404 MANUAL OF PHYSIOLOGY. 



The urine undergoes important changes after being voided, the 

 explanation of which is of much interest to the practitioner, and 

 must be understood by the student of medicine. (1.) Com- 

 monly enough the urine loses its transparency as soon as it gets 

 cold, though perfectly clear when passed, or when again heated 

 to the body-temperature, for the urates are soluble in warm but 

 almost insoluble in cold water. This "muddiuess," which soon 

 settles down, as a more or less brightly colored sediment, is 

 chiefly caused by the precipitation of acid sodium urate, stained 

 with a coloring matter derived from the urochrome. When 

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

 and if it be left standing for some time in a cool place, the acid- 

 ity 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 formation of lactic and acetic acids, and crystals of uric acid, 

 amorphous sodium urate, and crystals of lime oxalate are depos- 

 ited. 



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. 398) 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 

 ammonio-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, 



