THE METABOLISM OP PROTEINS. 113 



from the organism itself (endogenous). This result leads us to 

 infer that most of the amino substances of protein foods which 

 are not required as building stones for the tissues are broken 

 down so as to yield ammonia, which is excreted as exogenous urea 

 in the urine, but that the amino bodies that are really appropri- 

 ated by the tissues, although they may also produce some urea 

 (endogenous), cause other end-products to be formed. The most 

 important of these endogenous bodies is evidently Creatinin, for, 

 as will be seen from the above table, this substance is excreted in 

 the same absolute amount during both the starvation and the 

 protein-rich periods. 



Direct evidence that this conclusion is correct has been ob- 

 tained by examination of the blood and muscles for amino bodies, 

 ammonia and urea. The results have shown that the ammo 

 bodies absorbed from the intestine are carried through the liver 

 into the systemic blood, which transports them to the muscles, 

 where those that are not required for building up the tissues 

 are broken down into ammonia and a carbonaceous residue, which 

 is then burned just exactly as if it were carbohydrate or fat. 

 The useless ammonia becomes converted into urea in the manner 

 already described, either in the muscles themselves, or by being 

 carried to the liver, which, as we have seen, possesses to a very 

 high degree the power of producing urea. 



The Relative Importance of Proteins, Fats and Carbohy- 

 drates in Metabolism. The metabolism of fats and carbohy- 

 drates, with regard both to their importance as builders of living 

 tissues and the type of their metabolism, is very different from 

 that of proteins. That carbohydrates and fats are less impor- 

 tant in the animal economy than proteins is evidenced by the 

 fact that we can live perfectly well on protein food alone, but 

 not on either of the others. This does not, however, justify us 

 in concluding that carbohydrates and fats are merely materials 

 which are oxidized by the tissues, for the purpose of producing 

 energy, fuel as it were, and which can be dispensed with. They 



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are more than this, for no cell, in however starved a condition 

 it may be, is entirely free from either of them, thus indicating 

 that they must have been produced out of protein itself. Pro- 



