The whole history of our daily life is, briefly told, this: The food we eat becomes blood; the blood is carried all over the body, round and round, in the torrent of the circulation; as it sweeps past them or through them, the brain, nerves, muscles and skin pick out new food for their work, and give back the things they have used or no longer want; as they all have different work, some pick up what others have thrown away. There are also scavengers and cleansers to take up things which are wanted no longer, and to throw them out of the body.
Thus the blood is kept pure as well as fresh. Thus it is through the blood brought to them, that each part does its work.
But what is blood? It is a fluid. It runs about like water, but while water is transparent, blood is opaque. Under a microscope you will see a number of little round bodies—the blood-discs, or blood-corpuscles. All the redness there is in blood belongs to these. These red corpuscles are not hard, solid things, but delicate and soft, yet made to bear all the squeezing they get as they drive around the body. Besides these red corpuscles, are other little bodies, just a little larger than the red, not colored at all, and quite round. These are all that one can see in blood, but it has a strange property which we will study. Whenever blood is shed from a living body, within a short time it becomes solid. This change is called coagulation. If a dish be filled with blood, and you were to take a bunch of twigs and keep slowly stirring, you would naturally think it would soon begin to coagulate; but it does not, and if you keep on stirring you find that this never takes place. Take out your bundle of twigs, and you will find it coated all over with a thick, fleshy mass of soft substance. If you rinse this with water you will soon have left nothing but a quantity of soft, stringy material matted among the twigs. This stringy material is, in reality, made up of fine, delicate, elastic threads, and is called fibrin; by stirring you have taken it out. If the blood had been left in the dish for a few hours, or a day, you would find a firm mould of coagulated blood floating in a colorless liquid. This jelly would continue shrinking, and the fluid would remain; this fluid is called serum, and it is the blood out of which the corpuscles have been strained by the coagulation. All these various things, fibrin, serum, corpuscles, etc., make up the blood. This blood must move, and how does it move? You have had the different organs which assist in its circulation described, but let us illustrate.
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.
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.
From most places where there is oxidation, the venous blood comes away hotter than the arterial, and all the hot venous blood mingling, keeps the whole body warm. Much heat is given up, however, to whatever is touching the skin, and much is used in turning liquid perspiration into vapor. Thus, as long as we are in health, we never get hotter than a certain degree.
Everywhere this oxidation is going on. Little by little, every part of the body is continually burning away and continually being made anew by the blood. Though it is the same blood, it makes very different things: in the nerves it makes nerve; in the muscle, muscle. It gives different qualities to different parts: out of one gland it makes saliva, another gastric juice; out of it the bone gets strength and the muscle power to contract. But the far greater part of the power of the blood is spent in heat, or goes to keep us warm.
One thing more we have to note before we answer the question, why we move. We have seen that we move by reason of our muscles contracting, and that, in a general way, a muscle contracts because a something started in our brain by our will, passes through the spinal cord, through certain nerves, until it reaches the muscle, and this something we may call a nervous impulse. But what starts this?