3d. The lungs use fresh air in making the dark, impure blood, bright and
pure again.

4th. The skin carries away waste matter through the little perspiration
tubes.

All this work goes on, day and night, without our needing to think about
it at all; for messages are sent to the muscles by the nerves which keep
them faithfully at work, whether we know it or not.

REVIEW QUESTIONS.

1. What covers the body?

2. What lines the body?

3. Where are the nerves of the skin?

4. What is perspiration? What is the common name
for it?

5. What are the pores of the skin?

6. How does the perspiration help to keep you
well?

7. Of what use are the nails?

8. How should they be kept?

9. What care should be taken of the skin?

10. Why should you not wear rubber boots or
overshoes in the house?

11. Why should you change under-clothing night and
morning?

12. Where should the night-dress be placed in the
morning?

13. What should be done with the bed-clothes? Why?

14. Name the four kinds of work about which you
have learned.

15. How are the organs of the body kept at work?

CHAPTER XVII.

THE SENSES.

[Illustration: W]E have five ways of learning about all things around
us. We can see them, touch them, taste them, smell them, or hear them.
Sight, touch, taste, smell, and hearing, are called the five senses.

You already know something about them, for you are using them all the
time.

In this lesson, you will learn a little more about seeing and hearing.

THE EYES.

In the middle of your eye is a round, black spot, called the pupil. This
pupil is only a hole with a muscle around it. When you are in the light,
the muscle draws up, and makes the pupil small, because you can get all
the light you need through a small opening. When you are in the dark,
the muscle stretches, and opens the pupil wide to let in more light.

The pupils of the cat's eyes are very large in the dark. They want all
the light they can get, to see if there are any mice about.

[Illustration: _The eyelashes and the tear-glands._]

The pupil of the eye opens into a little, round room where the nerve of
sight is. This is a safe place for this delicate nerve, which can not
bear too much light. It carries to