Repair of Broken Bones. If bones are broken by a fall, or blow, they display a remarkable power of repair. The "skin" covering them (periosteum) pours out a quantity of living lime-cement, or animal-mortar, around the two broken ends, which solders them together, much as a plumber will make a joint between the ends of two pipes. This repair substance is called _callus_. The most remarkable thing about the process is that, when it has held the two broken ends together long enough for them to "knit" firmly--that is, to connect their blood vessels and marrow cavities properly--this handful of lime-cement, which has piled up around the break, gradually melts away and disappears; so that, if the ends of the bone have been brought accurately together, you can hardly tell where the break was, except by a slight ridge or thickening. TROUBLES OF THE NERVOUS SYSTEM The Nervous System is not easily Damaged. The nervous system is subject to a good many more diseases than are either the muscles or the bones; but, considering how complex it is, it is not nearly so easily damaged or thrown out of balance as we usually imagine, and has astonishing powers of repair. Instead of being one of the first parts of the body to be attacked by a disease, such as an infection or a fever, it is one of the very last to feel the effects of disease, except in the sense that it often gives early that invaluable danger signal, pain. Headache. Next after fatigue the most valuable danger signal given us by our nerves is that commonest of all pains, _headache_. Indeed, it is not too much to say that headache is the most useful pain in the world. It has little to do with the condition of the brain, but occurs in the head chiefly because the nerves of the head and face are the most sensitive of all those in the body, and the first ones, therefore, to "cry out" when hurt. Headache has been described as the cry of a poisoned or starved or over-worked nerve, and is simply nature's signal that something is going wrong. Toxins, or poisons, formed anywhere in the body, from any cause, get into the blood, are carried to the sensitive nerves of the head and face, and irritate them so that they ache. It is foolish to try to do anything to the head itself for the relief of headache, although cold cloths, or a hot-water bottle, may be soothing in mild cases. The thing to do is to clear the poison out of the blood, and the only way is to find what has caused it.