What do you breathe for? That is plain enough, you will say: I can not live without breathing. But why is it that your life depends on your breathing? This I will explain to you.
The blood changed from dark to red in the lungs.
You remember that I told you that the blood that comes back to the heart in the veins is dark; it is not good blood. It has been used while it was in the capillaries in building and repairing bone, and skin, and muscle, and nerve, etc. It is not fit to be used again so long as it is dark blood. What shall be done with it? It must be made in some way into good red blood again. Now the factory where this is done is the lungs.
Just as fast as the dark blood comes to the heart, it sends it to the lungs to be made into red blood, then it goes back to the heart to be sent all over the body. But how, you will ask, is the dark blood changed into good red blood in the lungs? It is done by the air that you breathe in; every time that you draw a breath, air goes down into the lungs and changes the blood that it finds there.
And now you see why it is that you have to breathe to keep alive. If the air does not go down into the lungs, the dark blood that is there is not changed into red blood: it goes back to the heart dark blood, and is sent all over the body; but this dark blood can not keep you alive: it is the red blood that does this.
Drowning.
You see, then, how death is caused in drowning; the air is shut out by the water, and the blood is not changed in the lungs; so the blood goes back to the heart dark instead of red, and is sent all over the body.
The heart and the lungs fill up your chest. The lungs cover up the heart, except a little part of it on the left side: this is where you can feel its beating so plainly. Here is a figure of the heart and lungs; the lungs are drawn apart, so that you can see the heart, and its large arteries and veins. You see, markeda, the windpipe by which the air goes down into the lungs. The lungs are light, spongy bodies. They are light because they are full of little cells for the air to go into. It is in these cells that the blood is changed by the air.
Situation of the heart and lungs of fishes.
Gills of fishes.
How fishes breathe.
And now I will tell you about the lungs of fishes. But perhaps you will say that fishes do not breathe, and it can not be that they have lungs, for they would be of no use to them. It is true that they do not have such lungs as we have; but they have lungs, and they really do breathe air. How is this, you will ask, when they live in the water? There is a good deal of air always mixed up with water, and the lungs of a fish are so made that the air in the water can change the blood in them. The gills of a fish are itslungs, and the way that they are used is this. The fish takes water into its mouth, and lets it run out through the gills, and so the air that is mixed with the water changes the blood in them. Our lungs are fitted to breathe air alone, but the fish may be said to breathe air and water together. Air alone does the fish no good; he can not live in it; he must have his air mixed with water, or it is of no use to him.
Breathing of the lamprey eel.
Here is a picture of the lamprey eel. You see that it has a row of holes on its neck: these are openings that lead to its lungs; there are seven on each side. It is from this that it is sometimes called seven-eyes. Insects have such openings into their lungs. The grasshopper has twenty-four of them, in four rows. So you see that there are different ways of breathing in different animals. They do not all breathe through their mouths and noses, as we do.
The voice.
You see that the chief use of breathing is to air the blood; but it is of use to us in another way. It makes the voice. We could not speak if we did not breathe. The sound of the voice is made in the top of the neck, in what we call Adam’s apple. This is a sort of musical box at the top of the windpipe: in this box there are two flat cords stretching right across it. Now when we speak or sing, the sound is made in this way: the air, coming up out ofthe lungs, strikes on these cords, and makes them shake or vibrate. It is just as the vibration of the fiddle-string makes a sound when the bow is drawn over it. If you look at an Æolian harp fixed in a window, you can see that the strings are made to quiver by the wind, and this causes the sound. In the same way, the wind that is blown up from your lungs makes the cords in the Adam’s apple vibrate; and the chest may be said to be the bellows of that little musical box or organ that you have in the throat.
The voices of animals.
The purring of the cat.
Many animals have a musical box in the throat similar to ours. The lowing of the cow, the barking of the dog, and the mewing and squalling of the cat are all done in such a box. You perhaps have wondered how the cat purrs. This noise is made in the same box where she does her mewing and squalling; for if you put your finger on her Adam’s apple while she is so quietly purring, you can feel a quivering motion there.
The croaking of the frog.
Fishes, you know, have no voice. They have no musical box. If they had they could not use it, for the only way in which it can be used is to blow air through it. The frog can not use his so long as he is under water; he has to stick his head up out of water when he wants to croak.
Questions.—What do you breathe for? How is the blood in the lungs changed? What would it do if it were not changed? How is death, caused in drowning? How are the heart and lungs situated? Why are the lungs so light? What is said about the lungs of fishes? What is said about the breathing of the lamprey eel? What about the breathing of the grasshopper? How is the breathing of use besides changing the blood? Tell how the voice is made. What is said about the voices of animals? Where is the cat’s purring done? Why do fishes have no musical box? What is said about the croaking of frogs?
Questions.—What do you breathe for? How is the blood in the lungs changed? What would it do if it were not changed? How is death, caused in drowning? How are the heart and lungs situated? Why are the lungs so light? What is said about the lungs of fishes? What is said about the breathing of the lamprey eel? What about the breathing of the grasshopper? How is the breathing of use besides changing the blood? Tell how the voice is made. What is said about the voices of animals? Where is the cat’s purring done? Why do fishes have no musical box? What is said about the croaking of frogs?