84 THE AMINO ACIDS 



genous, or plastic, foods, and the non-nitrogenous or 

 respiratory foods. In accordance with this classifica- 

 tion plastic foods were tissue formers and supplied 

 energy for muscular activity; the respiratory foods, 

 on the other hand, were essential for the respiratory 

 act and the constant temperature of the body, but 

 could not be transformed into organized tissue. 



VOIT 



The fundamental conception of Voit (1867) was 

 that all protein in the body is not decomposed with 

 equal ease. In accordance with this idea he divided 

 the protein material of the body into two groups, the 

 organized or tissue protein, that built up into living 

 protoplasm and difficult of disintegration and, secondly, 

 circulating protein existing in the fluids and tissues of 

 the organism without being an integral part. The 

 circulating protein may be more easily and readily 

 destroyed than the organized or tissue protein. In his 

 classic experiment designed to show the difference of 

 metabolism between tissue protein and circulating 

 protein, Voit allowed a well-fed dog to starve for 

 several days. He demonstrated that under these cir- 

 cumstances there is at first an abundant decomposition 

 of protein material which is later followed by a period 

 during which very little protein is catabolized. His 

 interpretation of these facts was to the effect that dur- 

 ing the time when a large protein disintegration ob- 

 tained only circulating protein was destroyed, whereas 



