19. What does this story teach you?

20. What happens if you lean over your desk or
work?

21. How will this position injure your lungs?

22. What other bones may be injured by wrong
positions?

23. Why do old people's bones break easily?

24. How should the feet be cared for?

25. How does tobacco affect the bones?

26. What do doctors say of its use?

27. What is said about cigarettes?

28. What about chewing tobacco?

29. To whom is tobacco a great enemy? Why?

30. What is always true of its use by youth?

CHAPTER II.

MUSCLES.

[Illustration: W]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 (m[)u]s'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.

TENDONS.

You can see muscles on the dinner table; for they are only lean meat.

[Illustration: _Tendons of the hand._]

They are fastened to the bones by strong cords,