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_.