How We Learn What Goes on about Us.The nervous system not only keeps all the different parts of the body working together, but constantly regulates and alters their activities to meet conditions in the world outside. We are all the time looking up to see something, reaching out to grasp something, listening to hear something. How does the nervous system get the messages from the outside world which enable it to do this work so quickly and accurately?
Have you ever noticed the queer things that children do when they are playing blindman’s buff or trying blindfolded to pin the tail on the donkey? Have you ever tried to do for five minutes with your eyes shut any of the things you do in your daily life? It is easy to see in this way how dependent we are upon our eyes. But the eyes are not the only doors through which messages enter the brain. With our eyes shut, we can still hear, feel, taste, and smell. We can hardly imagine what the world would be like to us, if all these senses were taken away.
There are many different senses by which we learn what is going on about us, but five of them are so much more familiar than the rest that they are generally called the five senses. These are sight, hearing, taste, smell, and touch.
The Senses Compared to the Telephone.In the ordinary telephone, there is at one end an instrument called the transmitter, into which a person speaks, and at the other end an instrument called the receiver, which another person holds to his ear. A wire runs between these two parts. In our bodies, the brain and spinal cord correspond to the receiver; the sense organssuch as the eyes and earsare like the transmitter; and the nerves run between these parts, like the wires. The sense organs respond to light or sound or some other influence from outside, and pass on a message that is sent up along the nerve to the spinal cord or brain.
The Structure of the Eye.The eyes are in some ways the most beautiful and most useful of all the sense organs of the body. They teach us the greater part of what we know about the world. Through them we perceive the beauty of sun, trees, and flowers, and through them we are able to learn what people in other ages have thought and done, by looking at the things they have made and reading the books they have written.
The eye (as shown in Fig. 72) is somewhat like a camera. In the front is a transparent lens, which makes a little picture at the back of the eye, just as the lens in the camera makes a picture on the film or plate, by bending the rays of light that pass through it. The lens in the eye is not hard like glass, however, and its shape can be changed so as to make it flatter or rounder, by the pull of tiny muscles and membranes attached to it.
At the back of the eye is the retina, which corresponds to the sensitive plate in the camera. There the light sets up chemical changes that make the picture. Color blindness, or the inability to distinguish certain colors (usually red and green), is due to the lack of substances which are found in the normal retina.
In the retina, the picture of what we are seeing is changed into a nerve message which passes to the brain by way of the large optic nerve. By means of this nerve message, and the connections that it makes in the brain, we know what the picture is that is formed on the retina. It is with the brain that we really see consciously, and in the brain the messages brought from the eye are passed on from one nerve cell to another till they lead to action. The picture on the retina would be nothing but a dead picture without the nerve and brain behind it.
The most important parts of the eye, the lens and the retina, are inclosed in a nearly spherical eyeball. There are three coats or layers in the wall of the eyeball, the sclerotic (skle rot’ik) coat, the choroid (ko’ roid) coat, and the retina itself.
The sclerotic is the tough outside coat which incloses the whole eyeball, except where the nerve passes in at the back. It is white in color, except in the middle of the front of the eye, where it becomes transparent. This transparent window is the cornea. – The part of the sclerotic coat around the cornea is what we call the white of the eye.
Inside the sclerotic lies the choroid coat, which has a circular opening in front, opposite the middle part of the cornea. The part of the choroid coat around this opening forms a kind of curtain, called the iris, which can be expanded so as to make the central opening smaller, or drawn back so as to make it larger. It is the iris which we see as a colored ring when we look into a person’s eye (the part which makes the eye black or blue or brown). The dark central opening, the pupil, is the opening behind the cornea by which the light passes to the lens.
The large space behind the lens is filled with a trans-parent jelly-like substance called the vitreous humor; the space between the lens and the cornea with a watery fluid, the aqueous humor.
How the Eye Accommodates Itself to Changing Conditions.–In a dark room the iris is drawn back so that the pupil is very large. In a bright light it shuts in, as if it were drawn by a puckering string on the inside edge, and makes the pupil smaller and smaller. In this way the amount of light that enters the eye is regulated. When you go into a dark room, you cannot see clearly till the iris has had time to open and let in more light.
