Men have killed animals after a long hunt and have found the blood of the arteries so changed in color that it looked like blood from the veins. It was dark and impure because it held an oversupply of carbon dioxid and had lost most of the oxygen.

When we are breathless most of the trouble is due to the fact that the heart is overtaxed by the large quantity of blood sent to it from the hard-working muscles to be forwarded to the lungs to be purified of its carbon dioxid, while at the same time the lungs are also overtaxed by their unusual work.

Those who train for athletic sports learn to keep the balance of the gases in their blood. They know how to manage their running and the work of heart and lungs in such a way that neither will be overtaxed until the end is near. They are willing to be breathless at the very last because they are soon to stop running and catch their breath again. But to get breathless at the beginning of the race means defeat.

The same is true in horseracing. No good jockey lets his horse get out of breath until the last part of the race. At that time, however, the horse is urged to work the muscles of his legs as hard and as fast as possible. It is safe to do this now, for as soon as he reaches the goal his muscles will stop producing such quantities of carbon dioxid and his heart will cease to be overtaxed by its work of pumping this impure blood to the lungs to be purified.

CHAPTER XVI

WHERE BLOOD CHANGES COLOR

Place one hand lightly on your chest; place the other on your back between the shoulder blades; inhale slowly until your lungs are full, then exhale slowly until they seem empty. While you

do this notice that the breastbone rises, and that the front and rear walls of your chest are forced gradually farther apart. While you take another

long breath and send it

MEASURED BY THE DOCTOR

out again stand with your hands resting lightly on each side of the body just over your lower ribs. Notice that it is expansion sideways this time; you also see that the capacity of your chest has increased greatly.

Take a tape measure and get the girth of your chest after you have exhaled all you can, and again after you have drawn in as large a supply of air as your lungs will hold. Learn from these tests that the size of your chest can be increased and diminished at will, and that its size can be increased permanently by frequent exercise of

WITH HIS CHEST EXPANDED

this kind. To prove this in your own case, measure your chest to-day; then for two months take fifteen deep, full breaths three times a day. With each breath expand your lungs as fully as you can without really straining them. At the end of the two months measure yourself again and you will find that your chest measure has increased. From this you have the right to conclude that your lungs also are larger. We often talk of the lungs as if they were a pair of big bags tucked in under the ribs somewhere, waiting to swell out or to sink in according as we use them. In a way the notion of the bag is rather correct, except that instead of two bags, one on each side, we must think of thousands upon thousands of microscopic bags called air sacs. We must recall what we learned in Good Health, and think of each one of these sacs as the expanded tip of a tiny tube that ends in it. We must remember that the tubes themselves are the small twigs of larger tube branches, and that within the

A HOLLOW CHEST

large chamber which the ribs make we have two sets of these branching tubes ending in air sacs. Each set is called a lung. The heart lies between the right and left lungs, and is a trifle more on the left than on the right side.

For the sake of saving time and space a few facts, new and old, must be given under numbered headings. They show how the lungs help us throughout our lives:

that it has gained a bright scarlet color, and we call it pure, as indeed it is. Even in pure arterial blood, however, there is some carbon dioxid.

2. Lungs are at work not because they themselves need air, but because they serve as a storehouse and a place where oxygen and carbon dioxid and carbon dioxid may change places. Such a central exchange is needed because, as we know, here and there over the entire body each smallest tissue is in need of oxygen and must be relieved of its carbon dioxid. It is in the lungs

that blood unloads itself of most of its carbon dioxid, loads itself up with oxygen, and streams off to some distant destination. Breathing, then, is mainly for the benefit of the tissues of the body, not for the sake of the lungs themselves.

3. All the blood of the body comes to the lungs and goes away again once every twenty-three seconds. While it passes through the lungs it does not leave the capillaries, but the capillaries themselves are so closely intertwined with the air sacs that the two cannot be separated. And while they lie thus near together, with capillaries close about the air sacs, rapid exchanges are taking place. Oxygen mixed with the other gases of the air is on one side of the animal membrane of the air sac; carbon dioxid, with a little oxygen, is in the blood on the other side of the membrane within the capillaries. And as the gases are side by side, two of them— the oxygen and the carbon dioxid-change places without delay. Oxygen enters the blood from the air sac; carbon dioxid enters the air sac from the blood; the red corpuscle carriers are loaded in the twinkling of an eye, and hasten off to unload where their cargo is called for. In the meantime, however, the large supply of carbon dioxid is as unwelcome in the air sac as it is everywhere else in the body. It is therefore expelled as promptly as possible by an outgoing breath.