564 MORBID CONCRETIONS. 



the urine is increased, or at least its tendency to precipitate is 

 very much augmented. This is partly, no doubt, owing to the 

 evolution of ammonia in the urine, which almost always takes 

 place when the inner coat of the bladder is diseased. Calculi 

 composed of phosphate of lime are rare, and in general they 

 contain no other ingredient than phosphate of lime cemented by 

 animal matter, and disposed in concentric coats. In some rare 

 cases, the external coat, or at least part of it, is uric acid ; but an 

 external coat of fusible calculus or ammonia-phosphate of mag- 

 nesia is rare. This would indicate that the urine in which phos- 

 phate of lime calculi are deposited is not ammoniacal. 



There can be little doubt that the nucleus of almost all the 

 calculi is formed in the kidney : and what is called a fit of the 

 gravel is the pain felt while that nucleus is passing from the 

 kidney through the ureter to the bladder. We must except those 

 cases in which any solid substance makes its way into the blad- 

 der ; because a urinary calculus almost always is deposited up- 

 on this solid matter. Thus in the Hunterian collection there is 

 a large fusible calculus, which has for its nucleus a piece of leaden 

 sound. I have seen a calculus formed upon a pin, which must 

 have been thrust into the bladder, (probably of a female,) through 

 the urethra. Dr Marcet gives an instance of a musket-ball lodg- 

 ed in the bladder, round which as a nucleus a urinary calculus 

 had concreted. 



As most of the constituents of urinary calculi exist in the 

 urine, there is no great difficulty in conceiving how they may ori- 

 ginate, either in the kidney or bladder. 



Uric acid being a constant constituent of urine, and being 

 very little soluble, we can easily see how it may be deposited 

 whenever the quantity of free acid, which urine contains, happens 

 to be augmented. If uric acid exists in urine (as Dr Prout 

 conjectures) in the state of urate of ammonia, that salt would 

 be deposited whenever it exists in greater than its usual quan- 

 tity in ammoniacal urine. It is very curious that this state of 

 the urine should be confined to children, and that the deposition 

 of such calculi produces so great a degree of irritation. 



Phosphate of lime exists in urine though in small quantity. 

 It is doubtless held in solution by the acid which healthy urine 

 contains in excess. Hence phosphate of lime can only be depo- 

 sited when the urine becomes alkaline by the evolution of am- 