If you have ever taken photographs or watched some one else taking them, you know that the camera has to be focused; that is, it has to be adjusted a little, according to the distance of the thing to be photographed. When you have taken a picture of an object near at hand and then want to take one of a building some distance away, you have to focus, by moving a screw which changes the distance between the lens and the plate. The eye has to do some-what the same thing. Look at a pencil a few inches from your eyes and then look quickly to the other end of the room. For a second, while your eye is focusing, everything will look blurred.
In the case of the eye, this focusing is done, not by changing the distance between the lens and the retina, but by a change in the shape of the lens itself. When the eye is at rest or looking at something a long way off, the lens is quite flattened; but when we look at an object near at hand, tiny muscles in the eye contract in such a way that the lens becomes more rounded. Rays of light are bent by a flat lens so as to make a picture of far-off objects, and by a more curved lens, so as to make a picture of things near at hand. This adjustment of the eye which is necessary when we change from looking at things far off to things near by, or the reverse, is called accommodation.
The Protection of the Eye: The eye is very carefully protected by a deep socket-setting with a bony ring around it, by a soft lining of fat, and by the eyebrow above. It is also protected by the eyelid, a fold of skin which helps to keep out too much light, and which closes instantly at any sudden motion that seems to threaten injury. The eye-lashes help to guard the eye from dust.
The Lachrymal Gland.In the outer corner of each eye is the lachrymal gland, which pours out a liquid to wash the eye and keep it clean. Ordinarily this liquid is formed very slowly, and after passing across the eye is carried off by a small channel into the back of the nose. Sometimes, if a person is angry or hurt or sad, a reflex nervous action causes the lachrymal gland to secrete very rapidly. The liquid gathers in the eye and overflows, and we call it tears.
The Function of Eyeglasses.The eye is a complicated organ and in many people it does not work perfectly. In order that we may see clearly, the lens must focus, or form its picture, exactly on the screen of the retina. In some eyes the lens is curved too little, or is too close to the retina. People with such eyes cannot see things near at hand without effort, in spite of the contraction of the eye muscles; we call such people far-sighted.
Other people have lenses that are curved too much. They can read a book in their hands but cannot see a blackboard across the room clearly. Such people are said to be near-sighted.
Another kind of eye trouble, called astigmatism, is caused by an irregular shape of the cornea or of the lens, which bends the light rays so that they do not come to a clear focus.
Sometimes a child seems to be stupid just because his eyes have some of these defects and he does not see clearly. If one keeps on trying to see with eyes that do not focus well, the muscles of accommodation are constantly strained, and bad headaches or nervousness or other troubles may result. All such difficulties may generally be cured at once by the use of eyeglasses. The lenses in the eyeglasses are shaped so as to correct the defects of the cornea and lens by bending the light rays so that they will make a clear picture. A good oculist can find out just what is wrong with the eyes and have a pair of glasses made to correct the trouble. With the right kind of glasses, a person can see clearly and without constant strain on the focusing muscles.
The use of eyeglasses often makes a wonderful change in a child’s life, makes him, better at school work and in games, and cures him of headaches and other troubles that were due
to weak eyes. If you have to hold a book very close to your eyes when you read, if the writing on the blackboard does not look distinct to you, if you have headaches, if your eyes hurt you after you have been reading for a while, or if they are red and inflamed, you should have them examined.
The examination of the eyes should always be made by a physician trained in this work (an oculist) and not in an eyeglass shop, for only a physician can do the work properly. Eyeglasses that are not just right will do more harm than good and may seriously injure the eyes. It is a good plan, as a precaution, to have the eyes examined about once a year, whether they are known to give trouble or not.
Examinations of the eyes of children in certain schools have shown that one out of every five needs eyeglasses.
