LUXUS CONSUMPTION. 435 



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 circumstances 

 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 elements. This urea 

 is thus indicative of a utilization of proteids 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 (p. 444) when a diet very rich in proteid alimentary 

 principles is taken. This luxus consumption may be com- 

 pared 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 pur- 

 pose is harder to store than the paper money and not so 

 convenient 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. 



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 

 animal 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 luxus-consumption urea is 

 leucin produced during intestinal digestion; and this is 

 very likely turned into urea in the liver. At any rate 

 the liver, to which the portal vein might carry all leucin 

 thus formed, contains urea, which no other gland does; and 

 when the liver is greatly altered, as in phosphorus poison- 

 ing and the disease known as acute yellow atrophy, urea 

 almost entirely disappears from the urine. This latter fact 

 seems to point to a final production of urea in the liver, what- 

 ever its immediate antecedents may be; whether muscle 

 kreatin, or intestinal leucin, or excess of peptones in the 

 diet. The latter might perhaps be broken up there into a 

 nitrogenous part (urea) and a non-nitrogenous part; and we 

 shall find that a non-nitrogenous substance (glycogen) is 

 stored in the liver. 



