=Use of Blood Cells.=--The red cells act like boats. They load up with oxygen in the lungs and carry it to all parts of the body. Here they trade it off for carbon dioxide, a waste substance. This they carry back to the lungs to be cast out of the body. There is one white blood cell to every four hundred red ones. The white cells are the body-guards. They change their shape and are able to crawl through the walls of the capillaries. Wherever the body is hurt, they collect in large numbers and eat the germs which are always trying to get into the body through sores. The white matter called _pus_ in a sore is largely made of white blood cells which came there to fight the germs and were killed in the battle. The germs of boils and fevers often get into the blood, but the white cells usually kill them before they have a chance to grow into large numbers and make the body sick. =How to stop Bleeding.=--Most of the larger arteries are deep in the flesh and seldom get cut. There are many veins just under the skin. If the blood comes out in spurts, it is from an artery; but if it flows steadily, it is from a vein. If the blood does not run out in a stream, it will stop without any special care. As soon as the blood gets to the air it forms a jellylike mass called a _clot_. This helps stop the flow. All hurt places in the skin should be tied up in a clean cloth. [Illustration: FIG. 72.--Stopping the flow of blood from an artery.] If a large artery is cut, a bandage twisted tight with a stick around the limb on the side of the wound next to the heart will stop the bleeding. If a vein is cut, the bandage should be placed on the side of the cut away from the heart. =Alcoholic Drinks weaken the Blood.=--It has been noticed for some years that when a user of beer or whisky is attacked with fever, the disease is more severe than in one not using alcohol. The reason for this has lately been explained by a well-known scientist working in Paris. He put certain disease germs in rabbits, but they did not become sick. When he gave them a little alcohol and put the same amount of disease germs in them as before, they became sick and died. By careful study he learned that the white blood cells had in the first case killed the germs. In the second experiment the blood cells were made so weak and lazy by the alcohol that they did not put up such a strong fight against the germs. =Tobacco and the Blood.=--Any one who chews or smokes tobacco regularly gets much of the poison into the blood. The vessels in the mouth and throat drink in some of the juice and also the poison from the smoke. How much this poison affects the blood cells is not known, but it is likely to do them some harm because it makes the growing cells of the body less active.