Veins and Arteries. These blood-tubes running from the walls of the food-tube to the heart are called _veins_; and the other tubes through which the heart pumps the blood all over the body are called _arteries_. If you will spell this last word "air-teries," it may help you to remember why the name was given to these tubes ages ago. When the body was examined after death, they were found to be empty and hence were not unnaturally supposed to carry air throughout the body, and "air-teries" they have remained ever since. While absurd in one way, the name is not so far amiss in another, for an important part of their work is to carry all over the body swarms of tiny baskets, or sponges, of oxygen taken from the air. Why the Blood is Red. The first and main purpose of the blood-pipes and the heart is to carry the dissolved food from the stomach and intestines to the cells all over the body. But the cells need air as well as food; and, to carry this, there are little basket-cells--the _red corpuscles_. Take a drop of blood and put it under a microscope, and you will see what they look like. The field will be simply crowded with tiny, rounded lozenges--the red cells of the blood, which give it its well-known color. [Illustration: BLOOD CORPUSCLES (Greatly magnified) _A_, red blood; _B_, white blood.] The White Corpuscles or Scavengers of the Blood. As the blood-tubes are not only supply-pipes but sewers and drainage canals as well, it is a good thing to have some kind of tiny animals living and moving about in them, which can act as scavengers and eat up some of the waste and scraps; and hence your microscope will show you another kind of little blood corpuscle, known, from the fact that it is not colored, as the _white corpuscle_. These corpuscles are little cells of the body, which in shape and behavior are almost exactly like an _ameba_--a tiny "bug," seen only under the microscope, that lives in ditch-water. Under the microscope the white corpuscles look like little round disks, about one-third larger than the red corpuscles, and with a large kernel, or _nucleus_, in their centre. They have the same power of changing their shape, of surrounding and swallowing scraps of food, as has the ameba, and are a combination of scavengers and sanitary police. When disease germs get into the blood, they attack and endeavor to eat and digest them; and whenever inflammation, or trouble of any sort, begins in any part of the body, they hurry to the scene in thousands, clog the blood-tubes and squeeze their way out through the walls of the smallest blood-tubes to attack the invaders or repair the damage. This causes the well-known swelling and reddening which accompanies inflammation.