=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. [Illustration: =Bacteria growing in a kidney and producing an abscess (×300).= _a_ kidney tube. _b_ white blood cell attacking bacteria. _c_ bacteria. _d_ blood vessel of the kidney.] =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.