THE SENSES
=162.= A man has five ways of knowing about things outside of the
body. He can feel, see, hear, smell, and taste.
=163. Feeling.=--Nerves go to nearly every cell in the body. They
carry news to the brain when anything touches them. The news produces
a feeling. Feelings are of three kinds:--
_First_, when anything touches the cells without harming them, we feel
a _touch_. We feel a touch by nerves in the skin. Those in the ends of
the fingers and tongue can feel the best. Those upon the back give but
little feeling.
Touch tells whether anything is hard, or rough, or round, or square,
or has other qualities and shapes.
_Second_, when anything touches the bare nerves or hurts the cells, we
feel a _pain_. We can feel a pain anywhere in the body. Pain tells us if
we are being harmed. If we had no feeling of pain, we might be killed
before we could know of our danger. Pain warns us away from danger.
_Third_, we can feel _heat_ and _cold_. Anything very hot or very
cold, however, makes only a pain and gives no feeling either of cold
or of heat.
=164. Sight.=--We see with our eyes. An eye is a hollow ball. In its
front is a clear window. Behind the window is a round curtain with a
round hole in its middle. When we speak of the color of the eye, we
mean the color of this curtain. Light passes through the hole in the
curtain and falls upon some nerves in the back of the eyeballs. There
it forms a picture like a photograph. The nerves carry this picture to
the brain, and we see it.
[Illustration: =The human eye.=
_a_ bony case of the eye.
_b_ muscle to move the eye.
_c_ and _d_ coverings of the eye.
_e_ lining or seeing part of the eye.
_f_ eyelid.
_g_ colored curtain or iris.
_h_ and _i_ clear windows of the eye.]
=165. Movements of the eyes.=--We can turn our eyes so as to look in any
direction. Sometimes a person has one eye turned sidewise. Such a person
is cross-eyed, and sees well out of only one eye at a time. Glasses may
help the eyes, but sometimes a surgeon has to cut a tiny muscle.
=166. Coverings of the eyes.=--The eyeballs lie in a bony case, upon a
soft bed of fat. In front each is covered with two lids. We can shut
the lids to keep out dust and insects. When we are sleepy, they come
together and cover the eyes. Little hairs at their edges help to keep
out the dust.
Sometimes a little dirt gets under the lids. Then the eye smarts or
itches, and we want to rub it; but this may grind the dirt in deeper.
Then you should get some one else to lift your eyelid and pick out the
dust with a soft handkerchief. If you cannot get help, lift the lid by
the eyelashes; blow your nose hard, and the tears may wash the dirt
away.
Dust and disease germs may get into our eyes and make them sore and
red. You should bathe your eyes well every time you wash your face.
You should use a clean towel, for a dirty one may carry disease germs
to your eyes. Some forms of sore eyes are catching. If any one has
sore eyes, no one else should use his towels or handkerchiefs.
=167. Tears.=--Clear salt water is always running over the eyes and
down a tube into the nose. The use of this water is to bathe the eyes
and keep them clean. It sometimes runs over the lids in drops called
_tears_.
=168. How to use the eyes.=--If using your eyes makes them painful or
gives you a headache, you are straining your eyes. Facing a bright
light strains the eyes. Shade your eyes while you study. A cap may be
used as a shade if you cannot get anything else. Never try to look at
the sun or a very bright light. You should have the light at one side
or behind you. The light should be steady. Reading in a dim light will
harm the eyes.
=169. Near sight.=--If you cannot read without holding your book less
than a foot from your eyes, you are nearsighted, and should wear
glasses all the time. If you do this, your eyes may be strong, and you
may be able to see well.
=170. Far sight.=--If you cannot read without holding your book at
arm's length, you are farsighted and need glasses. Most old persons
are farsighted.
=171. Alcohol and the eyes.=--Alcohol makes the eyes red. It weakens the
eyes and may produce blindness. A drunken person often sees double.
=172. Tobacco= causes dimness of sight and sometimes produces blindness.
=173. Hearing.=--We hear with the ears. Sound is made by waves in the
air. The part of the ear on the outside of the head catches the air
waves and throws them inside the ear. These air waves strike against a
little drum. Three little bones then carry the waves on to nerves
farther inside the head. Animals can turn their ears and catch sound
from any direction.
[Illustration: =Diagram of the ear.=
_a_ outer ear.
_b_ drum head.
_c_ _d_ and _e_ bones to carry sound to inner ear.
_f_ _g_ and _h_ inner ear.
_i_ tube to the mouth.
_j_ middle ear.]
=174. Ear wax.=--Wax is formed just inside the ear. It keeps flies and
insects from crawling into the ear. Boys in swimming sometimes get
cold water into their ears. This may make them have an earache.
=175. How the throat affects the ear.=--An air tube runs from the
inside of the ear to the mouth. Sometimes when you blow your nose, you
blow air into the ear. This makes you partly deaf and you hear a
roaring in your ears.
Sometimes when you have a cold in your throat, this little tube is
stopped. Then your ear may ache and may even discharge matter. This
may make you somewhat deaf. Earache and deafness are most often due to
a cold in the throat and a stoppage of this tube.
Many little boys and girls are deaf and do not know it. They cannot
hear the teacher well, and sometimes the teacher thinks they are bad
or careless because they do not answer.
=176. Care of the ears.=--Very loud noises may harm the ear and make
you deaf. When you expect a very loud noise, put your fingers in your
ears to shut out the sound.
Boxing the ears may break their tiny drums and make you deaf.
Do not get cold water in your ear. This may cause an earache and make
you deaf. If you get water in your ear while you are in swimming, turn
your head to one side and shake it. This will get the water out.
Do not put cotton or anything else into your ears.
=177. Smell.=--We smell with the nose. Some things give out a vapor to
the air. When we draw the air into the nose, this vapor touches the
nerves, and we perceive a smell. The nerves are high up in the nose.
In order to perceive smell clearly, we sniff the air far up the nose.
=178. Use of smell.=--Bad air and spoiled food smell bad. A bad smell
is the sign of something spoiled. The sense of smell tells us when
food or air is unfit for use. Some people try to hide a bad smell with
perfumery. To do this only makes the danger greater, for then the
smell does not tell us of the danger of food or air.
Some animals can smell much better than a man. A dog will smell the
track of a wild animal hours after it is made. Savages can smell much
better than civilized men.
=179. Taste.=--We taste with the tongue. Dry food has no taste, but it
must first dissolve in the mouth. Spoiled food tastes bad. Bad-tasting
food is not fit to eat. Taste tells us whether food is good or bad.
We can learn to like the taste of harmful things. At first no one
likes tobacco or strong drink, but the liking is formed the more one
uses these. We ought to be careful not to begin to use such things.
_Alcohol_ and _tobacco_ burn the mouth and harm the taste. Food does
not taste so good and we may eat spoiled food and not know it.
WHAT WE HAVE LEARNED
1. We can feel in every part of the body, but mostly in the ends
of the fingers.
2. Light makes a picture upon the nerves inside of the eye.
3. If the eyes ache, the light should be softened or the position
of the book or work changed, or else the eyes should be
rested.
4. Sound in the air goes into the ear and strikes against a drum.
Bones then carry the sound to the ear nerves.
5. Air snuffed up the nose gives the sense of smell. Smell tells
us if the air or food is fit for use.
6. Taste tells us whether food is fit for use. Men can learn to
like the taste of wrong things like tobacco or alcohol.
[Illustration: =The Human Skeleton, showing position of bones.=]