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. =16. In order that the arm may be bent, some force must be used.= The ulna or radius--for the two move together--must be pushed or pulled towards the humerus, or the humerus must be pushed or pulled towards the radius and ulna. How is this done in your own arm?