Look at the bones of the arm in a skeleton. (Frontispiece; also Fig. 3.) You will see that in the upper arm there is one rather large bone (_H_) reaching from the shoulder to the elbow, while in the fore-arm there are two, one (_U_) being wider and stouter than the other (_Ra_) at the elbow, but smaller and more slender at the wrist. The bone in the upper arm is called the =humerus=; the bone in the fore-arm, which is stoutest at the elbow, is called the =ulna=; the one which is stoutest at the wrist is called the =radius=. 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 curiously scooped out, in such a way that the one fits loosely into the other. [Illustration: FIG 3.--_The Bones of the Upper Extremity with the Biceps Muscle._ 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.