81. Blood.—After food becomes blood, it goes to every part of the body to feed the cells. Even a pin prick anywhere in the body draws blood. The blood makes the skin pink. There are five or six quarts of blood in a man's body. This is about 1/13 of his body.

Blood looks like a red liquid. But if you look at it through a strong microscope, it looks like water, and millions of little red cells. These cells carry air through the body. They make the blood look red. There are also a smaller number of white cells. Blood is made of red cells, white cells, and a liquid.

82. The liquid in blood.—The liquid part of the blood is albumin, and water, with a little fat, sugar, and minerals. It is food and drink for the cells of the body. When blood is drawn from the body it soon becomes like jelly. We call the jelly a clot. When you cut your finger, a clot forms in the cut and plugs up the bleeding place. If it did not, the blood would all run out of the body and we should die.

83. The heart.—The blood is held in tubes. A pump inside the body keeps it always moving. This pump is called the heart. The heart is a bag of muscle with thick sides. It is about as large as your fist. When it is full, it has the power to make itself smaller, and so it squeezes the blood out through a tube. We can feel each squeeze as a heart-beat. You can find the heart-beat just to the left of the middle of the body about two hand-breadths below the neck.

84. The heart-beat.—A man's heart beats about seventy times each minute. Boys' and girls' hearts beat much faster. Running or hard work of any kind makes the heart beat faster yet. Your heart will keep on beating until you die. It does not seem to rest at all, yet it works only while you feel it beat. Between each beat it rests while the blood is filling it again. So it really rests one half of the time.

85. Arteries.—The heart pumps the blood through a single tube. This tube opens into smaller tubes. These open into still smaller ones. You must use a strong microscope to see the finest blood tubes. The tubes reach every part of the body, and carry blood to its cells. They are called arteries. At each heart-beat a wave of blood can be felt in an artery. This wave is the pulse. It can be felt in the wrist, temples, and other places. By the pulse we can tell how often and how strongly the heart is beating.

86. Capillaries.—The smallest arteries divide into a fine network of small tubes. These tubes are the capillaries. They lie around every cell of the body. Their sides are very thin. As the blood flows through them, some of it soaks through the sides of the tubes. Blood contains all kinds of food for the cells. Each cell is always wet with food and can eat it at any time. The cells are like the tiny animal, the ameba, and can take in the food by any part of their bodies. The cells are better off than the ameba, for their food is brought to them. They pay the body for their food by working for it.

87. Veins.—The capillaries come together again to form large tubes. These tubes are called veins. Only a little of the blood goes through the sides of a capillary. The rest flows on into the veins. The veins unite to form two large tubes. These two tubes open into the heart.

88. How the blood flows.—The blood is pumped out of the heart, through the arteries to the capillaries. There some goes out to the cells. The rest flows into the veins and goes back to the heart. All the blood in the body passes through the heart every two minutes. It takes only twenty seconds for a drop of blood to go from the heart to the toes and back again. The arteries are deep in the flesh, but some of the large veins can be seen upon the back of the hands.

89. Bleeding.—If a large artery or vein is cut there is a great deal of bleeding. You can always stop a cut from bleeding by holding it fast between the hands. Do not be afraid of the blood when you see any one bleeding, but hold the sides of the cut tightly with both of your hands. This will stop any bleeding until help comes. You may keep a person from bleeding to death by doing this when other persons are afraid of the blood.

90. Healing cuts.—When your flesh is cut it soon grows together again. The work of the little white cells in the blood is to help heal cuts and wounds and bruises. These cells are like little amebas in the blood. They keep moving around with the blood, and now and then burrow outside the capillaries to see if all is well. If they find a cut, hundreds and thousands rush to the spot at once. Some eat up any specks of dirt on the cut. Others fit themselves into the sides of the cut and grow long and slender, like strings, and so bind the two edges of the cut together. In this way all cuts are healed.

