6.  All bodily motions are due to the action of the muscles.

7.  Most of the muscles act only when we wish them to do so.  Some muscles, however, act when it is necessary for them to do so, whether we will that they should act or not, and when we are asleep as well as when we are awake.

CHAPTER XXI.

HOW TO KEEP THE MUSCLES HEALTHY.

1.  How to Make the Muscles Strong.—­With which hand can you lift the more? with the right hand or with the left?  Why do you think you can lift more with the right hand than with the left?  A blacksmith swings a heavy hammer with his right arm, and that arm becomes very large and strong.  If we wish our muscles to grow large and strong, so that our bodies will be healthy and vigorous, we must take plenty of exercise.

2.  Effects of Idleness.—­If a boy should carry one hand in his pocket all the time, and use only the other hand and arm, the idle arm would become small and weak, while the other would grow large and strong.  Any part of the body which is not used will after a time become weak.  Little boys and girls who do not take plenty of exercise are likely to be pale and puny.  It is important that we should take the proper amount of exercise every day, just as we take our food and drink every day.

3.  Healthful Exercise.—­Some kinds of play, and almost all kinds of work which children have to do, are good ways of taking exercise.  A very good kind of exercise for little boys and girls is that found in running errands or doing chores about the house.

4.  Food and Strength.—­A great part of our food goes to nourish the muscles.  Some foods make us strong, while others do not.  Plain foods, such as bread, meat, potatoes, and milk, are good for the muscles; but cakes and pies, and things which are not food, such as mustard, pepper, and spices, do not give us strength, and are likely to do us harm.

5.  Over-Exertion.—­We ought not to exert ourselves too much in lifting heavy weights, or trying to do things which are too hard for us.  Sometimes the muscles are permanently injured in this way.

6.  The Clothing.—­We ought not to wear our clothing so tight as to press hard upon any part of the body.  If we do, it will cause the muscles of that part to become weak.  If the clothing is worn tight about the waist, great mischief is often done.  The lungs cannot expand properly, the stomach and liver are pressed out of shape, and the internal organs are crowded out of their proper places.

7.  Tight Shoes.—­People are often made very lame from wearing tight shoes.  Their muscles cannot act properly, and their feet grow out of shape.