~6.~ The white corpuscles have something to do with keeping the body in good repair. They are carried by the blood into all parts of the body and stop where they are needed to do any kind of work. They may be compared to the men who go around to mend old umbrellas, and to do other kinds of tinkering. It is thought that the white corpuscles turn into red ones when they become old. ~7.~ The corpuscles float in a clear, almost colorless fluid which contains the digested food and other elements by which the body is nourished. SUMMARY. 1. The blood contains very small objects called blood corpuscles. 2. There are two kinds of corpuscles, red and white. 3. The red corpuscles carry oxygen. 4. The white corpuscles repair parts that are worn. 5. The corpuscles float in a clear, almost colorless fluid, which nourishes the body. CHAPTER XI. WHY THE HEART BEATS. ~1.~ If you place your hand on the left side of your chest, you will feel something beating. If you cannot feel the beats easily, you may run up and down stairs two or three times, and then you can feel them very distinctly. How many of you know the name of this curious machine inside the chest, that beats so steadily? You say at once that it is the heart. [Illustration: THE HEART.] ~2.~ The Heart.--The heart may be called a live pump, which keeps pumping away during our whole lives. If it should stop, even for a minute or two, we would die. If you will place your hand over your heart and count the beats for exactly one minute, you will find that it beats about seventy-five or eighty times. When you are older, your heart will beat a little more slowly. If you count the beats while you are lying down, you will find that the heart beats more slowly than when you are sitting or standing. When we run or jump, the heart beats much harder and faster. ~3. Why the Heart Beats.~--We have learned in preceding lessons that the digested food is taken into the blood. We have also learned that both water and oxygen are taken into the blood. Thus the blood contains all the materials that are needed by the various parts of the body to make good the wastes that are constantly taking place. But if the blood were all in one place it could do little good, as the new materials are needed in every part of the body. There has been provided a wonderful system of tubes running through every part of the body. By means of these tubes the blood is carried into every part where it is required. These tubes are connected with the heart. When the heart beats, it forces the blood through the tubes just as water is forced through a pipe by a pump or by a fire-engine. ~4. The Heart Chambers.~--The heart has four chambers, two upper and two lower chambers. The blood is received into the upper chambers, and is then passed down into the lower chambers. From the lower chambers it is sent out to various parts of the body. [Illustration: THE INSIDE OF THE HEART.]