THE SENSES, OR DOORS OF KNOWLEDGE
=The Organs of Sense.=--In order that our body may keep out of the way
of other persons and find food and drink and do its work, the brain
must have some way of receiving news about what is near us, how it
looks, and of what it is made. Special organs for receiving knowledge
of people and things about us are scattered over the surface of the
body. They are called _sense organs_. The chief ones are the two eyes,
the two ears, the nose, and many organs of taste in the mouth, and the
thousands of tiny organs of feeling in the skin.
=The Eye.=--The eye consists of a globe called the _eyeball_ and parts
which move this and protect it from injury. Each eyeball is attached
at its back part to the large nerve of sight (Fig. 90). This carries
messages to the brain, telling it what the eye sees.
The eyeball is held in a socket in the front of the skull. A layer of
fat lines the socket and keeps the eye from being injured by jars. The
_eyebrows_ at the lower edge of the forehead prevent the sweat from
running into the eyeball.
[Illustration: FIG. 90.--Side of the face cut away to show the eyeball
in its socket. _n_ is the nerve of sight; the other letters show the
muscles which move the eyeball.]
The _eyelids_ can close over the front of the eyeball to shut out dirt
or anything else likely to hurt it. The lids have learned to do their
work so well that we do not need to think to close them when anything
flies toward the eye, for they are shut before we can think.
A salty fluid called _tears_ flows from the tear gland at the upper
and outer side of the eyeball. The tears keep the front of the eyeball
clean.
=Parts of the Eyeball.=--The outside of the eyeball is a tough white
coat except in front, where it is as clear as glass. Within the outer
coat is a very thin black lining to keep the light from scattering. In
front the lining is not against the outer coat, but hangs loose and
has in it a round hole called the _pupil_ to let the light pass
through. The part around the hole is the _iris_. It may be blue,
black, or brown, and can squeeze up so as to make the pupil very small
when the light is strong.
[Illustration: FIG. 91.--A slice from before backward through the eye.]
The end of the nerve of sight forms a tender pink covering over most
of the inner surface of the eyeball. The cavity within the eyeball is
filled with three clear substances. The _lens_, shaped like a flat
door knob, is fixed just behind the pupil. In front of the lens is a
_watery fluid_ and behind it is a clear _jellylike mass_. The use of
the lens and also the other substances is to bend the rays of light
together so that they will meet at one place.
=How the Eyeball is Moved.=--Six muscles fixed to the bones of the
socket holding the eye have their other ends fastened to the tough
coat of the eyeball. One muscle turns the ball upward, another turns
it downward, one turns it inward and another turns it outward. If an
inner or an outer muscle is too strong, a person may have cross eyes.
=Keeping the Eye Strong.=--Nearly all young children have perfect
eyes. After a year or two in school the eyes of some children become
weak. Many children get weak eyes after they are ten or twelve years
old. This is because they have not taken care of the eyes.
The eyes are often hurt by reading a book with fine print, reading in
a dim light, or by leaning over the book so that the eyes look
downward instead of straight forward. As the eyes are very weak after
measles and most other diseases, they should not be used much until a
week or more after recovery.
In reading the book should be held a little over a foot in front of
the chest and you should sit nearly straight and let the light fall on
the page from one side. Never read while lying down because it strains
the eyes. Stop reading as soon as the eyes smart.
=Helping the Eyes to See.=--Very few old people can see to read
without the help of glasses, because the lens of the eye hardens in
old age. To see things near by, the shape of the lens must be changed.
In some children, the shape of the eyes has become so changed by
straining them to read fine print or see things in a dim light that
the eyes hurt after being used for any kind of work, and the head may
often ache and make the whole body feel bad. Such eyes need help. You
should have them examined by an eye doctor who can fit you with
glasses which will help you see clearly without headache.
=Keeping the Eyes Well.=--Bits of dirt often get beneath the eyelids
and cause much pain. By taking hold of the eyelashes the lid may be
pulled out from the eye and any dirt removed with the corner of a
clean handkerchief passed gently along the lid.
The eyes sometimes become sore because they are rubbed with soiled
fingers on which are germs. These germs get inside the lids and grow,
and in this way poison the eyes. Unless care is used sore eyes are
likely to spread from one child to another in the school. The sick child
rubs its eyes and then handles a book or pencil on which the germs are
smeared by the fingers which touched the eyes. The next child picks up
the same book later, gets the germs on the fingers, and then rubs the
eyes. For this reason you should never rub the eyes. If you have sore
eyes, _be careful that no one else catches the sickness from you_.
