The joints, where two bones which are to play upon each other come in contact, as they do at the elbow or shoulder, are made in different ways. The elbow only moves to and fro like a hinge; the hip and shoulder, like a "ball and socket," move every way. You do not need to be told that each kind of joint is found just where it is needed for the work it has to do; for there is no mistaking or misplacing in God's workmanship, as there so often is in the very best of _ours_. I cannot at present tell you anything about the muscles, except that it is by their means that we move arms, legs, head, eyes--every part of the body, for bones cannot move of themselves, but are acted on by the muscles. Nor can we learn much about the nerves, because the subject is very difficult to understand. They come from the brain in the head, and from that part of it which runs all down the backbone, through the little bony rings of the vertebræ; and they are protected, because they are so very delicate, and so precious to us, by a strong bony sheath. At first these nerves are like coarse twine, but they divide and divide until they become as fine as threads of white silk--almost as fine as the stronger part of a spider's web--and they go all over the body, reaching to the very tips of the fingers. The first pair of nerves goes to the nose, for smell; the second to the eye, for sight; and so on for hearing and taste. These are the nerves called "sensory," which carry to the brain sensations from outside the body. The "motor" nerves are those which take orders from the brain, to be instantly obeyed by the muscles. In the hand, which has twenty-seven bones--one more than the foot--and is a more wonderful "tool" than any which God has given to the lower animals, wonderful as _their_ tools are, the sense of touch is stronger than in any other part of the body. Suppose you put your fingers upon something very hot or very cold. "Quick as thought," as we say, you draw them away again. But before you did so, what had happened? The nerves at the tip of your finger had sent a telegram straight home to the brain, "Too hot!" or "Too cold!" and the brain had telegraphed back to the fingers, "Keep out of the way of it!" whatever the hot or cold thing may have been. To think, even for a moment, of these lightning messages running backwards and forwards, to and from the brain, gives us some little idea how very wonderful the brain itself must be, and also how God has made one part of the body to depend upon another.