The cement (crusta petrosa, substantia ossea) is disposed as a thin layer on the roots of the teeth, from the termination of the enamel to the apex of each root, where it is usually very thick. In structure and chemical composition it resembles bone. It contains, sparingly, the lacunæ and canaliculi which characterize true bone; the lacunæ placed near the surface receive the canaliculi radiating from the side of the lacunæ toward the periodontal membrane; and those more deeply placed join with the adjacent dental canaliculi. In the thicker portions of the crusta petrosa, the lamellæ and Haversian canals peculiar to bone are also found.
As age advances, the cement increases in thickness, and gives rise to those bony growths or exostoses so common in the teeth of the aged; the pulp cavity also becomes partially filled up by a hard substance, intermediate in structure between dentin and bone (osteodentin, Owen; secondary dentin, Tomes). It appears to be formed by a slow conversion of the dental pulp, which shrinks, or even disappears.