The nerves are always carrying messages to and from the cells. The cells depend upon these messages to tell them when and how to work. If the nerve of any part of the body is hurt or cut, we cannot feel with the part or move it, and its cells do not act in the right way. We do not feel the nerves while they are carrying the messages. We wish the cells of the arm to work, and they work, but we do not feel the message as it goes from the mind to the cells of the arm. [Illustration: =A thin slice from the spinal cord with the cells and nerves magnified 200 diameters.= _a_ cells in the gray matter. _b_ fibers in the gray matter. _c_ nerve threads in the white matter.] =138. The spinal cord.=--The nerves start inside the backbone. The backbone is hollow. It has a soft, white cord inside, as thick as the little finger. Part of the mind lives in this cord. The cord is called the _spinal cord_. Some of the nerves start from cells of the spinal cord. These cells send word to the muscles to move and to all the cells of the body to eat and grow. They also send word to the arteries to carry the right amount of blood to the cells. From the nerves the spinal cord gets word when something hurts any part of the body. You may put your finger on a sharp pin. The spinal cord feels the prick, and quickly sends word to snatch the finger away. So the finger is taken away before you really feel the prick. When some one sticks a pin into you, you cannot help jumping. This is because the spinal cord sends word for you to jump away from the pin before it can harm you much. Thus the spinal cord keeps the body from being hurt. It acts while we are asleep as well as when we are awake. =139. Need of a spinal cord.=--We do not feel the spinal cord acting, and we cannot keep it from acting. It tells the cells when to eat and grow, and it tells the heart and arteries how much blood to send to each cell. If we had to think about feeding an arm or a leg, we should sometimes forget it, but the spinal cord keeps doing it without our thinking of it. We put food into the body, and the spinal cord tells the cells to use it. If it stops acting for an instant, the cells stop work and we die. We cannot change its action by any amount of thinking. [Illustration: =Regions of the head and action of the different parts of the brain.=] =140. The brain.=--The nerves of the body go to the brain as well as to the spinal cord. The brain lies in the top of the head. A hard cover of bone keeps it from getting hurt. It is a soft white mass, and weighs about three pounds. Its outside is made of cells, while its inside is the very beginning of the nerves of the body. =141. The mind.=--The mind is the real man. It is the thinking part of himself. It lives in the body and works by means of the cells of the brain. If these cells are hurt or killed, the body seems to have no mind, but yet it may keep on living. If all the mind leaves the body, the body is dead.