148 NUTRITIONAL PHYSIOLOGY 



animal in fair condition. Physiologists accepted the 

 evidence and granted that the proteins of the organism 

 could be synthesized sometimes at least from amino- 

 acids. Shortly after this change in our conceptions the 

 discovery of erepsin was announced. It was recognized 

 that the existence of this enzyme made it more probable 

 that digestion should normally run its full course rather 

 than it should be terminated at an early stage by the in- 

 tervention of the absorbing cells. It became apparent 

 that even though the material leaving the intestine might 

 have 'the relatively complex character which we associate 

 with the peptone stage of digestion, the products delivered 

 to the interior of the villi by the cells might have under- 

 gone further cleavage. The ability of the animal to turn 

 to account such simple bodies in synthesizing its own 

 proteins became clear. 



In the chapter on Intestinal Digestion (Chapter X) the 

 statement was made that the various amino-acids have 

 been called the "building-stones" of metabolism. In the 

 course of digestion they are separated, and after absorption 

 or during the very act of absorption, they are to some 

 extent assembled again. Many facts bearing upon this 

 process have been brought to our attention by the recent 

 studies of physiologic chemists concerning the constitu- 

 tion of protein molecules. All that has been done of late 

 in this direction has served to emphasize the variety of in- 

 dividual structure comprehended under the term protein. 

 When such compounds from various sources are subjected 

 to decomposition, either by digestion or by other means, 

 the assortment of amino-acids obtained differs with the 

 particular protein under investigation. 



Some, which one is tempted to regard as perfect proteins, 

 yield the full list of amino-acids as at present known. 

 Others, seemingly defective when judged by this rather 

 arbitrary standard, fail to yield certain members of the 

 series. This variation has an important bearing on mat- 

 ters of nutrition. It can no longer be maintained that all 

 substances characterized as proteins are equivalent in 



