to tax them long
at a time with fine work, or to use them in a poor light.
12. The nerves of smell are placed in the upper part of the inside of
the nose.
13. "Colds" often destroy the sense of smell.
14. The nerves of taste are placed in the tongue and palate.
15. Many things which we think we taste we really do not taste, but
smell or feel.
16. Objects which have a pleasant taste are usually healthful, while
those which have a bad taste are usually harmful.
17. Pepper, mustard, etc., as well as alcohol and tobacco, have an
unpleasant taste, and are not healthful. If we use them we shall injure
the nerves of taste as well as other parts of the body.
18. We feel objects by means of the sense of touch.
19. The sense of touch is most acute at the tip of the tongue and the
ends of the fingers.