If the dust is too irritating, or the air is foul and contains disease germs, these set up an inflammation in the nose, and we "catch cold," as we say. If we keep on breathing bad or dusty air, the walls of the nasal pa.s.sages become permanently thickened and swollen; the mucus, instead of being thin and clear, becomes thick and sticky and yellowish, and we have a catarrh.

Catarrh is the result of a succession of neglected "bad colds," caused, not by fresh, cold air, but by hot, stuffy, foul air containing dust and germs. The best and only sure way to avoid catarrh is by breathing nothing but fresh, pure air, day and night, keeping your skin clean and vigorous by cool bathing every day, and taking plenty of play in the open air.

So perfect is this heating, warming, and dust-cleansing apparatus in the nose, that by the time quite cold air has pa.s.sed through the nostrils, and got down into the back of the throat, it has been warmed almost to the temperature of the body, or blood-heat, and has been moistened and purified of three-fourths of its dust or disease germs. When you go out of doors on a cold, frosty morning, your nose is very likely to block up, because so much hot blood is pumped into these little steam-coils of blood vessels, in order to warm the air properly, that they swell until they almost block up the nostrils.

The Sense of Smell. The lower three-fourths of the nasal pa.s.sages have nothing whatever to do with the sense of smell; this is found only in the highest, or third, division of the pa.s.sages, right up under the root of the nose, where odors can readily rise to it. Here can be found a little patch of mucous membrane of a deep yellowish color, which is very sensitive to smells, and from which a number of tiny little nerve twigs run up to form the nerve of smell (_olfactory nerve_), which goes directly to the brain. The position of the smell area at the highest and narrowest part of the nose pa.s.sage explains why when you have a very bad cold, you almost lose your sense of smell; the lining of the lower part of the nose has become so inflamed and swollen as to block up the way to the highest part where the smelling is done.

[Ill.u.s.tration: ADENOIDS

A section through the nose and mouth: _A_, adenoid growth; _P_, soft palate; _T_, right tonsil.]

Adenoids. If colds are neglected and allowed to run on, the inflammation spreads through the nose back into the upper part of the throat, or pharynx. Here it attacks a spongy group of glands, like a third tonsil, which swells up until it almost blocks up the nose and makes you breathe through your mouth. These swollen glands are called adenoids, and cause not only mouth-breathing, but deafness, loss of appet.i.te, indigestion, headache, and a stupid, tired condition; so that children that are _mouth-breathers_ are often two or more grades behind in school, poor students, and even stunted and undersized. You can often tell them at sight by their open mouths and vacant, stupid look. A very simple and harmless sc.r.a.ping operation will remove these adenoids entirely, and what a wonderful improvement the mouth-breather will make!

He will often catch up two grades, and gain two inches in height and ten pounds in weight within a year.

[Ill.u.s.tration: MOUTH-BREATHERS

Note how swollen the face is under the eyes and how tired and dull the whole expression.]

Adenoids not only cause deafness by blocking up the tube (_Eustachian_) that runs from the throat to the ear,--the tube through which the air pa.s.ses when your ear "goes pop,"--but are also the commonest cause of ear-ache and gatherings in the ear, which may burst the drum.

THE TONGUE

The Tongue is not Used chiefly for Tasting. If you will notice the next time that you have a bad cold, you will find that you have almost lost your sense of taste, as well as of smell, so that everything tastes "flat" to you. This ill.u.s.trates what scientists have known for a long time, but which seems very hard to believe, that two-thirds of what we call taste is really smell. If you carefully block up your nostrils with cotton or wax, so that no air can possibly reach the smell region at the top of them, and blindfold your eyes, and have some one cut a raw potato, an apple, and a raw onion into little pieces of the same size and shape, and put them into your mouth one after the other, you will find that it is difficult to tell which is which.

The only tastes that are really perceived in the mouth are bitter, sweet, sour, and salty; and even these are perceived quite as much by the roof and back of the mouth, especially the soft palate, as they are by the tongue. All the delicate flavors of our food, such as those of coffee or of roast meat or of freshly baked bread, are really smells.

