Pain is useful in telling us that some organ is out of order and needs care. Killing the pain does not help the sick organ, and it may let the organ get so sick as to cause death.

One use of the nerves is to tell us when any part of the body is hurt or sick. Pain is nature's warning, and to numb the nerves which tell us about it is as foolish as to kill a person because he brings us bad news. _No medicine should ever be given children to make them sleep or stop their crying except by the advice of the physician._

=Powders and Pills.=--If you get sick, do not try to cure yourself with pills or powders bought at the store. Some of these medicines contain poisons which hurt the heart or other organs. A number of persons have been killed by taking such medicines. When you are sick, go to a good doctor who understands how the organs should work, and he will find which one is out of order and tell you exactly what medicine you need and what to eat in order to get well quickly.

=Tea and Coffee.=--These drinks usually wake up the brain and make it work better for a time. If too much of them is used, they may excite the brain in such a way as to make persons nervous. If taken for supper, they may prevent sleep. Children should not use either tea or coffee. Tea sometimes disturbs digestion, and coffee may injure both the stomach and the heart.

PRACTICAL QUESTIONS

1. What is a narcotic?

2. Name some narcotics.

3. What is a stimulant?

4. Name some stimulants.

5. How long before alcohol taken reaches the brain?

6. What effect does strong drink have on the brain?

7. Does alcohol help us think better?

8. What facts show that alcohol sends men to prison?

9. What shows that alcohol makes the mind sick?

10. Why is it dangerous for the young to take strong drink?

11. What shows that tobacco makes the brain work slower?

12. Why should you not use opium or morphine?

13. What do pain killers contain?

CHAPTER XXII

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.

[Ill.u.s.tration: 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 gla.s.s. 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 pa.s.s 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.

[Ill.u.s.tration: 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 k.n.o.b, is fixed just behind the pupil. In front of the lens is a _watery fluid_ and behind it is a clear _jellylike ma.s.s_. 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 gla.s.ses, 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 gla.s.ses 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 pa.s.sed 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.

[Ill.u.s.tration: 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 pa.s.s 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.