[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.
=How the Heart Does Its Work.=--The power back of blood distribution is the heart. It is an automatic pump, as it were, that sends blood to the lungs and through the arteries to all parts of the body. The heart is divided into four divisions: the left and right ventricles and the right and left auricles. The right auricle receives the blood from the upper half of the body through a large vein and the lower half of the body through another large vein, and the blood from both lungs empties into the left auricle through two left and two right pulmonary veins. The large arteries of the heart which carry the blood from the heart to the different organs arise from the ventricle.
The blood always flows in the same direction. It goes into the auricle from the veins, and from this into the ventricle. It then passes into the arteries, then to the veins and then to the capillaries.
The action of the heart is very much like a force pump; the dark blood flows into the right auricle, which contracts; when this is done, the blood is forced into the right ventricle; this in turn contracts and forces the blood into the lungs, where oxygen is taken on and carbonic acid gas and other impurities are thrown off. From the lungs the blood, now red and pure, passes into the left auricle and thence into the left ventricle, from which it is forced into the aorta to be distributed to all parts of the body.