The tongue, which is usually described as the organ of taste, is really a sort of fingerless hand grown up from the floor of the mouth--to help suck in or lap up water or milk, push the food in between the teeth for chewing, and, when it has been chewed, roll it into a ball and push it backward down the throat. It is not even the chief organ of speech; for people who have had their tongues removed on account of cancer, or some other disease, can talk fairly well, although not so clearly as with the whole tongue.

The tongue is simply a "tongue-shaped" bundle of muscles, covered with a thick, tough skin of mucous membrane, dotted all over with little k.n.o.b-like processes called _papillae_, which are of various shapes, but of no particular utility, except to roughen the surface of the tongue and give it a good grip on the food. If the mucous "skin" covering the tongue does not shed off properly, the dead cells on its surface become thickened and whitish, and the germs of the mouth begin to breed and grow in them, forming a sort of mat over the surface. Then we say that the tongue is badly coated. This coating is in part due to unhealthy conditions of the stomach and bowels, and in part to lack of proper cleaning of the mouth and teeth.

The Sense of Taste can usually be Trusted. Since the nose and the tongue have had about five million years' experience in picking out what is good and refusing what is bad, their judgment is pretty reliable, and their opinion ent.i.tled to the greatest respect. As a general thing, those things that taste good are wholesome and nutritious; the finest and most enjoyable flavors known are those of our commonest and most wholesome foods, such as good bread, fresh b.u.t.ter, roast meats, apples, cheese, sugar, fruit, etc.; while, on the other hand, those things that taste bad or bitter or salty or sour, or that we have to learn to like, like beer or pickles or strong cheese or tea or coffee, are more often unwholesome or have little nutritive value. Very few real foods taste bad when we first try them. If we used our noses to test every piece of food that went into our mouths, and refused to eat it if it "smelt bad,"

we should avoid many an attack of indigestion and ptomaine poisoning. It is really a great pity that it is not considered polite to "sniff" at the table.

THE EYE

How the Eye is Made. Next in importance after the smell and the taste of our food comes the appearance of it; hence, our need of eyes to help us in choosing what to eat, as well as how to avoid the dangers about us.

The eyes began as little sensitive spots on the surface of the head.

Like the nose pits, as they became more sensitive, they too sank in beneath the surface; but with this difference, that, instead of remaining open, the rims or edges of the eye-pit grew together and became transparent, forming a cover, or eye-gla.s.s, which became the clear part of the eye, called the _cornea_. At the same time, the little sensitive spot at the bottom of the eye-pit spread out into the shape of the bottom of a cup, called the _retina_; and then the hollow of that cup between the retina and the cornea filled up with a clear, soft, animal jelly called the _vitreous humor_, and we have the eye as it is in our heads to-day.

The sensitive retina, spreading out, as it does, to form the back of the eyeball, is the nerve-coat of the eye; and from its centre a thick round bundle of nerve fibres, known as the _optic nerve_, runs back to the brain.

[Ill.u.s.tration: THE APPARATUS OF VISION

A cross-section diagram, showing eye and optic nerve, the bones forming the orbit or socket, and the front lobes of the brain.]

The bones of the head, grown out in a ring in order to protect the eyes, are called the _orbit_ or _socket_.

To protect the delicate gla.s.s (cornea) of the eye, there are two folds of skin, one above and one below, known as the eyelids. The eyelids carry a row of extra long hairs at their edges, called the eyelashes, and a number of little glands, somewhat like those of the stomach, to pour out a fluid, which makes the lids glide smoothly over the eyeball and keeps them from sticking together. Underneath the upper lid a number of these glands become gathered together and "grow in," after the fashion of the salivary glands, to form a larger gland about the size of a small almond, which pours out large amounts of this fluid as tears.

It is called the tear gland (_lachrymal_ gland).

Whenever a cinder or a grain of sand or a tiny insect or any other irritating thing gets into the eye, this gland pours out a flood of tears, which washes the intruder down into the inner corner of the eye where it can be wiped out; or, if it be small enough, carries it down through a little tube in the edge of each eyelid, through a little pa.s.sage known as the _nasal_, or _tear, duct_, into the nose. So, if you get anything into your eye, much the best and safest thing to do is to hold the lids half shut, but as loose, or relaxed, as possible, and allow the tears to wash the speck of dust down into the inner corner of the eye. If you squeeze down too hard with the lids, and particularly if you rub the eye, you will be very likely to scratch the cornea with the speck of dust or sand, or, if the speck be sharp-edged, to drive it right into the cornea and give yourself a great deal of unnecessary pain and trouble, or even seriously damage the eye. If the cinder or dust doesn't wash down quickly, pull the upper lid gently away from the eyeball by the lashes and hold it there a minute or so, when often the cinder will drop or wash out.

