IT IS GOOD TO PLAY OUT OF DOORS TILL THE BELL RINGS—EVEN IN WINTER
The muscles, the brain, the skin, and other parts of the body get liquid food from the blood by “sucking” it through the walls of the smallest of the blood vessels, for these walls are very thin. In the same way, when waste passes from the muscles or the skin into the blood, it, too, soaks through the thin walls of the tiniest blood tubes, called capillaries.
Your heart beats or throbs about seventy-five times in a minute when you are well. Look at the second hand of a watch, while you count the beats in your wrist or in your neck.
Does your heart ever become tired? Not while you keep well, unless you over-drive it by running or wrestling too hard. It can rest between the beats. But the heart muscle, like any other muscle, must have plenty of good red blood to feed on. You put food into the blood by eating good breakfasts and dinners. The more you run and jump and play, the more work the heart has to do and the stronger it grows; and a good morning romp before school will send the blood flowing so merrily round from top to toe that you will feel fresher and brighter all the day.
III. FRESH AIR—WHY WE NEED IT
The heart is not the only thing that goes faster and harder when you run about in the morning and play hard. You are breathing faster and deeper as well, as if there were something in the air outside that you needed in your body as much as food.
But, of course, you know that air is not good to eat. It has no strength in it, as food has; it isn’t even a liquid like milk or coffee or tea. It is so thin and light that we call it a gas. Indeed, I suppose it is pretty hard for you to believe that air is a real thing at all. But all outdoors is full of the gas called air, and everything that seems to be empty, like a room or an empty box, is full of it.
You cannot even smell it, as you can that other gas which comes through pipes into our houses and burns at the gas jets; nor can you see it like the gas that comes out of a boiling kettle or from the whistle of a locomotive, and which we call steam. This is simply because air is so pure that it has no smell, and is so perfectly clear that we can see right through it. Almost the only way that we can recognize it is by feeling it when it is moving. But it is a very real thing for all that; and, like sunshine and food, is one of the most important things in the world for us.
What is it that air does in the body? We must need it very much, for we die quickly when we cannot get it: it takes us only about three minutes to suffocate, or choke to death, if we can’t get it.
You remember that the blood is pumped out from the heart, all through the body. Everywhere it goes,—to the feet and the hands and the head,—it is carrying two things: food that it has sucked up from the food tube, and hundreds and hundreds of tiny red sponges called red corpuscles. These little sponges are full of air which they sucked up as the blood passed through the lungs. When we stop breathing,—that is, taking in air,—the little red sponges of course can’t get any air to carry to the different parts of the body.