396 HUMAN PHYSIOLOGY 



per day does not vary very greatly, but the amount of fluids 

 imbibed varies within much wider limits. For this reason, 

 while the amount of water excreted in the urine per diem 

 varies enormously, the amount of solids is more fixed. In a 

 man on an average diet, it may be stated that something 

 like 1500 c.cm. of water and 60 to 70 grms. of solids are 

 daily eliminated. 



I. Nitrogenous Substances. 



A. Urea. The chemistry and mode of formation of urea 

 have been discussed on p. 369. Since it is as urea that, 

 on an ordinary diet, nearly 90 per cent, of the waste nitrogen 

 is eliminated in the urine, the amount excreted depends 

 upon the amount of proteids taken in the food. For this 

 reason, during fasting, the excretion of urea may fall as low 

 as 6 grms. per diem, while on a diet containing the ordinary 

 amount of proteids, about 33 grms. 15*4 grms. of nitrogen 

 are excreted. On a normal diet from 86 to 90 per cent, of 

 the waste nitrogen is excreted as urea, but, when the nitrogen 

 intake is decreased, the proportion of urea-nitrogen may fall 

 to as low as 60 per cent. 



When the urine is allowed to stand, certain micro- 

 organisms are apt to get into it, and to cause a hydration of 

 the urea, whereby it is changed into ammonium carbonate 



TT T_T 



H H) [H 



H N-O-C-O-N ' 



H' IH 



The urine is thus made alkaline, and the phosphates of 

 the earths are precipitated. The phosphate of magnesia 

 combines with the ammonia to form ammonio-magnesium- 

 phosphate, NH 4 MgP0 4 -f 6H 2 O, which crystallises in charac- 

 teristic prism-like crystals. 



B. Non-Urea Nitrogen. The 10 or 12 per cent, of nitrogen 

 which on an ordinary diet is not excreted as urea is dis- 

 tributed in : 



1. Ammonium Salts. About 4 or 5 per cent, of the total 



