Life is Shown by Breathing. If you wanted to find out whether a little black bunch up in the branches of a tree were a bird or a cluster of leaves, or a brown blur in the stubble were a rabbit or a clod, the first thing you would probably look for would be to see whether it moved, and secondly, if you could get close enough without its moving away, whether it were breathing. You would know perfectly well if you saw it breathing that it was alive, and that, if it were not breathing at all, it would probably be dead, or very nearly so.

Why is breathing so necessary to life that it lasts practically as long as life does, and when it stops, life stops too? Animals can stop eating for days, or even weeks, and yet live, especially if they were fairly fat when they began to fast. Indeed, some animals, like woodchucks, bears, and marmots, will go to sleep in the fall, and sleep right on through to spring without eating a mouthful. But if any animal or bird is prevented from breathing for three minutes, it will die.

Short Storage Supply of Air. There is a difference between the kind of things that you take in when you breathe and the kind of things you take in when you eat or drink. Food and drink are solids and liquids; and the body is a great sponge of one soaked full of the other, so that large amounts of food and water can be stored up in the body. But what you take in when you breathe is, of course, air—which is neither a solid nor a liquid, but a gas, very light and bulky. Of gases the body can soak up and hold only a very small amount; so its storage supply of them will be used up completely in about three minutes, and then it dies if it cannot get more air.

Why our Bodies Need Air-Oxidation. The body is made up of millions of tiny living animals called cells, which eat the food that is brought to them from the blood and pour their waste and dirt back again into the same current. Now, what would happen if we were to throw all the garbage from the kitchen, and the wash water from the kitchen sink, and the dirty water from the bathroom right into the well out of which we pumped our drinking water? We should simply be poisoned within two or three days, if indeed we could manage to drink the disgusting mixture at all. That is exactly what would happen to our body cells if they were not provided with some way of getting rid of their waste and dirt.

Part of the waste that comes from our body cells is either watery, or easily dissolved in water; and this is carried in the blood to a special set of filter organs—the liver and the kidneys—and poured out of the body as the urine. Another part of it, when circulating through the skin, is passed off in the form of that watery vapor which we call perspiration, or sweat. But part of the waste can be got rid of only by burning, and what we call burning is another name for combining with oxygen, or to use one word—oxidation; and this is precisely the purpose of the carrying of oxygen by the little red blood cells from the lungs to the deeper parts of the body—to burn up, or oxidize, these waste materials which would otherwise poison our cells. When they are burnt, or oxidized, they become almost harmless.

Why the Red Cells Carry only Oxygen to the Body. But why do not the red cells carry air instead of just oxygen? This is simply a clever little economy of space on nature's part. As a chemist will tell you the air which we breathe is a mixture of two gases—one called nitrogen and the other oxygen; just as syrup, for instance, is a mixture of sugar and water. Then too, as in syrup, there are different amounts of the two substances in the mixture: as syrup is made up of about one-quarter sugar and three-quarters water, so air is made up of one-fifth oxygen and four-fifths nitrogen. Now the interesting thing about this mixture, which we call air, is that the only really "live" and vital part of it for breathing purposes is the one-fifth of oxygen, the four-fifths of nitrogen being of no use to our lungs. In fact, if you split up the air with an electric current, or by some other means, and thus divide it into a small portion of pure oxygen (one-fifth), and a very much larger portion (four-fifths) of nitrogen, the latter would as promptly suffocate the animal that tried to breathe it as if he were plunged under water.[18]

It may perhaps be difficult to think of anything burning inside of your bodies where everything is moist, especially as you do not see any flame; but you do find there one thing which always goes with burning, and that is warmth, or heat. This slow but steady and never-ceasing burning, or oxidation, of the waste and dirt inside your bodies is what keeps them warm. When you run fast, or wrestle, or work hard, your muscle-cells work faster, and make more waste, and you breathe faster to get in the oxygen to burn this up—in other words, you fan the body fires, and in consequence you get a great deal hotter, and perhaps perspire in order to get rid of your surplus heat.

The Ocean of Air. Where does the blood in the body go in order to get this oxygen, which is so vital to it? Naturally, somewhere upon the surface of the body, because we are surrounded by air wherever we sit, or stand, or move, just as fishes are by water. All outdoors, as we say, is full of air. We are walking, just as fishes swim, at the bottom of an ocean of air some thirty miles deep; and the nearer we get up toward the surface of that ocean, as, for instance, when we climb a high mountain, the lighter and thinner the air becomes. Above ten thousand feet we often have great difficulty in breathing properly, because the air is so thin and weak in oxygen.

How the Lungs Grew Up. In the simplest forms of life, any part of the soft and delicate surface will do for the blood to reach, in order to throw off its load of carbon "smoke" and take on its supply of oxygen. In fact, animals like jellyfish and worms are lungs all over. But as bodies begin to get bigger, and the skin begins to toughen and harden, this becomes more and more difficult, although even the highest and biggest animals like ourselves still throw off a certain amount of this carbon dioxid and other gases through the skin. Accordingly, certain parts of the surface of the body are set apart specially for this business of breathing; and as we already have an opening into the body provided by the mouth and food tube, the simplest thing to do is to use the mouth for taking in air, when it is not being used for taking in food, and to set aside some part of the food tube for breathing purposes.

