Part 7

The radius and ulna form the lever; its fulcrum is the end of the humerus in the elbow (Fig. 3, F); the weight to be moved is the weight of the radius and ulna (with that of the bones of the hand if present), and this may be represented by a weight applied at about the middle of the fore-arm; the power is the pull you give the string, and that is brought to bear on your lever at the point where the string is fastened to the radius, i.e. nearer the fulcrum than the point where the weight is applied; and you know that when you have the fulcrum at one end and the power between the fulcrum and the weight, you have a lever of the third order.

Now, in order to make the thing a little more like what takes place in your own arm, instead of boring a hole through the humerus, let the string glide in a groove which you will see at the top of the humerus, and fasten the end of it to the shoulder-blade or anything you like above the humerus, and let the string be just long enough to let the arm be quite straightened out, but no longer, so that when the arm is straight the string is just about tight, or at least not loose.

Now shorten the string by pinching it up into a loop. Whenever you do this you will bend the fore-arm on the arm. Suppose you used a string which you had not to pinch up, but which, when you pleased, you could make to shorten itself. =Every time it shortened itself it would pull the fore-arm up and would bend the arm--and every time it slackened again, the arm would fall back into the straight position.=

In your arm there is not a string, but a body, placed very much as our string is placed, and which has the power of shortening itself when required. Every time it shortens itself it bends the arm, and when it has done shortening and lengthens again, the arm falls back into its straight position. This body which thus can shorten and lengthen itself is called a muscle.

If you put one hand on the front of your other upper arm, about half-way between your shoulder and elbow, and then bend that arm, you will feel something rising up under your hand. This is the muscle, which bends the arm, shortening, or, as we shall learn to call it, =contracting=.

In your own arm, as in the limb of the rabbit which you studied in your last lesson, the flesh is arranged in masses or bundles of various sizes and shapes, and each mass or bundle is called a muscle. There are several muscles in the arm, but there is in particular a large one occupying the front of the arm, called the =biceps=. It is a rounded mass of red flesh, considerably longer than it is broad or thick, and tapering away at either end. It is represented in Fig. 3.

You may remember that while examining the leg of the rabbit you noticed that in many of the muscles, the soft flesh, which made up the greater part of the muscle, at one or both ends of the muscle suddenly left off, and changed into much firmer material which was white and glistening. This firmer white part you were told was called the tendon of the muscle. The rest of the muscle, generally called “the belly,” is made up of what you are accustomed to call flesh, or lean meat, but which you must now learn to speak of as =muscular substance=. Every muscle, in fact, consists in the first place of a mass of muscular substance. =This muscular substance is made up of an immense number of soft strings or fibres, all running in one direction and done up into large and small bundles.= At either end of the muscle these soft muscular fibres are joined on to firmer but thinner fibres of connective or fibrous tissue. And these thinner but firmer fibres make up the cord or band of tendon with which the muscle finishes off at either end.

=It is by these tendons that the soft muscles are joined on to the hard bones, or to some of the other firm textures of the body.= The tendons are sometimes round and cord-like, sometimes flat and spread out. Sometimes they are very long, sometimes very short, so as to be scarcely visible. But always you have some amount of the firmer fibres of connective tissue joining the soft muscular fibres on to the bones, and generally the tendons are not only firmer but much thinner and more slender than the belly of the muscle.

The muscular belly of the biceps is placed in the front of the upper arm. Some little way above the elbow-joint it ends in a small round strong tendon which slips over the front of the elbow and is fastened to, i.e. grows on to, the radius at some little distance below the joint (Fig. 3, P). The upper part of the muscular belly ends a little below the shoulder, not in one tendon but in two tendons (Fig. 3, a), which gliding over the end of the humerus are fastened to the shoulder-blade (or =scapula= as it is called), into which the humerus fits with a joint.

