Calculi (calx, lime) are masses of crystalline matter of stony hardness formed in the interior of hollow organs, such as the bladder, the bowels, the pelvis of the kidney, the ducts of glands, etc. Their form, character, and even position varies with the organ in which they are produced. As a rule they are mainly composed of salts natural to the secretion of the organ in which they are found. Why the salts of these fluids should crystallize out and aggregate together to form large stones it is not easy to say, but there is ground for the belief that they are induced to do so in part, if not wholly, for one of two reasons - either that they are in excess of the normal quantity, or that they are brought into contact with some kind of matter by which they are attracted and upon which they gather, or both these conditions may be concerned in the action.

To what extent, if any, the nature of the food conduces to the formation of stone cannot be stated. It is not found that stone in the bladder, which is chiefly composed of lime, is more prevalent on the chalk formation than elsewhere, and the statement that animals "pastured where lime has recently been laid "become the subjects of calculus requires confirmation. It may, however, be noticed that all conditions which favour the long retention of concentrated saline solutions in an organ predispose to calculus formation, hence it arises that stones or gravel are frequently found to occur where paralysis exists in the bladder or other organs.