8. The heat also warms the body. It keeps it at the same warmth on a cold day as on a hot day. 9. We wear clothes to keep the heat in, and so to keep us warm. 10. The air of a room needs to be changed often. It is made stuffy by our breath. 11. The voice is made by the breath in a box in the neck. 12. Alcohol uses air belonging to the cells of the body. 13. Tobacco smoke has the same poisons as tobacco. It can poison the whole body through the lungs. CHAPTER XI THE SKIN AND KIDNEYS =122. Waste matters.=--The food is burned in the cells. As this burning goes on, the _smoke_ goes off by the lungs and the unburned substances, the _ashes_, go off by the skin and kidneys. The ashes are mostly the minerals of the cells, but there are also some from the burned albumin. All these go back to the blood and are carried to the skin and kidneys. [Illustration: =The skin (×100).= _a_, _b_ and _c_ epidermis. _d_ and _g_ tough and thick part of skin. _e_ sweat gland. _f_ blood tubes. _h_ fat pockets.] =123. The skin.=--The skin covers the whole body. It is strong and keeps the body from being hurt. =124. The epithelium.=--The skin is covered with a thin layer of cells like fine scales. These scales are called _epithelium_, or _epidermis_. They have no blood tubes or nerves and so have no feeling. You can run a pin under them without feeling pain. They are always growing on their under side and wearing off on their upper side. They keep the nerves and blood tubes of the skin from being hurt. =125. The nails.=--The top scales of epithelium at the ends of the fingers become matted together to make the nails. The nails keep the ends of the fingers from being hurt. They can also be used to hold or cut small things. The new parts of the nails form under the skin and push down the older parts. So the nail grows farther than the end of the finger and needs to be cut off. Biting the nails leaves their ends rough. Then they may catch in the clothes and tear into the tender flesh. We ought to keep the nails cut even with the ends of the fingers. The nails are not poisonous, but the dirt under them may be. We ought to keep them clean. Clean nails are one mark of a careful boy or girl. =126. Hair.=--Some of the scales of epithelium over some parts of the body dip into tiny holes in the skin. In each hole they become matted together to form a _hair_. Fine short hair grows on almost every part of the body. On the top of the head it grows long and thick. When boys become men, it also grows long upon their faces. The skin pours out a kind of oil to keep the hair soft and glossy. [Illustration: =A hair (×200).= _a_ the surface of the skin. _b_ a hair. _c_ an oil gland. _d_ a muscle to make the hair stand on end. _e_ and _g_, the growing cells of the hair. _f_ fat in the skin.]