134 PRINCIPLES OF PHYSIOLOGY 



One of the substances produced by the 

 decomposition of muscle-protoplasm is a 

 nitrogenous body called creatine. By union 

 with the elements of water, it splits into urea 

 and sarcosine, showing a relationship to the 

 former substance, and possibly part of it 

 may ultimately be converted into urea, as it 

 does not appear normally in urine. A 

 closely allied substance, differing from 

 creatine as regards its formula by the loss 

 of the elements of water, is creatinine, 

 which always exists in urine. It is in all 

 probability formed, not from the creatine 

 of muscle, as was once supposed, but from 

 the metabolism of protein in the liver, 

 and as it is poisonous it is thrown out in the 

 urine. There is still obscurity on this point. 

 Another nitrogenous waste product in the 

 urine is uric acid, of which from seven to ten 

 grains are separated daily. Unlike urea, it is 

 highly insoluble, but being an acid it unites 

 with the alkalies, soda and potash, to form 

 urates, which are highly soluble, and in this 

 form it is thrown out. If in excess, or if there 

 is not sufficient base to unite with it, uric acid 