As the light rays cannot be bent, or drawn into the eyes as smells can into the nostrils, it is necessary that the eyes should be able to roll about so as to turn in different directions; and so nature has made them round, or globular, attaching to their outer coat or sh.e.l.l (the _sclerotic_ coat) little bands of muscle, each of which pulls the eyeball in its particular direction. There are four straight bands--one for each point of the compa.s.s: one fastened to the upper surface of the eye to roll it upward; another to the lower to roll it downward; another to the outer to roll it outward; and another to the inner side to roll it inward for near vision.[29]

There is another reason for the rounded shape of the eye--that it may act as a lens in condensing the rays of light. In order that we may see things clearly, the rays of light must be brought to a focus upon or close to the retina, at the back of the eye; and our eyes are so shaped that they form a lens of proper thickness, or strength, to do this.

You can see how this is done with an ordinary magnifying gla.s.s, or burning-gla.s.s. The little sharply lighted and heated point to which the light-rays can be brought is the focus of the lens, and the distance it lies behind the lens is called the focal distance. The thicker the lens, or burning-gla.s.s, is in the middle, the shorter its focal distance, and the more strongly it will magnify.

A healthy, or normal, eye is of just such shape and "bulge" that rays of light entering the eye are brought to a focus on, or close to, the retina at the back of the eyeball. Some people, however, are unfortunately born with eyes that are too small and flat, or do not "bulge" enough; and then the rays of light are focused behind the retina instead of upon it, and the image is blurred. This is known as "long sight" (_hyperopia_), and can be corrected by putting in front of the eyes lenses of gla.s.s, called spectacles, which bulge sufficiently to bring the rays to focus on the retina.

An eye that is too large and round and bulging brings the rays to a focus in front of the retina, and this also blurs the image. This form of poor sight is called "short sight" (_myopia_), and can be relieved by putting in front of the eye a gla.s.s that is concave, or thinnest in the middle and thickest at the edges, in the right proportions to focus the image where it belongs, right on the retina. This kind of gla.s.s is sometimes called a "minifying" gla.s.s, from the fact that it makes objects seen through it look smaller. It is also called a "minus" gla.s.s, while the magnifying gla.s.s is called a "plus" gla.s.s. The shape of the gla.s.ses or spectacles prescribed for an eye is just the opposite of that of the eye. If the eye is too flat (_long-sighted_), you put on a bulging, or convex, gla.s.s; and if the eye is too bulging (_short-sighted_), a hollow, or concave, gla.s.s. Other eyes are irregularly shaped in front and bulge more in one direction than another, like an orange. This defect is called _astigmatism_ and is very troublesome, making it hard to fit the eye with gla.s.ses, as the gla.s.ses have to be ground irregular in shape.

[Ill.u.s.tration: A SCHOOL EYE-TEST

A normal eye should be able to read the smaller type easily at a distance of twenty feet.]

We have just seen how the eye deals with rays of light coming from a distance, which are practically parallel. When, however, books or other objects are brought near the eye, the rays of light coming from them do not remain parallel, but begin to spread apart, or diverge; and a stronger lens is required to bring them to a focus upon the retina. To provide for this, there is in the middle of the eyeball a firm, elastic, little globular body about the size and shape of a lemon-drop, called the _crystalline lens_. Around this is a ring of muscle, which is so arranged that when it contracts it causes the lens to change its shape and become more bulging, or thicker in the middle. This makes the eyeball a "stronger" lens so that the rays of light can be brought to a focus upon the retina.

This action is known as _accommodation_, or adjustment; and you can sometimes feel it going on in your own eye, as when you pick up a book or a piece of sewing and bring it up quickly, close to the eye, in order to see clearly.

