=The Cells have a Liquid Home.=--The cells in the body of man, like the ameba, live in a watery liquid. This liquid is called _lymph_. The cells cannot move about as the ameba does to obtain food, so the _blood_ brings the food near them and it soaks through the blood tubes into the lymph spaces next to the cells (see colored Fig. 3). The ameba gives off waste material into the water; the cells of the body give it off into the lymph to be carried off by the circulation. The blood, then, has _two functions_: (1) to take nourishment to the tissues; (2) to take away waste material from them. =The Organs of Circulation.=--These are the _heart_, which propels the blood; the _arteries_, which take blood away from the heart; the _veins_, which take the blood back to the heart; and the _capillaries_ (Fig. 53), which take the blood from the arteries to the veins. [Illustration: FIG. 53.--CAPILLARIES, connecting artery (_b_) with vein (_a_).] =The heart= is a cone-shaped organ about the size of its owner’s fist. It lies in a diagonal position behind the breastbone, with the small end of the cone extending toward the left. The smaller end (Exp. 1) taps or beats against the chest wall at a point between the fifth and sixth ribs on the left side. The breastbone and ribs protect it from blows. An inclosing membrane called the _pericardium_ secretes a serous fluid and lessens the friction from its beating. =Why the Heart is Double.=--_There must be a pump to move the impure blood from the body to the lungs_ to get oxygen from the air, and there must be _another pump to send the pure blood from the lungs back to the body_. Hence there are two pumps bound together into one heart, beating at the same time like two men keeping step, or like two carpenters keeping time with their hammers. There are valves in the heart, as in other pumps. These valves are so arranged that when any part of the heart contracts and forces the blood onward, the blood cannot return after that part of the heart relaxes. Are the pumps placed one behind the other? Or is one above the other? Neither; they are side by side, with a fleshy partition between them (Fig. 54). The pump on the right moves the impure blood from the body to the lungs, and the one on the left moves the pure blood from the lungs to the body. There is no direct connection between the right and left sides of the heart. [Illustration: FIG. 54.--DIAGRAM OF HEART. Notice the two dark spots in the right auricle, and four dark spots in left auricle, where the veins enter. Does the aorta pass in front of, or behind, the pulmonary artery?]