4. The heart pumps or beats about seventy times a minute. 5. The blood flows through arteries to all parts of the body. 6. The arteries open into the capillaries. Capillaries make a network around each cell of the body. 7. Some of the liquid parts of the blood go out through the sides of the capillaries and become food for the cells of the body. 8. From the capillaries the blood flows into the veins and back to the heart. 9. Bleeding can be stopped by holding the cut tightly between the hands. 10. The white blood cells grow into the sides of cuts, and so heal them. They also guard the body against the seeds of many diseases. 11. The red blood cells carry air to the cells of the body. 12. Alcohol weakens the heart and arteries. 13. Tobacco harms the heart. CHAPTER X BREATHING, HEAT, AND CLOTHING =100. The lungs.=--Our food becomes blood and feeds the cells of our body, but we grow only a little heavier. What becomes of the food? [Illustration: =The air tubes and lung.= _a_ larynx or voice box. _b_ trachea or windpipe. _d_ air sacs, each like a tiny frog's lung.] Besides food, air is always getting into our bodies. In breathing, air passes through the nose into a tube in the neck. This tube is called the _windpipe_. You can feel it as a pile of hard rings in the front part of the neck. The windpipe divides into many branches. At the end of its smallest branches are little bags or sacs. The branches and the sacs make the two lungs. So a lung is a soft and spongy piece of flesh, and can be blown up like a rubber bag. A frog's lung is a single, thin bag, about half an inch across it. Each little sac of a man's lung is like a tiny frog's lung. [Illustration: =A frog's lung (×4).=] =101. The diaphragm.=--The lungs fill the upper part of the body just below the neck. They are covered by the bony ribs, and rest upon a broad muscle. This muscle is called the _diaphragm_. It divides the inside of the body into two parts. The upper part is the _chest_, and holds the heart and lungs. The lower part is the _abdomen_, and holds the stomach, intestine, and liver, and a few other parts. [Illustration: =The parts inside the body.= _a_ lungs. _b_ heart. _c_ diaphragm. _d_ stomach. _e_ liver. _f_ intestine.] =102. Breathing.=--When the diaphragm lowers itself, or the ribs are raised, the chest is made larger. Then the air rushes through the nose and swells out the lungs to the size of the chest. This is taking a breath. Then the chest becomes smaller again, and blows the air out. A man breathes about eighteen times a minute. He does not seem to rest in breathing, but as he works only when he takes in breath, he rests one half of the time.