Some of the Ways in Which Eyes are Strained. Whether we wear glasses or not, it is important to remember not to strain the eyes by using them too long at a time for close work, or by trying to read, sew, or do any other fine work in a poor light. Children often hurt their eyes by reading in the late afternoon when the light is failing. Too bright a light may be just as bad as too dim a light; and it is harmful to read with sunlight or lamplight glaring into the eyes or reflected directly from the paper. If you cannot see the writing on the blackboard clearly or if there is a glare of light in your eyes, you should ask to have the shades drawn up or down, or perhaps to have your seat changed.
When one is reading, writing, or sewing, the light should come from the left side and from above, because in this way less shadow is cast on the work by the active right hand. The book or work should always be held at least twelve inches away from the eyes.
A flickering, unsteady light is very trying, and reading on trains or cars is often harmful for this reason. We should not read when lying down, for this makes the eyes work in an unnatural position and always strains them.
Directions as to the removal of cinders or other foreign bodies which may get into the eye will be found on page 243.
The Ear and Its Functions.When a stone is thrown into the water, waves spread out from it in all directions. When a person calls out, when a bell rings, or when any other noise is made, waves of air are produced which are some-what like these water waves. They are called sound waves. We cannot see them with the eyes or feel them with the hands, as in the case of the water waves, but the ear is made in such a way that it is affected by them. There are only certain kinds of sound waves that the ear can catch, and people vary somewhat in their power of detecting them. The cry of the bat, for instance, is so shrill that some people cannot hear it at all.
The ear which we see on the outside of the head is just a kind of trumpet to catch the sound waves, and is not the real organ of hearing. It opens into a tube, at the end of which is a thin membrane called the ear drum. On the other side of the ear drum is a space called the middle ear, and beyond this is the inner ear, another curiously shaped space filled with liquid.
When sound waves come into the outer ear, they make the delicate ear drum quiver. This moves a chain of three little bones that stretch from the drum across the middle ear. These bones carry the motion along to the fluid in the inner ear, where the nerves of hearing are situated, and where the movement of the fluid causes messages to pass along these nerves to the brain.
From the middle ear there is an opening called the Eustachian tube, which enters the back of the throat. After a cold in the head, germs sometimes work their way up through this tube into the middle ear and cause serious disease and sometimes deafness. Pain or rumbling in the ears or discharge from the ears are signs of danger, and indicate that a good physician should be consulted. If a person has any reason to suspect that his hearing is not good, the source of the trouble should at once be sought.
Wax in the ears is entirely normal. It is formed by small glands in the outer ear and helps to guard the approach to the ear drum. It should not be removed except when too much forms, and then only by a physician, as digging into the ear with pointed instruments may injure the delicate drum.
The Organs of Taste and Smell.There are many other organs of sensation in the body, much smaller and less familiar than the eyes and ears,the organs of taste and smell, for example:
If you look at your tongue closely in the glass, you will see many little ridges like tiny mountains and valleys covering the whole upper surface. The organs of taste, called the taste buds, are situated in these little valleys. They are tiny rounded masses of cells, sensitive to the taste of certain sub-stances. The organs of smell are similar groups of sensitive cells with nerve connections, situated in the upper part of the nose. They are affected by different substances from those which affect the taste buds.
It is with the sense organs of the tongue that we taste and distinguish sweet and salty things, sour and bitter things; but most of the flavors of food which we call “tastes” are really smells, perceived by the sense organs in the nose. (You remember that the nose and throat are directly connected by an opening at the back of the throat.) If you hold your nose so that no air can get up to these sense organs, most of the flavors of the things you eat will not be noticed at all. Sometimes during a severe cold in the head, when there is difficulty in breathing through the nose, these so-called tastes, which are really smells, are almost lost.
The organs of smell are perhaps the most marvellously delicate sense organs of the body. Particles carried by the air, so small that they cannot be seen, upon coming in con-tact with these organs of smell will make one aware of odors. A bowl of dried rose leaves may stand in a room for a long time without losing enough of their substance to be detected by weighing on the most delicate scales, yet through all that time microscopic amounts of fragrant substances have been given off from the rose leaves and have affected the sense organs of the nose, so as to send an impulse up the nerves and produce in the brain the sensation of their characteristic odor.
