URINARY CALCULI 381 



passages, although the center of the concretion most often con- 

 sists of uric acid or urates. Infection does not always play so 

 important a part here as it seems to in the formation of biliary 

 calculi ; calculi formed because of changes in the urinary com- 

 position independent of infection are often called " primary/' in 

 contradistinction to those arising from changes in composition 

 brought about by infection and ammoniacal decomposition. 

 Because of the injury produced by a primary calculus, infection 

 frequently results, and then the primary calculus may become 

 the nucleus of a secondary calculus ; indeed, on account of the 

 change of reaction, the primary calculus may be dissolved out, 

 and its place taken by the secondary deposit (metamorphosed 

 calculi). In structure urinary calculi usually show both radi- 

 ating and concentric lines of formation, and when the chief 

 constituents are dissolved away, an organic framework remains. 

 They are generally classified according to their composition, as 

 follows : 



Uric-acid Calculi. Uric acid is but slightly soluble, only 

 one part dissolving in 39,480 of pure water at 18, and it is even 

 less soluble in the presence of acids. The presence of sodium 

 diphosphate in the solution makes it much more soluble, and 

 various organic bodies also favor its solution, among them 

 being the urinary pigments. As can be seen, the maintenance 

 of uric acid in solution is by a small margin, even in normal 

 conditions ; hence the mere cooling of the urine frequently suf- 

 fices to cause an abundant deposition of uric acid combined 

 with pigment, as the familiar " brick-dust " deposit. The for- 

 mation of uric-acid calculi is, therefore, not only a question of 

 the amount of uric acid in the urine, but depends even more 

 upon the amount of the substances that hold it in solution, and 

 as both these factors are subject to wide variations under both 

 physiological and pathological conditions, uric-acid and urate 

 calculi are the commonest of urinary concretions. Uric acid is 

 eliminated combined chiefly with sodium, potassium, and am- 

 monium ; according to some authors, as a biurate, according to 

 others, as a quadriurate. If the urine is excessively acid, it con- 

 tains much acid phosphates, which withdraw part of the bases 

 from the uric acid, and this, when free, crystallizes out if in 

 excess. Hence the formation of uric-acid concretions is favored 

 by high acidity of the urine, by concentration of the urine, or 

 by an increased elimination of the uric acid. The last may 

 result from excessive nuclein-rich food, or from excessive katab- 

 olism of the tissue nucleoproteids (e. g. y leucocytosis from in- 

 flammatory diseases or leukemia), which conditions are also 



