150 



PRACTICAL PHYSIOLOGY. 



[XXIV. 



portion of blood-clot, or some albuminoid matter in which crystals of oxalate 

 of lime or globular urates become entangled. Layer after layer is then de- 

 posited. In certain cases the nucleus may con- 

 sist of a foreign body introduced from without. 

 Calculi are sometimes classified as primary and 

 secondary; the former are due to some general 

 alteration in the composition of the urine, whilst 

 the latter are due to ammoniacal decomposition 

 of the urine, resulting in the precipitation of 

 phosphates on stones already formed. This of 

 course has an important bearing on the treat- 

 ment of calculous disorders. Calauli occur in 

 acid and alkaline urine. A highly acid urine 

 favours the formation of uric acid calculi, because 

 that substance is most insoluble in very acid 

 urine. A highly alkaline urine favours the for- 

 mation of calculi consisting of calcium phosphate or triple phosphate, as these 

 substances are insoluble in alkaline urine. 



FIG. 78. Cystin. 



4. Method of Examining a Calculus. 



(a.) Make a section in order to see if it consists of one or more 

 substances ; examine it with the naked eye, and a portion micro- 

 scopically. 



(b.) Scrape off a little, and heat it to redness on platinum foil 

 over a Bunsen- burner. 



79. a. a. Leucln balls ; b.b. Tyrosin sheaves ; . Double balls of 

 ammonium urate. 



(A.) If it be entirely combustible, or almost so, it may consist of 

 uric acid or urate of ammonium, xanthin, cystin, coagulated fibrin 

 or blood, or ureostealith. 



