422 TEXT-BOOK OF PHYSIOLOGY. 



colorless neutral substance, crystallizing under varying conditions 

 in long silky needles or in rhombic prisms. It is soluble in water 

 and alcohol. It is composed of CON 2 H 4 . When subjected to pro- 

 longed boiling, it combines with water, giving rise to ammonium 

 carbonate. The presence of Micrococcus urea in urine will also 

 convert the urea, by combining it with two molecules of water, into 

 ammonium carbonate, CON 2 H 4 + 2H 2 O=(NH 4 ) 2 CO 3 . 

 MJ The average amount of urea excreted daily varies from 30 to 34 

 Nkp grams. ^As urea is now known to be the principal end-producl; of 

 proteid metabolism wi^i'n tiie bodv^it is evident that the quantity 

 produced and eliminated in the twenty-four hours will depend on the 

 quantity of proteid food consumed and on the extent to which the 

 proteid constituents of the tissues are metabolized^ In the condition of 

 nutritive equilibrium, when the proteid ingested is 100 grams and the 

 urea egested 31.5 grams, it is difficult to state the percentage of urea 

 which is derived from the metabolism of the proteid food (circulating 

 proteid) and that derived from the metabolism of the proteids of the 

 tissues (organ proteid). In this condition, however, it is found that 

 if the proteid consumed is varied within limits above or below the 

 standard amount of 100 grams, the quantity of urea excreted rises and 

 falls in practically the same ratio, indicating apparently that the 

 production of urea is directly dependent on the proteid supply. On 

 the contrary, it has been observed in human beings in the fasting 

 condition that for a period of ten days there is a daily excretion of 

 about 21 grams of urea, equivalent to about 70 grams of profeid. 

 Again, contrary to former views, the metabolism of proteid and the 

 production of urea are practically independent of muscular work. 

 Even after severe labor extending over a period of some hours there 

 is no noticeable increase in the urea eliminated. 



Seat of Urea Formation. It is quite certain in the light of 

 present knowledge that urea is partly foimed JDJ^JTiverNDV the action 

 of the cells out of cleavage" products of proteid metabolism. The par- 

 ticular compounds out of which the cells synthetize urea are the 

 ammonium salts, especially the carbamate and carbonate. The 

 experimental reasons for this view have already been stated on page 

 410. 



Uric acid is one of the constant ingredients of the urine. It is a 

 crystalline nitrogen-holding body closely resembling urea, its formula 

 bein g C 5 H 4 N 4 O r The total quantity excreted daily varies from 0.2 

 to^i gram. It is doubtful if uric acid exists in a free state in the 

 urine,^the indications being that it is combined with sodium and 

 potassium in the form of a quadriurate. The urates are frequently 

 ^ deposited when in excess from the urine as a brick-red sediment, 

 the color being due to their combination with the coloring-matter 

 uroerythrin.^ When pure, uric acid crystallizes in the rhombic form, 



