106 PRINCIPLES OF PHYSIOLOGY 



hydrogens of the radicle is replaced by a 

 molecule containing two atoms of hydrogen 

 and one of nitrogen. Thus take caproic acid, 

 one of the fatty acid series. It is represented 

 by the chemical formula C 5 H ir COOH; 

 substitute for one of H n a group NH 2 , and 

 we have C 5 H 10 .NH 2 .COOH, or amino- 

 caprcic acid, a well-known body called 

 leucin. It is important to note that these 

 amino-acids, of which there is a large num- 

 ber known to chemists, are always among the 

 final products of the decomposition of proteins. 

 Hence it was inferred that proteins were ab- 

 sorbed as amino-acids. These, however, have 

 not been found in the blood, possibly owing 

 to great technical difficulties, and it is still 

 a matter undecided as to (1) whether proteins 

 are so absorbed, and (2) how and where they 

 are re-transformed into the proteins of the 

 blood. It is a fact, however, that they all 

 pass to the liver, and that during the absorp- 

 tion of proteins the nitrogen in the blood 

 increases. 



Further, it would seem that protein matter 

 may be used up in two ways. A certain portion 