We have then in the biceps a thick fleshy muscular belly placed in the front of the arm and fastened by tendons, at one end to the shoulder-blade, and at the other to the fore-arm. What would happen if when the arm is straight and the shoulder-blade fixed, the biceps were suddenly to grow very much shorter than it was? Evidently the same thing that happened when you pinched up and shortened the string which, if you look back you will see, we supposed to be placed very much as the biceps with its tendons is placed. =The radius and ulna would be pulled up, the fore-arm would be bent on the arm.=

Now tendons have no power of shortening themselves, but muscular substance does possess this remarkable power of suddenly shortening itself. Under certain circumstances each soft muscular fibre of which the muscle is made will suddenly become shorter, and thus the whole muscle becomes shorter, and so pulls its two tendinous ends closer together, and if one end be fastened to something fixed, and the other to something moveable, the moveable thing will be moved.

This way that a muscle or a muscular fibre has of suddenly shortening itself is called a =muscular contraction=. =All muscles, all muscular fibres, have the power of contracting.= Now a mass of substance like the biceps might grow shorter in two ways. It might squeeze itself together and become smaller altogether, it might squeeze itself as you would squeeze a sponge into a smaller bulk. Or it might change its form and not its bulk, becoming thicker as it became shorter, just as you might by pressing the two ends together squeeze a long thin roll of soft wax into a short thick one. It might get shorter in either of these two ways, but it does actually do so in the latter way; it gets thicker at the same time that it gets shorter, and gets nearly as much thicker as it gets shorter. And that is why, when you put your hand on the arm which is being bent, you feel something rise up. =You feel the biceps getting thicker as it is getting shorter in order to bend the arm.=

The shortening does not last for ever. Sooner or later the muscle lengthens again, getting thinner once more, and so returns to its former state. The lengthened condition of the muscle is the natural condition, the condition of rest. The shortening or contraction is an effort which can only be continued for a certain time. The contraction bends the arm, and as long as the muscle remains shortened the arm keeps bent; but as the muscle lengthens, the weight of the hand and fore-arm, if there is nothing to prevent, straightens the arm out again.

=It is in the muscle alone, in the belly made up of muscular fibres, that the shortening takes place.= The tendons do not shorten at all. On the contrary, if anything they lengthen a little, but only a very little, when the muscle pulls upon them. Their purpose is to convey to the bone the pull of the muscle. They are not necessary, only convenient. It would be possible but awkward to do without them. Suppose the fleshy fibres of the biceps reached from the shoulder-blade to the fore-arm: you could bend your arm as before, but it would be very tiresome to have the muscle swelling up in the inside of the elbow, or on the top of the shoulder; in either place it would be very much in the way. By keeping the fleshy, the real contracting muscle, in the arm, and carrying the thin tendons to the arm and to the shoulder, you are enabled to do the work much more easily and conveniently.

Well, then, we have got thus far in understanding how the arm is bent. The biceps muscle contracts and shortens, tries to bring its two tendinous ends together. The upper tendons, being fastened to the fixed shoulder-blade, cannot move; but the lower tendon is fixed to the radius; the radius, with the ulna to which it is fastened, readily moves up and down on the elbow-joint--the shape of bones in the joint and all the arrangements of the joint, as we have seen, readily permitting this. When the muscle, then, pulls on its lower tendon, its pulls on the radius at the point where the tendon is fastened on to the bone. The radius thus pulled on forms with the ulna a lever of the third order, working on the end of the humerus as a fulcrum; and thus as the tendon is pulled the fore-arm is bent.

=17.= But now comes the question. What makes the muscle shorten or contract? You willed to move your arm, and moved it, as we have seen, by making the biceps contract; but =how did your will make the biceps contract=?

If you could examine your arm as you did the leg of the rabbit, you would find running into your biceps muscle, one or more of those soft white threads or cords, which you have already learnt to recognize as =nerves=.

These nerves seem to grow into and be lost in the biceps muscle. We need not follow them any further in that direction, but if we were to trace them in the other direction, up the arm, we should find that they soon meet with other similar nerves, and that the several nerves joining together form stouter and thicker nerve-cords. These again join others, and so we should proceed until we came to quite stoutish white nerve-trunks as they are called, which we should find passed at last between the vertebræ, somewhere in the neck, into the inside of the vertebral canal, where they became mixed up with the mass of nervous material we have already spoken of as the spinal cord.

