396 CONSTITUENTS OF URINE. [BOOK n. 



is alkaline. The latter, when fasting, are for the time being 

 carnivorous, living entirely on their own bodies, and hence their 

 urine becomes under these circumstances acid. 



The natural acidity increases for some time after the urine has 

 been discharged, owing to the formation of fresh acid, apparently 

 by some kind of fermentation. This increase of acid frequently 

 causes a precipitation of urates, which the previous acidity has 

 been insufficient to throw down. After a while however the acid 

 reaction gives way to alkalinity. This is caused by a conversion 

 of the urea into ammonium carbonate through the agency of a 

 specific ferment. This ferment as a general rule does not make its 

 appearance except in urine exposed to the air ; it is only in un- 

 healthy conditions that the fermentation takes place within the 

 bladder. 



Abnormal constituents of Urine. The structural elements 

 found in the urine under various circumstances are blood, pus and 

 mucus corpuscles, epithelium from the bladder and kidney, and 

 spermatozoa. Serum-albumin, fibrin (frequently as ' casts '), alkali- 

 albumin, globulin, a peculiar form of albumin (the so-called hemi- 

 albumose), fats, cholesterin, sugar, leucin, tyrosin, oxalic acid, bile 

 acids and bile pigment, may be enumerated as the most important 

 metabolic products abnormally present in urine. Besides these the 

 urine serves as the chief channel of elimination for various bodies, 

 not proper constituents of food, which may happen to have been 

 taken into the system. Thus various minerals, alkaloids, salts, 

 pigmentary and odoriferous matters, may be passed unchanged. 

 Many substances thus occasionally taken suffer changes in passing 

 through the body ; the most important of these will be considered 

 in a succeeding chapter. 



