Each time the heart contracts what happens? Let us begin with the right ventricle full of blood. It contracts; the pressure comes on all sides, and were it not for the flaps that close and shut the way, some of the blood would be forced back into the right auricle. As it is, there is but one way,—through the pulmonary artery. This is already full of blood, but, because of its wonderful elasticity, it stretches so that it holds the extra fluid. The valve at its mouth closes, and the blood is safely shut in, but the artery so stretched contracts and forces the blood along into the veins and capillaries of the lungs, in turn stretching them so that they must force ahead the blood which they already contain. This blood is forced into the pulmonary vein, thence to the left auricle; the auricle forces it into the ventricle, and the latter pumps it into the aorta; the aorta overflows as the pulmonary artery did, and the blood goes through every capillary of the body into the great venæ cavæ, which forces it into the right auricle; thence to the ventricle where we started. In this passage every fragment of the body has been bathed in blood. This stream rushing through the capillaries contains the material from which bone, muscle, and brain are made, and carries away all the waste material which must be thrown off. The actual work of making bone or muscle is performed outside of the blood in the tissues. You say, the capillaries are closed, and how can the blood get to the tissues? It will be necessary here to speak of a certain property of membranes in order that you understand how the tissues are built up by the blood apparently closed within the veins. If a solution of sugar or salt be placed in a bladder with the neck tied tightly, and this placed in a basin of pure water, you will find that the water in the basin will soon taste of sugar or salt and after a time will taste as strong as the water in the bladder. If you substitute solid particles, or things that will not dissolve, you will find no change. This property which membranes, such as a bladder, have, is called osmosis. It is by osmosis chiefly that the raw, nourishing material in the blood gets into the flesh lying about the capillaries. It is by osmosis chiefly that food gets out of the stomach into the blood. It is by this property that the worn-out materials are drained away from the blood, and so cast out of the body. By osmosis the blood nourishes and purifies the flesh. By osmosis the blood is itself nourished and kept pure. We must now understand how we live on this food we eat. Food passing into the alimentary canal is there digested. The nourishing food-stuffs are dissolved out of the innutritious and pass into the blood. The blood thus kept supplied with combustible materials, draws oxygen from the lungs, and thus carries to every part of the body stuff to burn and oxygen to burn it with. Everywhere this oxidation is going on, changing the arterial blood to venous.