=The Ear.=--The ear is made of three parts called the _outer ear_, the
_middle ear_ or _eardrum_, and the _inner ear_. The outer ear is made
of a plate of skin and gristle and a slightly bent tube about one inch
long. At the inner end of this tube is a thin membrane or _drumhead_.
Beyond the drumhead is the cavity of the middle ear about as large as
a pea. A chain of three tiny bones stretches from the outer drumhead
across this cavity to a tiny _inner drumhead_. Beyond the inner
drumhead is the inner ear.
[Illustration: FIG. 92.--View of the ear from in front. Three little
bones stretch across the middle ear.]
The middle ear is kept full of air by means of a tube leading from it
to the throat. A cold or other sickness may cause this tube to fill up
and make you deaf. The inner ear consists of a sac and four bent tubes
filled with a watery fluid. They are also surrounded by watery fluid
contained in channels in a bone of the skull. The end of the nerve of
hearing is on one of the tubes.
=How we Hear.=--Throwing a stone in the water makes waves which move
farther and farther outward. In the same way a noise causes waves in
the air. These waves pass into the ear tube, strike the outer
drumhead, and make it move. This moves the chain of bones in the
middle ear so that they cause motion in the inner drumhead. This in
moving back and forth makes waves in the fluid of the inner ear which
strike on the ends of the nerve of hearing and cause messages to be
carried to the brain.
=Care of the Ears.=--The ears should not be struck or pulled, as the
eardrum is easily broken. Do not put pencils, pins, or anything else
in your ears. Wax naturally forms in the ear tube to keep out bugs and
flies. The outer part of the tube may be kept clean by wiping it with
a moist cloth over the little finger. If you often have earache or a
running ear, you should have it examined by a physician. _Neglecting a
sick ear may cause deafness._
Some persons are deaf in one ear and do not know it. Test each ear by
covering the other one with a heavy cloth and note how far off you can
hear the ticks of a watch.
=The Nose.=--The nose has a skin-like lining, but it is always kept
moist by little glands which give out a watery fluid. The endings of
the nerve of smell are in the lining in the upper part of the nose.
Two nerves lead from the nose to the brain.
When we catch cold, much blood rushes to the lining of the nose and it
becomes swollen. It then gives out a thick white mucus. This covers
the nerve endings, so that we cannot smell.
Smell is of great use in telling us whether our food is good, by
helping us to enjoy food with a pleasant odor, and by warning us
against bad air.
=The Sense of Taste.=--The nerves by which we taste end in the soft
covering of the tongue and some other parts of the mouth. A food
cannot be tasted while it is dry. For this reason much slippery fluid
flows into the mouth from glands under the ears and tongue. This
fluid, called _saliva_, softens the solid food when it is well chewed,
so we can taste it.
=The Senses of the Skin.=--There are endings of nerves in the skin all
over the body. They are of three or four different kinds. Some of them
tell us about heat, others tell us about cold. Some tell us about the
shape, the smoothness, or hardness of objects, while others tell us
when the skin gets hurt.
Most of the nerve endings are in the deeper part of the skin, so that
they are covered by the epidermis and cannot be hurt by the rough
things handled.
=Alcohol and the Senses.=--The senses are but little affected by a
small amount of alcoholic drink. The sense of taste, after being
accustomed to the sharpness of strong drink, may be less easily
pleased with the taste of common food and drink.
The use of large amounts of alcohol blunts all the senses. In a
drunken man the senses of the skin are so numbed that he does not know
when anything touches him, and he may be badly burned before he feels
the pain.
Heavy drinking makes the hearing less keen, enlarges the blood vessels
of the eyes, and makes them appear red and bloodshot.
=Tobacco and the Senses.=--The use of tobacco does not injure the
senses of the skin and usually has no effect on hearing. Both chewing
and smoking, if much practiced, make the sense of taste less delicate,
so that one cannot enjoy his food to the fullest extent.
Much smoking of tobacco may hurt the nerve of sight and in a few cases
it has made men blind. Many boys have weakened their eyes by the use
of cigarettes.
PRACTICAL QUESTIONS
1. Name the chief sense organs.
2. Of what use are the eyelids and tears?
3. Name four parts of the eyeball.
4. What is the iris?
5. Of what use is the lens?
6. What moves the eyeball?
7. When do children get weak eyes?
8. How are the eyes often hurt?
9. How may poor eyes be helped?
10. What makes the eyes sore?
11. How do germs get into the eyes?
12. Name the three parts of the ear.
13. What does the inner ear contain?
14. What may result from neglecting a sick ear?
15. Of what use is smell?
16. Why should food be well chewed?
17. In what part of the skin are most of the nerve endings?
18. What effect does tobacco have on the sense of taste?