ORIGIN OF UREA. 407 



this occurs the urine will always be found to he distinctly acid, and 

 if it be left standing for some time in a cool place, the acidity will 

 be found to increase, owing to the presence of a peculiar fungus 

 which sets up acid fermentation. This is said to depend on the for- 

 mation of lactic and acetic acids, and crystals of uric acid, amor- 

 phous sodium urate, and crystals of lime oxalate are deposited. 



After a certain time (which is shorter when the urine is not 

 very acid and is exposed to a warm atmosphere) the development 

 of bacteria occurs in it, and causes the urea to unite with water, 

 and to change in the manner already mentioned (p. 400) into 

 ammonium carbonate. This gradually neutralizes the acidity, 

 and finally renders the urine alkaline. At the same time an 

 amorphous precipitate of lime phosphate appears, and crystals of 

 ammouio-magnesium phosphate and of ammonium urate are pro- 

 duced. 



URINARY CALCULI. 



Various ingredients of the urine, which are difficult of solution, 

 sometimes become massed together as concretions, particularly if 

 there exist any small foreign body in the bladder which by act- 

 ing as a nucleus lays the foundation of a stone. Sometimes small 

 concretions are formed in the tubes or pelvic recesses of the kidney, 

 and, when these make their way into the bladder, they com- 

 monly grow larger and larger. The structure and composition of 

 a calculus often give the history of its own transit from the kidney, 

 and also of various changes in the metabolism of the individual, 

 for successive layers of different substances are generally found 

 in a stone that has attained any great size. The chief materials 

 found in calculi are uric acid, ammonium urate, calcium oxa- 

 late and carbonate, ammonio-magnesium phosphate, etc. 



SOURCE OF UREA, ETC. 



The question as to whether the chief materials of the urine pre- 

 exist in the blood and are therefore merely removed by the kid- 

 ney, or are manufactured by the special powers of the renal cell, 

 has been widely discussed, and though the great weight of evi- 

 dence is in favor of the former view, some of the experimental 

 results on the subject are rather conflicting. 



