When there is too little air, a fire burns slower, and makes a blacker smoke and more ashes. Alcohol takes some air from the cells of the body. So they burn with smoke and ashes of the wrong kind. The skin has to work harder to get rid of these, and sometimes it cannot do it well. Then the body is poisoned. The alcohol is burned and cannot poison the body any more. But it causes the body to make poisons, and so it is to blame. The poisons do great harm to the skin and kidneys. Alcohol causes more kidney disease than all other things put together. WHAT WE HAVE LEARNED 1. Little tubes in the skin are always giving off ashes and waste matters in the perspiration. 2. Perspiration dries on the skin. So the skin must be washed often. 3. The kidneys get rid of more water and waste matter than the skin does. 4. Perspiration also gets upon the clothes and bed sheets. These must be washed too. 5. Dirty water from washing should be thrown out where it cannot run into a well. 6. The skin is thick and strong and keeps the body from being hurt. 7. The skin is covered with a layer of scales. The scales have no feeling. 8. The scales form the nails on the ends of the fingers. 9. The scales also form the hair. CHAPTER XII THE NERVES, SPINAL CORD, AND BRAIN =136. Need of nerves.=--The cells of the mouth, stomach, and intestine digest food; the cells of the liver change the food to blood; the cells of the heart pump the blood to feed all the cells of the body; the red blood cells carry air for the cells to breathe; and the cells of the skin and kidneys carry away the waste of the rest of the cells. Each set of cells works for all the rest. If the cells of the body were only tied together, each one would do as it pleased, and no two would work together. But something tells each cell of the body to work with the others. The cells all obey the mind. A tiny thread goes to each cell of the body. Each thread is a _nerve_. The mind and the cells signal to each other over the nerves. By means of the nerves the mind makes the cells work together. [Illustration: =A nerve thread (×400).= _a_ central conducting fiber. _b_ covering of fat.] [Illustration: =A thin slice for the end of a cut nerve (×200).= _a_ nerve thread. _b_ connective tissue binding the threads into a cord.] =137. Nerve messages.=--The nerve threads run in bundles and form nerves large enough to be seen. The mind uses the nerves to tell the cells to do work. It tells the muscles to move the arms and legs. It tells the heart to beat and stomach to pour out gastric juice; and it tells each of the cells to eat. The cells also send word over the nerves to the mind. They tell the mind when they are touching anything, and whether it is hard, or smooth, or hot, and many other things about it. The cells also tell the mind if they need more food, or are tired.