If this little muscle is worked too hard, as when we try to read in a bad light, it becomes tired and we get what is called "eye-strain"; and if the strain be kept up too long, it will give us headache and may even make us sick at the stomach. The commonest cases of eye-strain are in eyes that are too flat (_hyperopic_) where this little muscle has to "bulge" the lens enough to make good the defect and bring the rays to a focus. This, however, of course keeps it on a constant strain; and the eye is continually giving out, and its owner suffering from headache, neuralgia, dyspepsia, sleeplessness, and other forms of nervous trouble, until the proper lens or spectacle is fitted.[30]

A surface as delicate and sensitive to light as the retina, would, of course, be damaged by too bright a glare; so in the front of the eye, just behind the cornea, a curtain has grown up, with an opening or "peep-hole" in its centre, which can be enlarged or made smaller by little muscles. This opening is the _pupil_; the curtain, which is colored so as to shut out the rays of light, is known as the _iris_, for the quaint, but rather picturesque, reason that _Iris_ in Greek means "rainbow," and this part of the eye may be any one of its colors.

[Ill.u.s.tration: DISINFECTING A BABY'S EYES AT BIRTH]

It is the iris which, according to the amount of coloring matter (pigment) in it, makes the eye, as we say, blue, gray, green, brown, or black. Blue eyes have the least; black, the most.[31]

The Care of the Eyes. The most dangerous diseases of the eye are caused by infectious germs, which get into them either from the outside, as in dust, or by touching them with dirty fingers; or through the blood, as in measles, smallpox, tuberculosis, and rheumatism. The more completely we can prevent these diseases, the less blindness we shall have in the nation. About one-sixth of all cases of blindness in our asylums is caused by a germ that gets into babies' eyes at birth, but can be done away with by proper washing and cleansing of the eyes.

THE EAR

Structure of the Ear. Next after sight, hearing is our most important sense; without it, speaking, and consequently reading and writing, would be impossible. Man learned to speak by hearing the sounds made by other people and things, and then by listening to his own voice and practicing until he could imitate them. Children who are unfortunate enough to be born _deaf_ also become _dumb_, not because there is anything the matter with their voice organs, but simply because, as they cannot hear the sounds they make, they do not form them by practice into words and sentences. By proper training, deaf mutes can now be taught to speak, though their voices sound flat and "tinny," like a phonograph.

As in the nose and the eye, the important part of the ear is the nerve spot that can "feel" the air waves that we call sound, just as the retina "feels" light. It is from this sensitive spot that the _auditory nerve_ carries the sound to the brain. This spot has grown into quite an elaborate structure, buried, for safety, deeply in the bones of the skull, close to the base of the brain. It is made up of a long row of tiny little nerve rods, laid side by side like the keys of a piano, only there are about three thousand of them. Each one of these is supposed to respond, or vibrate, to a particular tone, or sound. This keyboard, from the fact that, to save s.p.a.ce, it is coiled upon itself like a sea-sh.e.l.l, instead of running straight, is called the _cochlea_ (Greek for "snail-sh.e.l.l"); it is also called, because it is the deepest, or innermost, part of the hearing apparatus, the _internal ear_.

Just as the retina has a lens and a vitreous humor in front of it to act upon the light, so the internal ear has an apparatus in front of it to act upon the sound waves. This is called the _drum_ (_tympanum_). It consists of a fold of thin, delicate skin stretched tightly across the bottom of the outer ear ca.n.a.l, as parchment is stretched across the head of a drum. If you should take a hand-mirror--best a hollow, or concave, one--and throw a bright ray of light deep into some one's ear, you would be able, after a little trying, to see this drum-skin stretched across the bottom of it and about an inch and a quarter in from the surface of the head.

[Ill.u.s.tration: THE APPARATUS OF HEARING

A cross-section diagram from the outer ear to the lobes of the brain.]

When the sound waves go into the ear ca.n.a.l and strike upon this tiny drum, which is about two-thirds the size of a silver dime and really more like a tambourine or the disk of a telephone or phonograph than a drum, they start it thrilling, or vibrating, just as a guitar string vibrates when you thrum it. These little vibrations are carried across the hollow behind the drum by a chain of tiny bones, known as the _ear-bones_ (called from their shapes, the _hammer_, the _anvil_, and the _stirrup_), and pa.s.sed on to the keyboard of the cochlea.