PROTEIN METABOLISM 97 



liver, each splitting being such as to further the forma- 

 tion of urea." 



"In these special hydrolytic decompositions, the 

 result of which is to remove the unnecessary nitrogen, 

 we have an explanation of why and how the animal 

 organism tends to maintain nitrogen equilibrium even 

 when excessive amounts of protein are furnished with 

 the food. This excess of protein is not stored up in 

 the organism, as such, because the actual need of nitro- 

 gen is so small that an excess is always furnished with 

 the food. . ." 



PRESENT-DAY THEORY OF METABOLISM 



The proof of the presence of amino acids in the 

 blood through the investigations of Folin, Van Slyke, 

 and others has rendered necessary some slight modifi- 

 cation of our views concerning metabolic processes. 

 Although Folin in his original theory foretold the 

 probable importance of the lower protein decomposi- 

 tion products it was not until the actual presence of 

 these substances in the blood and tissues was demon- 

 strated that acceptance of this idea was general. Since 

 it has been proven beyond question that amino acids 

 are normally absorbed directly into the blood from the 

 intestine and are distributed to the tissues, it is assumed 

 that each tissue rebuilds itself from the mixture of 

 amino acids thus received. That portion of amino 

 acids which is not necessary for synthesis is changed 

 into urea and carbonaceous residues presumably by a 



