OUTPUT OF WASTE MATTER 133 



notice, more especially as illustrating the 

 modes of excretion. It is evident that 

 nitrogenous waste matters should be soluble 

 so that they may be carried off in the urine 

 Urea, of which about five hundred grains are 

 eliminated daily, is readily soluble in water 

 It is a carb-amide, and is represented by the 

 formula CO(NH 2 ). It contains the same ele- 

 ments as cyanate of ammonia, but it has not 

 the same molecular structure. Under the influ- 

 ence of various enzymes, it takes up water, 

 and is changed into ammonium carbonate, 

 which gives the ammoniacal odour to decom- 

 posing urine. In such urine we always have a 

 precipitate of phosphates of lime, phosphate of 

 magnesia, and the ammoniaco-magnesian, or 

 triple, phosphate, as these are not soluble in an 

 alkaline fluid. As already pointed out, a large 

 proportion if not the whole of the urea is 

 formed in the liver by the splitting up of pro- 

 tein matter that has come from the intestine, 

 and, to be more precise, it is formed from 

 amino-acids. This is sometimes spoken of as 

 the exogenous formation of urea. Along with 

 urea we always find traces of ammonia. 



