Physiology

by Sir M. Foster

The two tendons by which this muscle is attached to the scapula, or shoulder-blade, are seen at a. P indicates the attachment of the muscle to the radius, and hence the point of action of the power; F, the fulcrum, the lower end of the humerus on which the upper end of the radius (together with the ulna) moves; W, the weight (of the hand).

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 if you notice you will see that as you move the ulna up and down, its end and the end of the humerus slide over each other. But they will only slide one way, what we may call up and down. If you try to slide them from side to side, you will find that they get locked. They have only one movement, like that of a door on its hinge, and that movement is of such a kind as to double the ulna on the humerus.

Moreover, if you look a little more carefully you will find that, though you can easily double the ulna on the front of the humerus, and then pull it back again until the two are in a straight line, you cannot bend the ulna on the back of the humerus. On examining the end of the ulna you will find at the back of it a beak-like projection ([Fig. 3], also Frontispiece), which when the bones are straightened out locks into the end of the humerus, and so prevents the ulna being bent any further back. This is the reason why you can only bend your arm one way. As you very well know, you can bend your arm so as to touch the top of your shoulder with your fingers, but you can’t bend it the other way so as to touch the back of your shoulder; you can’t bring it any further back than the straight line.

[14.] Well, then, at the elbow the two bones, the humerus and ulna, are so shaped and so fit into each other that the arm may be straightened or bent. In the skeleton the two bones are quite separate, i.e. they have to be fastened together by something, else they would fall apart. Most probably in the skeleton you have been examining they are fastened together by wires or slips of brass. But they would hold together if you took away the wire or brass slips and bound some tape round the two ends, tight enough to keep them touching each other, but loose enough to allow them to move on each other. You might easily manage it if you took short slips of tape, or, better still, of india-rubber, and placed them all round the elbow, back, front, and sides, fastening one end of each slip to the humerus and the other to the ulna. If you did this you would be imitating very closely the manner in which the bones at the elbow are kept together in your own arm. Only the slips are not made of india-rubber, but are flat bands of that stringy, or as we may now call it fibrous stuff, which in the preceding lessons you learnt to call connective tissue. These flat bands have a special name, and are called ligaments.

At the elbow the two ends of the ulna and humerus are kept in place by ligaments or flat bands of connective tissue.

In the skeleton, the surfaces of the two bones at the elbow where they rub against each other, though somewhat smooth, are dry. If you ever looked at the knuckle of a leg of mutton before it was cooked, you will have noticed that you have there two bones slipping over each other somewhat as they do at the elbow, and will remember that where the bones meet they are wonderfully smooth, and very moist, so as to be quite slippery. It is just the same in your own elbow; the end of the ulna and the end of the humerus are beautifully smooth and quite moist, so that they slip over each other as easily as possible. You know that your eye is always moist. It is kept moist by tears, though you don’t speak of tears until your eyes overflow with moisture; but in reality you are always crying a little. Well, there are, so to speak, tears always being shed inside the wrapping of ligaments around the elbow, and they keep the two surfaces of the bones continually moist.

The ends of bones where they touch each other are also smooth, because they are coated over with what is called gristle or cartilage. Bone is very hard and very solid; there is not much water in it. Bones dry up very little. Cartilage is not nearly so hard as bone; there is very much more water in it. When it is quite fresh it is very smooth, but because it has a good deal of water in it, it shrinks very much when it dries up, and when dried is not nearly so smooth as when it is fresh. You can see the dried-up cartilage on the ends of the bones in the skeleton—it is somewhat smooth still, but you can form no idea of how smooth it is in the living body by simply seeing it on the dried skeleton.

At the elbow, then, we have the ends of two bones fitting into each other, so that they will move in a certain direction; these ends are smoothed with cartilage, kept moist with a fluid, and held in place by ligaments. All this is a called a joint.

[15.] There are a great many other joints in the body besides the elbow-joint: there is the shoulder-joint, the knee-joint, the hip-joint, and so on. These differ from the elbow-joint in the shape of the ends of the bone, in the way the bones move on each other, and in several other particulars, but we must not go into these differences now. They are all like the elbow, since in each case one bone fits into another, the surfaces are coated with cartilage, are kept moist with fluid (what the grooms call joint-oil, though it is not an oil at all), and are held in place by ligaments.

I dare say you will have noticed that though I have been speaking only of the humerus and ulna at the elbow, the other bone of the fore-arm—the radius—has something to do with the elbow too. I left it out in order to simplify matters, but it is nevertheless quite true that the end of the humerus moves over the end of the radius as well as over the end of the ulna, and that the end of the radius is also coated with cartilage and is included in the wrapping of the ligaments. I might add that the radius also moves independently on the ulna, but I don’t want to trouble you with this just now. What I wanted to show you was that the elbow is a joint, a joint so constructed that it allows the fore-arm to be bent on the upper arm.