Stones are formed within the urinary tract under a variety of conditions, and no single mechanism suffices to explain the process. It is difficult to distinguish the actual cause from the predisposing factors, and impossible to distinguish it from those factors which may be regarded as secondary.
Cystine and uric acid stones cannot be explained on any other basis than faulty metabolism. The experimental production of oxamide calculi by feeding large quantities of oxalate can probably be explained on a physiochemical basis. That a foreign body, regardless of its structure or composition, acts as a nucleus for the deposition of urinary salts is well established, so that the part played by mechanical factors and obstruction cannot be ignored. It is apparent that the chemical substances which go to make up urinary calculi vary under different conditions, and are dependent in part on the etiologic factor.
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