COMPOSITION OF THE URINE. 433 



With oxalic acid urea forms urea oxalate, CO(NH 2 ) 2 (COOH) 2I 

 which crystallizes as short rhombic prisms. 



Urea is converted into ammonium carbonate by the action of 

 the ferment Micrococcus urece; this change which takes place in 

 urine declares itself by the ammoniacal odor which is developed. 

 It is represented by the following equation : 



CO(NH 2 ) 2 + 2H 2 = (NHO.CO, 



Urea. Water. Ammonium carbonate. 



Urea is the end-product of the proteid metabolism of the body 

 and of the albuminoids of the food. When a meal containing 

 considerable proteids has been ingested, and the urea determined, 

 the amount will be at a maximum at the third or fourth hour ; 

 this is supposed to indicate the absorption of peptones from the 

 stomach. A second maximum occurs at the sixth or seventh hour, 

 and this is attributed to the absorption of peptones from the 

 intestine. 



Formation of Urea. Urea exists in the blood when that fluid 

 reaches the kidney, and the function of this organ is, so far as 

 urea is concerned, simply to eliminate it. Without ^ recounting 

 the experimental evidence, which is ample and satisfactory, it is 

 enough to say that urea is formed by the cells of the liver. The 

 most satisfactory explanation of the manner of this formation is 

 that given by Drechsel, and is substantially as follows : Proteids 

 undergo hydrolytic cleavage, and leucin, tyrosin, aspartic acid, and 

 other amido-bodies are formed : these undergo oxidation, forming 

 NH 3 , CO 2 , and H 2 O ; NH 3 and CO 2 unite to form ammonium 



~M"TT 



carbonate, CO<Q^fr , which, by the loss of a molecule of 

 water, brought about by the liver-cells, becomes urea : 



Ammonium carbonate. Water. Urea. 



DrechseFs view is that this is accomplished in two stages : 

 First, two atoms of hydrogen are removed, and then one atom of 

 oxygen. 



A series of experiments by various physiologists have demon- 

 strated that when the liver is removed from dogs, although this 

 operation is ultimately fatal, carbonates appear in the urine and 

 there is a diminished amount of urea. That any urea is found 

 demonstrates that the liver is not its only source ; but where else 

 it is formed is not known. 



Uric Acid (C 5 N 4 H 4 O 3 ). The amount of this substance in human 

 urine is very small, being on an average about 0.8 gram, but it is 

 the principal nitrogenous constituent of the urine of birds and 



28 



