First Book In Physiology And Hygiene Part 16

CHAPTER XXV.

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 _ca.n.a.l_ 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 ca.n.a.l of the ear there is stretched a thin membrane like the head of a drum. The ear-drum is also called the _middle ear_.

[Ill.u.s.tration: 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 sh.e.l.l.

[Ill.u.s.tration: 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-sh.e.l.l 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.

[Ill.u.s.tration: 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 gla.s.s. It is about the shape of a watch crystal, only very much smaller. This is to the eye what the gla.s.s 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 gla.s.ses 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.)

[Ill.u.s.tration: 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 pa.s.s 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 gla.s.s 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 gla.s.s 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 spygla.s.s or an opera-gla.s.s? 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 gla.s.ses to see objects near by, as in reading. Children should not try to wear old persons' gla.s.ses, 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 pa.s.sing 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.

[Ill.u.s.tration: 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.