The age-related subperiosteal expansion of long bones partially compensates mechanically for the endosteal resorption and resultant cortical thinning and increased porosity that occurs with aging. This radial outward displacement of the cortical mass provides an increase in a bone’s moment of inertia, thereby serving to protect the long bones from bending and torsional stresses. This can best be understood by envisioning a solid rod of a certain cross-sectional area. If the material in the solid rod were displaced radially from the central axis of the rod to create a hollow tube, the result would be a structure that was stronger in both bending and torsion and thus better able to resist fracture. Thus, rearranging the same amount of material into a hollow tube improves the structural properties. It is no coincidence that the long bones are hollow and that the osteon is also essentially a hollow tube with a central haversian canal. By a process of natural selection, a structure has evolved that best accommodates the local biomechanical requirements of a system whose metabolic resources are under strict systemic control.