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(also concretions or stones), solid masses in the hollow organs and glandular ducts of man.
Calculi may be of different sizes, shapes, and consistencies. The chemical composition varies with the place where they are formed and with the composition of the fluid in which lithogenesis (formation of stones) takes place. Calculi in the gall bladder and bile ducts consist of cholesterol, bile pigments, calcium phosphate, and calcium carbonate; urinary calculi are formed in the urinary tract and are divided according to the acid radicals into urates (the commonest type), oxalates, phosphates, and carbonates. Urinary calculi may contain xanthine, cystine, indigo, sulfur, soap, and cholesterol. The causes of calculi formation may be general (for example, disturbance of metabolism or pregnancy) or local (change in the chemical composition of a secretion, specifically, its protective colloids and pH, stagnation of a secretion, inflammation). Lithogenesis is most often caused by a combination of these factors. The formation of biliary and urinary calculi is the basis of cholelithiasis and urolithiasis, respectively.