462 THE HUMAN BODY. 



for the primary nitrogen waste of many tissues. Chemically 

 leucin is an ammonium derivative, being the amide of caproic 

 (a fatty) acid. 



"While the urea resulting from further changes in the 

 kreatin, leucin, or similar substances formed in the tissues, is 

 a measure of the wear and tear of their protoplasm, part of 

 the urea excreted has probably a different source; being due 

 to the oxidation of proteids as energy liberators or respira- 

 tory foods, before they have ever formed a tissue. When 

 abundant proteid food is taken the urea excretion is largely 

 increased and that very rapidly, within a couple of hours for 

 example, and before we can well suppose the proteids eaten 

 to have been built up into tissues, and these in turn broken 

 down; in fact there need be, and usually is, under such cir- 

 cumstances no sign of any special activity of any group of 

 tissues, such as one would expect to see if the urea always 

 came from the breaking down of formed histological ele- 

 ments. This urea is thus indicative of a utilization of pro- 

 teids for other than plastic purposes; and the same fact is 

 indicated by the storage of carbon and elimination of all 

 the nitrogen of the food when a diet very rich in proteid 

 alimentary principles is taken. This luxus consumption may 

 be compared to the paying out of gold by a merchant instead 

 of greenbacks when he has an abundance of both. Only the 

 gold can be used for certain purposes, as settling foreign 

 debts, but any quantity above that needed for such a purpose 

 is harder to store than the paper money, and not so con- 

 venient to handle; so it is paid out in preference to the 

 paper money, which is really somewhat less valuable, as 

 available at par only for the settlement of domestic debts. 

 Similarly, only proteids can be used for certain final stages of 

 tissue building, but an excess of them is more difficult to 

 store than fats or carbohydrates, and so is eliminated in pref- 

 erence to them. 



In artificial pancreatic digestions, when long carried on, 

 two bodies, called leucin and tyrosin, are produced from 

 proteids. It is found that when leucin is given to an ani- 

 mal in its food, it reappears in the urine as urea; so the Body 

 can turn leucin into that substance. Hence a possible source 

 of some of the luxv.s-consumption urea is leucin produced dur- 

 ing intestinal digestion; and this is very likely turned into 

 urea in ^the liver. Mammalia rapidly die when the liver is 