Other Sense Organs.Scattered over the surface of the body are tiny end organs of the nerves with which we feel the touch of things, and others by which we distinguish heat and cold.
These three kinds of sense organs appear to be distinct, some perceiving heat only and some cold only, while others serve for what we ordinarily call feeling. If careful studies are made of the power to feel very small hot and cold objects, it is found that only certain areas of the skin (having cold sense organs) are sensitive to cold, while others (having heat sense organs) are sensitive to heat. Sense organs of touch are particularly numerous in some parts of the skin. The ends of the fingers, for instance, have a delicate sense of touch, but on the back and shoulders the sense of touch is very imperfect. If two blunt points an inch apart are placed on the back, you cannot tell whether two points or one are touching you.
There are various other sensationssuch as the sensations of position, pain, hunger, and thirstall of which are felt by means of nerves connected with special sense organs in various parts of the body.
QUESTIONS FOR DISCUSSION AND REVIEW
1. By what means do we learn what is going on in the world about us?
2. Try this game. Place a person at one side of a large room, and tell him to look directly across at a certain point on the other wall. While he is looking, blindfold him, and then have him walk to the point indicated. Watch and see how reliable a guide his muscles are. If some one stood at the point of destination and spoke to the blindfolded person, what difference would it make?
3. Name the special sense organs. Which of them seems to you the most valuable?
4. What is the name of the outer coat of the eye? The middle? The inner? Describe each.
5. Stand beside a person and look across the front of his eye. What is the transparent, slightly convex window which you see?
6. What part of the eye regulates the amount of light that enters the eye? To what part of a camera does this correspond?
7. Why is it that when you first go from a brightly lighted room into a dark room you cannot see so well as you can after you have been in the dark room a few moments?
8. Have you ever tried “bending” a ray of light with a mirror? With a prism? A lens? What happens to the rays of light when you start a fire with a burning glass?
9. In a camera, what does the lens do? How do you focus the lens so that the picture is formed clearly on the plate or film?
10. Where must the picture be formed in the eye, in order that it may be seen clearly?
11. How is the lens in the eye changed in focusing for different distances?
12. Stand at a window and look at the glass, or better still at the window screen. Then look in the same direction at some point as far away as possible. See how the screen, or any mark on the glass, disappears. Look alternately at the screen and away, until you can feel the difference. What is happening in your eye? What name do we give to it?
13. What must happen in the eye and behind it, in order that the brain may be conscious of the picture on the retina?
14. In what ways is the eye protected from injury?
15. What is the function of tears? Where do they come from and where do they go?
16. What do we mean by near-sightedness? Far-sightedness? Astigmatism? What are glasses for?
17. How can poor eyes make a child seem stupid? Does a child always know that he is not seeing as well as he should? How may such cases be tested?
18. What are some of the indications of eyestrain? Should we wait for eyestrain to appear before we care for the eyes?
19. If a person has to bring his work nearer than twelve inches from his eyes, what is probably the matter?
20. Why is it harmful to the eyes to read on a train? By firelight?
21. Make a set of rules for the care of the eyes. Also a set of “Don’ts.”
22. What is sound? Do sounds exist which the human ear cannot hear?
23. What is the ear that we see on the outside of the head? What separates the outer ear from the middle ear?
24. How do the vibrations from the air reach the middle ear? Explain the arrangement whereby these vibrations reach the nerves of hearing and, finally, the brain.
25. What is the Eustachian tube? How does it sometimes become a source of danger?
26. Of what use is the sense of taste? Where are the organs of taste located? What influence does taste have on digestion?
27. Of what practical use is the sense of smell? How do the senses of taste and smell overlap one another? Give examples of so-called sensations of taste which are really sensations of smell.
28. How do blind people make the sense of touch take the place of sight? Find out all you can about Helen Keller. She was blind, deaf, and dumb because she could not hear, but she learned to talk, and has gained a good education.