440 MORBID PRODUCTS. 



niaco-magnesian phosphate is the predominating ingredient 

 exhibit a porous internal surface, studded here and there with 

 crystals. 



In calculi consisting of a nucleus and of laminae deposited 

 round it, it is important to ascertain whether the nucleus and 

 the concentric laminae are identical or different in their com- 

 position, 



The nucleus may consist either of one of the ordinary con- 

 stituents of urinary concretions or of a foreign body introduced 

 into the bladder, as for instance, a fragment of wood, a grain 

 of corn, 1 &c. The laminae may have the same chemical consti- 

 tution as the nucleus, and only differ from it in the period of 

 their deposition, as is the case with calculi of uric acid, urate 

 of ammonia, uric oxide, and earthy phosphates : or they may 

 differ from the nucleus in their composition ; in this case we 

 may always infer that changes have taken place in the character 

 of the urinary secretion ; for instance, if the nucleus consist of 

 uric acid, and is surrounded by a concentric layer of the 

 earthy phosphates, the urine must have been first constantly 

 acid, and subsequently neutral or alkaline. 



I now proceed to the consideration of the most common 

 urinary calculi. 



Combustible calculi. 



I. Calculi of uric acid are by no means rare; their cha- 

 racter have been already described in page 431; they may be 

 distinguished from calculi of urate of ammonia by the solu- 

 bility of the latter, and the insolubility of the former in a suf- 

 ficient quantity of boiling water. They are of every possible 

 size, their colour is sometimes (but very rarely) white, most 

 commonly yellow, rose-coloured, or brown; their surface is 

 smooth, sometimes even polished, and occasionally presents 

 rounded verrucose protuberances. Their fractured surface pre- 

 sents either a crystalline appearance, or is dense with concentric 

 strata merging into each other. The nucleus is crystalline, 

 and the surrounding laminae hard. I have found a minute 



1 [Professor Malago has recently extracted a calculus, of which the nucleus was a 

 globule of mercury. Filiatre Sebezio, 1845.] 



