HAT makes the limbs move?

You have to take hold of the door to move it back and forth; but you need not take hold of your arm to move that.

What makes it move?

Sometimes a door or gate is made to shut itself, if you leave it open.

This can be done by means of a wide rubber strap, one end of which is fastened to the frame of the door near the hinge, and the other end to the door, out near its edge.

When we push open the door, the rubber strap is stretched; but as soon as we have passed through, the strap tightens, draws the door back, and shuts it.

If you stretch out your right arm, and clasp the upper part tightly with your left hand, then work the elbow joint strongly back and forth, you can feel something under your hand draw up, and then lengthen out again, each time you bend the joint.

What you feel, is a muscle (mŭs´sl), and it works your joints very much as the rubber strap works the hinge of the door.

One end of the muscle is fastened to the bone just below the elbow joint; and the other end, higher up above the joint.

When it tightens, or contracts, as we say, it bends the joint. When the arm is straightened, the muscle returns to its first shape.

There is another muscle on the outside of the arm which stretches when this one shortens, and so helps the working of the joint.

Every joint has two or more muscles of its own to work it.

Think how many there must be in our fingers!

If we should undertake to count all the muscles that move our whole bodies, it would need more counting than some of you could do.