The mobility of the calcium and phosphorus of bone has been demonstrated in recent years to be much greater than we formerly thought,1 and it has come to be recognized that it probably is this increasing and decreasing salt content of bone that is responsible for the fluctuations that we have observed in its breaking strength.2 The pliable bone of youth and the brittle chalky bone of the aged have for a long time been considered as differing in their respective strengths because of variations in their mineral content, but it has been only lately that we have been able to show that years are not necessary for the production of marked changes in the strength of bone. On the contrary, we have found fluctuations in breaking strength of as much as 40 per cent occurring in the short space of six days.3
In recent publications we