All over the body there are, though you can not see them, networks of capillaries. All the arteries end in capillaries, and in them begin all veins. Supposing a little blood-corpuscle be squeezed in the narrow pathway of a capillary in the muscle of the arm. Let it start in motion backward. Going along the narrow capillary it would hardly have room to move. It will pass on the right and left other capillary channels, as small as the one in which it moves; advancing, it will soon find the passage widening and the walls growing thicker. This continues until the corpuscle is almost lost in the great artery of the arm; thence it will pass but few openings, and these will be large, until it passes into the aorta, or great artery, and then into the heart. Suppose the corpuscle retrace its journey and go ahead instead of backward. It will go through passages similar to the other, and it would learn these passages to be veins. At last the corpuscle would float into the vena cava, thence to the right auricle, from there to the right ventricle, by the pulmonary artery to the lungs; there it, with its attendant white corpuscles, serum and other substances, would be purified, then sent by pulmonary vein to the left auricle and ventricle, and then pumped over the body again. Some one may ask, What is the force that drives or pumps the blood? Suppose you had a long, thin muscle fastened at one end to something firm, and a weight attached to it. Every time the muscle contracted it would pull on the weight and draw it up. But instead of hanging a weight to the muscle, wrap it around a bladder of water. If the muscle contract now, evidently the water will be squeezed through any opening in the bladder. This is just what takes place in the heart. Each cavity there, each auricle and ventricle is, so to speak, a thin bag with a number of muscles wrapped about it. In an ordinary muscle of the body the fibers are placed regularly side by side, but in the heart, the bundles are interlaced in a very wonderful fashion, so that it is difficult to make out the grain. They are so arranged that the muscular fibers may squeeze all parts of each bag at the same time. But here is the most wonderful fact of all. These muscles of the auricles and ventricles are always at work contracting and relaxing of their own accord as long as the heart is alive. The muscle of your arm contracts only at your will. But the heart is never quiet. Awake or asleep, whatever you are doing or not doing, it keeps steadily on.