THE URINE. 175 



account of its sparing solubility, it may lead to stone in the 

 bladder. 



It may be further remarked, that in animals which void 

 the urine solid, uric acid takes the place of urea; thus from 

 serpents uric acid is obtained in masses of pure crystals. Other 

 matters are contained in the urine in small quantity, such as 

 a peculiar pigment, various extractive matters, among which 

 may be mentioned kreatin, and mucus from the bladder. 



The kidney is both a separator of urea from the blood, and 

 a manufacturer of more. That some of the urea is formed 

 elsewhere, and carried to the kidneys in the circulating 

 stream, is shown from that substance being always present in 

 the blood, although in small quantity, and from its accumu- 

 lating in animals from which the kidneys have been extracted. 

 But that it is in large part manufactured by the kidneys, is 

 proved by its accumulating far more rapidly in animals 

 whose ureters have been tied, than in those that have had 

 the kidneys removed. 



