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.