91. The white blood cells kill disease germs.—There are tiny living beings everywhere in the air, and soil, and water. Some of them can grow inside a man and make him sick. These tiny things are called disease germs. One kind gives a man typhoid fever, and another diphtheria. Another kind grows on cuts, and sometimes makes them very sore. The white cells of the blood are always watching for these enemies, like a cat hunting mice, and when they find them they at once try to kill them. But sometimes the white blood cells get killed. Then they look like cream in the cut. We call this creamy liquid matter or pus, and say "We have caught cold in the cut." In most pricks and cuts the white cells of the blood can kill all these enemies and also heal the cut.

92. Catching cold.—Sometimes the cold air blows on our head and hurts the cells of the nose. If there are disease germs in the air, they may grow in the injured part of the nose and make us have a "cold in the head." Then the white blood cells gather at the spot so as to kill the disease germs. Also the arteries bring a great deal of blood to the nose so as to heal the injured parts. Some of the white blood cells and the liquid from the blood run out, and we have to blow the nose. The white blood cells help to make us well whenever we catch a cold or other kind of sickness. 93. Red blood cells.—The red blood cells are like tiny flat plates. They float in the liquid part of the blood and make the blood look red. They carry air from the lungs to the cells of every part of the body, and thus help all the cells to breathe.

94. Why the heart beats hard when we run.—When we work hard, the cells of our bodies need a great deal of food. So the heart beats much harder, and sends them much more blood. We can feel our heart beat when we run hard.

When the cells work they get more blood in another way. The arteries become larger and hold more blood. Then the part looks red and feels warm. Thus your face gets red when you run hard. This is because your heart and arteries bring more blood to feed the working cells.

95. Need of a strong heart.—The heart must keep sending blood to feed the cells. If it should stop for only a little while, the cells would starve to death and we should die. We need strong hearts. When we work very hard for a long time, the heart gets tired. Then the cells do not get enough food and we feel weak all over. Boys ought not to run and lift till they are tired out, for this hurts their hearts.

96. What alcohol does to the blood.—Alcohol hinders the digestion of food. Then too little food will reach the blood, and so the cells of the body will get too little. Alcohol does not add strength to the body, but it takes it away. It seems to make men stronger, for it takes away their tired feelings. But it makes them really weaker, for it harms the blood.

97. How alcohol affects the heart.—Alcohol at first makes the heart beat more strongly and quickly, but it tires it out and makes it weaker. Then the heart pumps too little blood to the rest of the body, and a man is weaker all over.

If a drinker tries to run or work hard, his heart may not pump enough food to the working cells of his arms and legs. Strong drink takes away a man's strength and makes him less able to endure a long strain.

98. How alcohol harms the arteries.—Alcohol causes the arteries to become larger and to carry more blood. Then the face will be red and the skin will become warm. This makes a person feel well, and he seems to be helped. His blood seems to be flowing faster because his face is red. But really it is flowing slower.

When the arteries have been made large very often, they stay large all the time. A drinker's nose is often red from this cause.

Alcohol sometimes causes the arteries to become hard, and even to change to a kind of bone. Then they cannot change their size to carry just so much blood as each part needs.

99. How tobacco affects the heart.—Tobacco weakens all the body, but it harms the heart more than the rest. It often makes the heart beat slowly at one time and fast at another. It weakens the heart and keeps it from working harder when the working cells need more food. A smoker gets out of breath quickly. He cannot run far or work very hard. Chewing is a still more harmful form of using tobacco. When men train for a game or a race they never use tobacco.

Boys are not so strong as men, and so tobacco is more hurtful to them. Boys are harmed by tobacco far more than men. Cigarette smoke harms their stomachs and keeps food from their blood. If boys smoke, they become pale and weak. The poisonous smoke weakens the heart, and they cannot run or work so hard as they should. Even if a father uses tobacco, he should not allow his boys to use it.

WHAT WE HAVE LEARNED