As water in the plant is the carrier of plant food throughout the plant, so is blood the carrier and distributor of food in the animal. When food is absorbed, it either passes into the lymphatic system or into the capillaries of the blood system. If in the former, it is carried to the thoracic duct, which extends along the spinal column and enters one of the main blood vessels. If collected by the capillary system, it is carried to the portable vein, thence to the liver and finally to the heart, where it meets with the blue blood collected from all parts of the body. At this point, the blood contains both the nutriment and the waste matter of the body. Before it can be sent through the body again the waste material must be thrown out of the system by means of the lungs. This is accomplished by the heart forcing to the lungs the impure blood with its impurities collected from all parts of the body and also the nutriment collected from the digestive tract. The chief organs, therefore, of the circulatory system are the blood and lymphatic vessels containing respectively blood and lymph. The only difference between these two materials is in the fact that lymph is blood without the red-blood corpuscles. The body, after all, really depends upon this lymph for nourishment, since it wanders to all parts of the body, surrounds all the cells in all of the tissues and in this way carries to the cells the very kinds of food that they need. =Lymph Passes Through Cell Walls.=--The blood vessels have no openings into the body at all. In this respect the blood system is like the digestive system; it is separate and distinct in itself. The blood, however, does creep through the walls of the blood vessels. In so doing the blood corpuscles are left behind and lymph is the result. [Illustration: HOW THE BLOOD CIRCULATES THROUGH THE BODY] The center of the blood system is the heart. It is the engine of the body. Going out from it is the great aorta, which subdivides into arteries and farther away further subdivides until there is a great network of little arteries; these in turn become very tiny and take the name of capillaries. Thus the red blood, by means of arteries and capillaries, is carried to all parts of the body. This plan of distribution would not be complete unless some way were provided for the return of the blood to the heart and lungs for purification. And just such an arrangement has been provided. Another kind of network collects this scattered blood at the extremities into separate vessels, which gradually increase in size and finally empty their possessions into the heart. These are the veins of the body, and have to do with the impure blood of the body.