The little air-cells of the lungs have very delicate muscular (mŭs´ku lar) walls. Every time we breathe, these walls have to move. The muscles of the chest must also move, as you can all notice in yourselves, as you breathe. All this muscular work, as well as that of the stomach and heart, is directed by the nerves. You have learned already what alcohol will do to muscles and nerves, so you are ready to answer for stomach, for heart, and for lungs. Is alcohol a help to them? REVIEW QUESTIONS. 1. Besides carrying food all over the body, what other work does the blood do? 2. Why does the blood in the veins look blue? 3. Where is the blood made pure and red again? 4. Where is it sent, from the lungs? 5. What must the lungs have in order to do this work? 6. When do the lungs rest? 7. Why should we not wear tight clothes? 8. How does the air in a room become spoiled? 9. How can we keep it fresh and pure? 10. How should we breathe? 11. Why is it better to breathe through the nose than through the mouth? 12. Why is alcohol not good for the lungs? CHAPTER XVI. THE SKIN. [Illustration: T]HERE is another part of your body carrying away waste matter all the time--it is the skin. The body is covered with skin. It is also lined with a more delicate kind of skin. You can see where the outside skin and the lining skin meet at your lips. There is a thin outside layer of skin which we can pull off without hurting ourselves; but I advise you not to do so. Because under the outside skin is the true skin, which is so full of little nerves that it will feel the least touch as pain. When the outer skin, which protects it, is torn away, we must cover the true skin to keep it from harm. In hot weather, or when any one has been working or playing hard, the face, and sometimes the whole body, is covered with little drops of water. We call these drops perspiration (pẽr spĭ rā´shŭn). [Illustration: _Perspiratory tube._] Where does it come from? It comes through many tiny holes in the skin, called pores (pōrz). Every pore is the mouth of a tiny tube which is carrying off waste matter and water from your body. If you could piece together all these little perspiration tubes that are in the skin of one person, they would make a line more than three miles long. Sometimes, you can not see the perspiration, because there is not enough of it to form drops. But it is always coming out through your skin, both in winter and summer. Your body is kept healthy by having its worn-out matter carried off in this way, as well as in other ways. THE NAILS. The nails grow from the skin.