Our last article gave us a complete description of the human organization. In the present number we will inquire how we move this complex system of bones, nerves, flesh, and tissues.

We will take a particular motion and see if we can understand that. For instance, you bend your arm. You know that when your arm is lying on the table you can bend the forearm on the upper arm (or part above the elbow) until your fingers touch your shoulder. How is this done?

Look at the arm in a skeleton; you will see that the upper part is composed of one large bone, called the humerus, the fore part of two bones, the radius and ulna. If you look carefully you will see that the end of the humerus, at the elbow, is curiously rounded, and the end of the ulna, at the elbow, is scooped out in such a way that one fits loosely into the other. If you try to move them about, one on the other, you will find that you can easily double the ulna very closely on the humerus, without their ends coming apart; and as you move the ulna you will notice that its end and the end of the humerus slide over each other. But they will slide only one way—up and down. If you try to slide them from side to side they get locked. At the elbows, then, we have two bones fitting into each other, so that they will move in a certain direction; their ends are smoothed with cartilage, kept moist with a fluid and held in place by ligaments, and this is all called a joint.

In order that this arm may be bent some force must be used. The radius and ulna (the two move together) must be pushed or pulled toward the humerus, or the humerus toward the radius and ulna. How is this done in your arm? Imagine that a piece of string were fastened to either the radius or ulna, near the top: let the string be carried through a little groove, which there is at the upper end of the humerus, and fastened to the shoulder-blade. Let the string be just long enough to allow the arm to be straightened out, so that when the arm is straight the string will be just about tight. Now draw your string up into a loop and you will bend the fore-arm on the humerus. If this string could be so made that every time you willed it so, it would shorten itself, it would pull the ulna up and would bend the arm; every time it slackened the arm would fall back into a straight position.

In the living body there is not a string, but a band of tissues placed very much as our string is placed, and which has the power of shortening itself when required. Every time it shortens the arm is bent, every time it lengthens again the arm falls back into its straight position. This body, which can thus lengthen and shorten itself, is called a muscle. If you put your hand on the front of your upper arm, half way between your shoulder and elbow, and then bend your arm, you will feel something rising up under your hand: this is the muscle shortening or, as we shall now call it, contracting.

But what makes the muscle contract? You willed to move your arm, and moved it by making the muscle contract; but how did your will accomplish this? If you should examine, you would find running through the muscle soft white threads, or cords, which you have already learned to recognize as nerves. These nerves seem to grow into and be lost in the muscle. If you trace them in the other direction you would find that they soon join with other similar nerves, and the several cords joining together form stouter nerve-cords. These again join others, and so we should proceed until we came to quite stout white nerve-trunks, as they are called, which pass between the vertebrÆ of the neck into the vertebral column, where they mix in the mass of the spinal cord. What have these nerves to do with the bending of the arm? Simply this: If you should cut through the delicate nerves entering the muscle, what would happen? You would find that you had lost all power of bending your arm. However much you willed it, the muscle would not contract. What does this show? It proves that when you will to move, something passes along the nerves to the muscle, which something causes the muscle to contract. The nerve, then, is a bridge between your will and the muscle—so that when the bridge is broken the will can not get to the muscle.

If, anywhere between the muscle and the spinal cord, you cut the nerve, you destroy communication between the will and the muscle. If you injure the spinal cord in your neck you might live, but you would be paralyzed; you might will to bend your arm, but could not.

In short, the whole process is this: by the exercise of your will a something is started in your brain. That something passes from the brain to the spinal cord, leaves the spinal cord and travels along certain nerves, picking its way along the bundles of nervous threads which run from the upper part of the spinal cord, until it reaches the muscle. The muscle immediately contracts and grows thick. The tendon pulls at the radius, the radius with the ulna moves on the fulcrum of the humerus at the elbow-joint, and the arm is bent.

Why does the muscle contract when that something reaches it? We must be content to say that it is the property of the muscle. But it does not always possess this property. Suppose you were to tie a cord very tightly around the top of the arm close to the shoulder. If you tied it tight enough the arm would become pale, and would very soon begin to grow cold. It would get numb, and would seem to be heavy and clumsy. Your feeling in it would be blunted, and soon altogether lost. You would find great difficulty in bending it, and soon it would lose all power. If you untied the cord, little by little the cold and clumsiness would pass away, the power and warmth would come back, and you would be able to bend it as you did before. What did the cord do to the arm? The chief thing was to press on the blood-vessels, and so stop the blood from moving in them. We have seen that all parts of the body are supplied with blood-vessels, veins and arteries. In the arm there is a very large artery, branches from which go into all parts of the muscle. If, instead of tying the cord about the arm, these branches alone were tied, the arm, as a whole, would not grow cold or limp, but if you tried to bend it, you would find it impossible. All this teaches that the power which a muscle has of contracting may be lost and regained as the blood is stopped in its circulation, or allowed to circulate freely.

Our next question is, What is there in the blood that thus gives to the muscle the power of contracting, or that keeps the muscle alive? The answer is easy. What is the name given to this power of a muscle to contract? We call it strength. Straighten out your arm upon the table and put a heavy weight in your hand; then bend your arm. Find the heaviest weight that you can raise in this way, and try it some morning after your breakfast, when you are in good condition. Go without dinner, and in the evening when tired and hungry, try to raise the same weight in the same way. You will not be able to do so. Your muscle is weaker than it was in the morning, and you say that the want of food makes you weak; and that is so, because the food becomes blood. The things which we eat are changed into other things which form part of the blood, and this blood going to the muscle gives it strength. What is true of the relations of the blood to the muscles is true of all other parts of the body. The brain and nerves and spinal cord have a more pressing need of pure blood. The faintness which we feel from want of food is quite as much weakness of the brain and of the nerves as of the muscles, perhaps even more so.

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?

All the nerves do not end in the muscles, but many in the skin. These nerves can not be used to carry nervous impulses from the brain to the skin. By no effort of yours can you make the skin contract. For what purpose then are these nerves? If you prick your finger, you feel the touch, or say that you have sensation in your finger. If you were to cut the nerve leading to the finger you would lose this power of feeling. These nerves, then, ending in the fingers have a different use from those ending in the muscles. The latter carry an impulse from the brain to the muscles, and are called motor nerves. The former carry impulses from the skin to the brain, and are called sensory nerves, or those which bring about sensations. Motor nerves are of but one kind, but there are several kinds of sensory nerves, each kind having a special work to do. The several works which these nerves do are called the senses. By means of these sensory nerves we receive impressions from the external world, sensations of heat, cold, roughness, good and bad odors, taste, sound, and the color and form of things. Thus impressions of the external world are made upon the brain, and it is these impressions that stimulate the brain to action. The brain worked on by them, through ways that we know not of, governs the muscles, sends commands by the motor nerves, and rules the body as a conscious, intelligent will.