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 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.

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?

To trace one complete circuit of the blood (Fig. 54), let us begin with the blood in the capillaries of the outer tissues, such as the skin or muscles. The blood goes through small veins which unite into two large veins, through which it enters the receiving chamber, or right auricle, goes through the tricuspid valve into the expelling chamber, or right ventricle, then through a semilunar valve into the pulmonary artery leading to the lungs. Becoming purified while