CHAPTER XVI.
MORE ABOUT THE BONES.
Bones of the arm and hand.
Here are the bones of the arm and the hand. The head of the arm bone that goes into the socket at the shoulder is, as you see, a smooth round ball. It fits into a sort of cup. The joint here is what we call a ball-and-socket joint. The ball turns in the socket very easily in making any whirling motion with your arm, as you do when you jump the rope.
Shoulder joint and elbow joint.
The joint at the elbow is of a different kind: it is what we call a hinge joint. You can not make any whirling motion at your elbow as you can at the shoulder; the motion is all one way, like a hinge. The chief motion at the wrist also is a hinge motion, as you can see by working your hand back and forth. There are two bones, you notice, in the arm below the elbow: these roll on each other in such a way that you can turn the palm of your hand in different directions.
There are a great many little bones in the body of the hand and in the fingers. There is a very great variety in their motions, so that the hand can do almost any thing that you want it to do. I shall have something more to tell you about this when you come to the chapter on the hand.
Bones of the leg and foot.
You have here the bones of the leg and foot. You see only the lower end of the stout thigh bone, at the knee joint: it makes a hinge joint with the large bone of the leg. The motion of this joint is only one way, backward and forward, as you see in walking. The small, thick bone, called the knee-pan, is left out in the figure. One of the uses of this bone is to be a shield to the joint. If you fall down in running, you are apt to come upon the knee, and this shield keeps the joint from being hurt.