The lungs sprout out from the front of the gullet, just below the root of the tongue, in the days when we are getting ready to be born. The sprout divides into two, forming the beginning of the pair of lungs. Each lung sprout again divides into two, and each of the two smaller buds again into two, until finally we have the whole chest filled up with a "lung-tree" whose trunk stems and leaves are hollow. The stem of the tree or bush becomes the windpipe (trachea). The first two branches into which it divides form the right and left lung tubes, known as bronchi. The third, fourth, fifth, sixth, etc., divisions, and so on, form what are known as the bronchial tubes. These keep on splitting into tinier and tinier twigs, until they end, like the bush, in little leaves, which in the lung, of course, are hollow and are called the air cells (alveoli). This budding off of the lungs from the gullet is the reason why the air we breathe and the food we swallow go down the same passage. Every mouthful of our food slides right across the opening of the windpipe, which has to be protected by a special flap, or trap-door of gristle, called the epiglottis. If you try to eat and talk at the same time, the epiglottis doesn't get warning of the coming of a swallow of food in time to cover the opening of the windpipe, and the food goes down the wrong way and you cough and choke.

Now, if you will just place your fingers upon the front of your neck and slide them up and down, you will, at once, feel your windpipe—a hard, rounded tube with ridges running across it,—while, no matter how carefully you feel, or how deeply you press, you cannot feel your gullet or esophagus at all. Just take a mouthful of water, however, put your fingers deeply on each side of the windpipe, and swallow, and you will feel something shoot down the esophagus, between your fingers, toward the stomach.

Both of these tubes were made of exactly the same materials to begin with. Why have they become so different? A moment's thought will tell you. One, the gullet, has only to swallow solid food or drink, so that its walls can remain soft, and indeed fall together, except when it is actually swallowing. The other tube, the air-pipe or windpipe, has to carry air, which neither will fall of its own weight, nor can readily be gulped down or belched up. It is absolutely necessary that its walls should become stiff enough to keep it open constantly and let the air flow backward and forward. So we find growing up in the walls of this air pipe, cells which turn themselves into rings of gristle, or cartilage.

What the Breath Is. As you know, your "breath," as you call it,—that is to say, the used-up air which you blow out of your lungs,—is different in several ways from pure, or unused air. In the first place, it is likely to have a slight musky or mousy odor about it. You never like to breathe any one else's breath, or have any one breathe in your face. This dislike is due to certain gases, consisting of impurities from the blood, the cells of the lungs, the throat, the nose, and, if the mouth is open, the teeth. These are not only offensive and disagreeable to smell, but poisonous to breathe.

Then your breath is much warmer than the rest of the air. In fact, on a very cold morning you may have tried to warm up your fingers by breathing on them; and you have also noticed that if a number of people are shut up in a room with doors and windows closed, it soon begins to feel hot as well as stuffy. This heat, of course, is given off from the blood in the lungs and in the walls of the throat and nose, as the air passes in and out again.

When you stand at the window on a cold day, the glass just in front of your mouth clouds over, so that you can no longer see through it; and if you rub your finger across this cloud, it comes away wet. Evidently, the air is moister than it was when you breathed it in; this moisture also has been given off from the blood in the lungs.

But what of the principal waste gas that the blood gives off in the lungs—the carbon "smoke," or carbon dioxid? Can you see any trace of this in the breath? No, you cannot, for the reason that this gas is like air, perfectly clear and transparent, and never turns to moisture at any ordinary temperature. But it has a power of combining with certain other things and forming substances which, because they are combinations of carbon, are called carbonates. The commonest substance with which it will do this is lime. If you take a glass or a bottle two-thirds full of lime water, and breathe into it through a glass tube or straw, you will see in a very few minutes that it is becoming milky or cloudy from the formation of visible carbonate of lime, which, when you get enough of it, makes ordinary limestone. So, although you cannot see, or smell, this carbon "smoke" in your breath, you can readily prove that it is present.

How and Why our Breathing Varies. When you run or wrestle, you breathe faster in order to draw more air into the lungs. At the same time, your heart beats faster in order to drive a larger amount of blood through the lungs. If you run too far, or wrestle too hard, your heart and your lungs both go faster and faster, until finally they reach a point when they cannot go any quicker, and the poisonous waste substances are formed in your muscles faster than they can possibly be burned up, even by the quickest breathing and the hardest pumping of your heart. Then you begin to get "out of breath"; and if you were compelled—in order to save your life, for instance—to keep on running, or fighting, you would at last be suffocated by your own waste and dirt, and fall exhausted, or unconscious.

On the other hand, by carefully training your muscles and your heart and your lungs by exercises of various sorts in the open air, beginning with easy ones and going on to harder and longer ones, you can "improve your wind," so that your heart will be able to pump more blood through the lungs per minute, and your lungs will be able to expand themselves more fully and more rapidly without fatigue.

If you can recall having had a fever of any sort, even a slight one, such as comes with a sore throat or a bad cold, you may remember that you breathed faster and that your heart beat faster, and yet you were not doing any work with your muscles. The cause, however, is the same; namely, the amount of waste that is being produced in the body—in this case, by the poisons (toxins) of the germs that cause the fever. The more waste that is formed in the body, the more effort the heart and lungs will make to try to get rid of it.

The Ribs. How does the air get in and out of the lung tubes? Evidently you do not and cannot swallow it as you would food or drink; and as it will not run down of its own accord when you simply open your mouth, nature has had to devise a special bit of machinery for the purpose of sucking it in and pressing it out again. This she has done in a rather ingenious manner by causing certain of the muscle-rings in the wall of the chest to turn first into gristle, or cartilage, and then later into bone, making what are known as the ribs; these run round the chest much as hoops do round a barrel, or as the whalebone rings did in the old-fashioned hoop skirt. When the muscles of the chest pull these ribs up, the chest is made larger,—like a bellows when you lift the handle,—air is sucked in, and we "breathe in" as we say; when the muscles let go, the ribs sink, the chest flattens and becomes smaller, the air is driven out, and we "breathe out."