434 THE HUMAN BODY. 



Urea (N 2 H 4 CO) is the chief nitrogenous waste product 

 of the human Body and is related to the ammonia group, 

 being readily converted into ammonium carbonate by hydra- 

 tion, a change which occurs under the influence of some 

 living ferments when stale urine becomes alkaline and ac- 

 quires its well-known offensive ammoniacal odor 



N 2 H 4 CO + 2H 2 = (NH 4 ) 2 C0 3 . 



On another side urea is allied to the cyanogen group of sub- 

 stances, being isomeric with ammonium cyanate, which is 

 converted into it by simple heating. 



Uric acid (C 5 H 4 N 4 3 ) is present in but small quantity in 

 normal human urine, but is the chief nitrogenous excretion 

 of birds and reptiles. Its molecular structure is more com- 

 plex than that of urea, and when it is decomposed by various 

 methods urea is very frequently one of the products. It is a 

 less complete product of proteid degradation than urea. 

 Some of its decompositions indicate relations to oxalic acid 

 and to amido-acetic acid (glycin), and through this latter to 

 the ammonias and the fatty acids series. In human urine 

 uric acid exists chiefly in the form of salts of potassium and 

 sodium; these are less soluble in cold than in warm water, 

 and are sometimes deposited as a flocculent precipitate when 

 originally clear urine is left to cool. The precipitate dis- 

 appears on reheating the liquid. 



Hippuric acid (C 7 H 6 2 ) is scanty in normal human urine 

 but abundant in the urine of herbivora. Chemically it is 

 related to the aromatic series, being formed when benzoic 

 acid and glycin are made to unite with dehydration; and it 

 is broken up into those substances when boiled with mineral 

 acids. Certain aromatic bodies allied to benzoic acid are 

 found in hay and similar foods and account for the large 

 amount of hippu rates in herbivorous urine. But proteids 

 when broken up by putrefaction also yield bodies of the ben- 

 zoic group, and the hippu ric acid of human urine probably 

 has its origin in the liberation of benzoic residues in metabolic 

 activities of some of the living cells of the Body; these 

 residues being then combined with glycin to form hippu ric 

 acid. That glycin is formed in the Body is shown by the j 

 fact that benzoic acid given in food reappears in human urine I 

 as hippuric acid, having been somewhere united to a glycin 

 residue. 



