This message-and-answer system runs all through our body. For instance, if we run fast, then the muscle cells in our legs burn up a good deal of sugar-fuel, and throw the waste gas, or smoke, into the blood. This is pumped by the heart all over the body, in a few seconds. When this carbon dioxid reaches the breathing centre in the medulla, it stirs it up to send promptly a message to the lungs to breathe faster and deeper, while, at the same time, it calls upon the circulation centre close to it, to stir up the heart and make it beat harder and faster, so as to give the muscles more blood to work with. If some poisonous or very irritating food is swallowed, as soon as it begins to hurt the cells lining the stomach, these promptly telegraph to the vomiting centre in the brain, we begin to feel "sick at the stomach," the brain sends the necessary directions to the great muscles of the abdomen and the diaphragm, they squeeze down upon the stomach, and its contents are promptly pumped back up the gullet and out through the mouth, thus throwing up the poisons. And so on all over the body--every tiniest region or organ in the body, every square inch of the skin, has its special wire connecting it with the great telephone exchange, enabling it to report danger, and to call for help or assistance the moment it needs it. FOOTNOTES: [26] To give you an idea of what real things nerve-trunks are, this sciatic nerve is as large as a small clothes-line, or, more accurately, as a carpenter's lead pencil, and so strong that when the surgeon cuts down upon it and stretches it to cure a very bad case of sciatica, he can lift the lower half of the body clear of the table by it. This strength, of course, is not due to the nerve-fibres and cells themselves but to the tough, fibrous sheath, or covering, with which all the nerves that run outside of the brain and spinal cord are covered and coated. The spinal cord, though it is between one-half and three-fourths of an inch across, or about the size of an ordinary blackboard pointer, has little or none of this fibrous tissue in it, and is very soft and delicate, easily torn when its bony case is broken; hence its old name, the _spinal marrow_, from its apparent resemblance to the marrow, or soft fat, in the hollow of a bone. CHAPTER XXI THE HYGIENE OF BONES, NERVES, AND MUSCLES HOW TO GET AND KEEP A GOOD FIGURE Erect Position is the Result of Vigorous Health. Naturally and properly, an erect, graceful figure and a good carriage have always been keenly desired; and much attention has been paid to the best means of acquiring them; as we say, we try to "get the habit" of carrying ourselves straight and well. But it must be remembered that an erect figure and a good carriage are the results of health and vigor, rather than the cause of them.