DIVERSE COMPONENTS 



329 



measurement ; while ad libitum maintenance is also a balance, more 

 expensive to hold. Many days are required to demonstrate a bal- 

 ance at either, after a transition of intakes is allowed. A consider- 

 able range of carbon contents is thereby permitted, such as was not 

 found for water or heat. 



For nitrogen, one part of the story is very similar (fig. 160), 

 namely, the gradual adjustment of output to equal intake. But in 

 contrast to carbon exchange, when nitrogen intake ceases, then its 

 output becomes very low. Hence, over a wide range of intake rates 

 for nitrogen, output rates approximately equal them (fig. 161). 



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Fig. 160. Rate of exchange of nitrogen and of carbon in successive days. A dog 

 of 5.86 kg. was deprived of food for 2 days, then fed an excessive quantity of meat 

 (500 gm.) each day for 6 days, then starved for 2 days. Rates of outputs gradually 

 adjusted toward the rates of intake. Data of Rubner ('02, p. 246). 



Yet, exact balances occur at only one point for each individual. 

 The fraction of any excess nitrogen taken in that is actually stored 

 is small, and the rate of unstoring it during deficient intake is small, 

 as compared with carbon or with total energy. Variability of con- 

 tent is least for that component in which modifiability of exchange 

 is greatest. Where constancy of output is maintained (energy), 

 storage is provided, with wide range of contents. 



Very many studies, especially of nitrogen metabolism, have 

 ignored the pre-existing content of nitrogen in the body, and worked 

 entirely with rates of exchange (fig. 161). For example, Voit and 

 Korkunoff (1895) tried to find the minimal intake of meat for 