What have these nerves to do with the bending of the arm? Why simply this. Suppose you were able without much trouble to cut across the delicate nerves going to your biceps, and did so: what would happen? You would find that you had lost all power of bending your arm; however much you willed it, there would be no swelling rise up in your arm. Your biceps would remain perfectly quiet, and would not shorten at all, would not contract in obedience to your will.

What does this show? It proves that when you will to bend your arm, something passes along the nerves going to the biceps muscle, which something causes that muscle to contract? The nerve, then, is a bridge between your will and the muscle--so that when the bridge is broken or cut away, the will cannot get to the muscle.

If anywhere between the muscle and the spinal cord you cut the nerve which goes, or branches from which go, to the muscle, you destroy the communication between the will and the muscle.

The spinal cord, as we have seen, is a mass of nervous substance continuous with the brain; from the spinal cord nearly all the nerves of the body are given off; those nerves whose branches go to the biceps muscle in the arm leave the spinal cord somewhere in the neck.

If you had the misfortune to have your spinal cord cut across or injured in your neck, you might still live, but you would be paralysed. You might will to bend the arm, but you could not do it. You would know you were willing, you would feel you were making an effort, but the effort would be unavailing. The spinal cord is part of the bridge between the will and the muscle.

When you bend your arm, then, this is what takes place. =By the exercise of your will a something is started in your brain. That something--we will not stop now to ask what that something is--passes from your brain to the spinal cord, leaves the spinal cord and travels along certain nerves, picking its way among the intricate bundles of delicate nervous threads which run from the upper part of the spinal cord to the arm until it reaches the biceps muscle. The muscle, directly that “something” comes to it along its nerves, contracts, shortens, and grows thick; it rises up in the arm; its lower 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.=

You wish to leave off bending the arm. Your will ceases to act. The something to which your will had given rise dies away in the brain, dies away in the spinal cord, dies away in the nerves, even in the finest twigs. The muscle, no longer excited by that something, ceases to contract, ceases to swell up, ceases to pull at the radius, and the fore-arm by its own weight falls into its former straightness, stretching, as it falls, the muscle to its natural length.

=18.= So far I hope you have followed me, but we are still very far from being at the bottom of the matter. Why does the muscle contract when that something reaches it through the nerves? We must content ourselves by saying that it is the property of the muscle to do so. Does the muscle always possess this property? No, not always.

Suppose you were to tie a cord very tightly round the top of your arm, close to the shoulder. What would happen? If you tied it tight enough (I don’t ask you to do it, for you might hurt yourself) the arm would become pale, and very soon would begin to grow cold. It would get numbed, and would gradually seem to grow very heavy and clumsy; your feeling in it would be blunted, and after a while be altogether lost. When you tried to bend your arm you would find great difficulty in doing so. Though you tried ever so much, you could not easily make the biceps contract, and at last you would not be able to do so at all. You would discover that you had lost all power of bending your arm. And then if you undid the cord you would find that after some very uncomfortable sensations, little by little the power would come back to you; the arm would grow warm again, the heaviness and clumsiness would pass away, the feeling in it would return, you would be able to bend it, and at last all would be as it was before.

What did you do when you tied the cord tight? The chief thing you did was to press on the blood-vessels in the arm and so stop the blood from moving in them. If instead of tying the cord round the whole arm you had tied a finer thread round the blood-vessels only, you would have brought about very nearly the same effect. We saw in the last lesson how all parts of the body are supplied with blood-vessels, with veins, and arteries. In the arm there is a very large artery, branches from which go all over the arm. Some of these branches go to the biceps muscle. What would happen if you tied these branches only, tying them so tight as to stop all the blood in them, but not interfering with the blood-vessels in the rest of your arm? The arm as a whole would grow neither pale nor cold, it would not become clumsy or heavy, you would not lose your feeling in it, but nevertheless if you tried to bend your arm you would find you could not do it. You could not make the biceps contract, though all the rest of the arm might seem to be quite right.