It is well known that adult bone is not merely an inert structural material but is a living tissue with a mineral content which fluctuates under the influence of endocrine and enzyme activity. Hevesy (1) showed, by the use of radiophosphorus, that there is active metabolism in bone. He found, for example, that within fifty days 29 per cent of the mineral constituents were renewed in the femoral and the tibial epiphyses of rabbits. In pregnancy there may be a transfer of calcium and phosphorus from the bones and teeth of the mother to the tissues of her child. Over 60 per cent of the skeletal calcium of the newborn child is laid down during the last two months of prenatal life. The trabeculae of the bone constitute a calcium store which is available for maintenance of the calcium requirements of other tissues when the ingested supply is insufficient. The ratio of calcium to phosphorus remains essentially constant. The mineral content of bone may fluctuate even in health, but particularly in certain cases o...