In the pathological ossification, as well as its physiological prototype, the carbonates and phosphates of calcium and magnesium are present in a specially formed tissue of the nature of bone-cartilage, whereas calcification occurs independently of such a new-formed tissue. The deposition of the calcareous salts takes place either in the cells or intercellular substance of living or dead tissues, when the terms calcification or ossification are applied, or as accumulations of various size in tissues or canals, which are known as concretions and calculi. The immediate causes of the physiological deposition in the formation of bone are so obscure that only more or less probable explanatory theories are advanced, to all of which obvious objections arise. The causes of a pathological precipitation may be regarded as equally hidden. It is apparent, however, that old age usually furnishes the necessary factors. This in part may be due to the feeble nutrition associated with impairment of function in advancing years. In part it may be the result of the numerous opportunities offered in a long life for the occurrence of inflammation, the products of which are frequently infiltrated with calcareous salts. The latter are apparently kept in solution by the action of living cells, for, though presented to all in the fluids of the body, they are precipitated most constantly in dead parts or in the vicinity of those cells whose function is presumably lessened from disease or age. The solvent action of living cells is further demonstrated by the effect of the giant-cells in removing calcium salts from living or dead bone. The causes of calcification are therefore to be regarded as local, depending upon a destruction or weakening of the cells of a part--conditions which are directly attributable to an interference with nutrition. The deposition of calcium salts thus represents a disorder of nutrition, and may be experimentally produced by agencies which occasion a necrosis of tissues.