11.  The Use of Pain.—­It is not pleasant for us to have pain, but if the nerves gave us no pain when we are hurt we might get our limbs burned or frozen and know nothing about it until too late to save them.

12.  Nerves of Feeling.—­We have different kinds of nerves of feeling.  Those we have learned about feel pain.  Others feel objects.  If you take a marble or a pencil in the hand you know what it is by the feeling of the object.  This kind of feeling is called the sense of touch.

13. There are other nerves of feeling by means of which we are able to hear, see, taste, and smell, of which we shall learn in another lesson.  Besides these we have nerves which tell us whether objects are cold or hot, and heavy or light.  Nerves of feeling also tell us when we are hungry, or thirsty, or tired, and when we need more air to breathe.

14.  Nerves of Work.—­There are other nerves which are made just like the nerves of feeling, but which do not feel.  These nerves have a very different use.  They come from cells in the brain which have charge of the different kinds of work done in the body, and they send their branches to the parts which do the work; hence we call them nerves of work.

15. One set of cells sends nerves to the heart, and these make it go fast or slow as is necessary.  Another sends nerves to the liver, stomach, and other digestive organs, and causes them to do their part in the digestion of the food.  Other cells send branches to the muscles and make them act when we wish them to do so.  Thus you see how very useful the brain and nerves are.  They keep all the different parts of the body working together in harmony, just like a well-trained army, or a great number of workmen building a block of houses.  Without the brain and nerves the body would be just like an army without a commander, or a lot of workmen without an overseer.

16.  How we Use the Nerves.—­If you happen to touch your hand to a hot stove, what takes place?  You will say that your arm pulls the hand away.  Do you know why?  Let us see.  The nerves of feeling in the hand tell the nerve cells in the brain from which they come that the hand is being burned.  The cells which feel cannot do anything for the hand, but some of their branches run over to another part of the brain, which sends nerves down to the muscles of the arm.  These cells, through their nerve branches, cause the muscles to contract.  The cells of feeling ask the cells which have charge of the muscles to make the muscles of the arm pull the hand away, which they do very quickly.

17. So you see the nerves are very much like telegraph or telephone wires.  By means of them the brain finds out all about what is happening in the body, and sends out its orders to the various organs, which may be called its servants.