INTRODUCTION 3 



is very different from what was held to be the case even a few 

 years ago. The indiffusible colloid substance, protein, was 

 formerly supposed to be modified in such a manner by the hydro- 

 lyzing agencies of the intestinal tract that, as a final product, 

 peptone a diffusible substance was formed. The trypsin of 

 the pancreatic juice was known to be able to carry the hydro- 

 lysis beyond the stage of peptone, but Kuhne, who first investi- 

 gated its action, arrived at the conclusion that trypsin was only 

 able to affect half of the protein molecule. It is now known that 

 both pepsin and trypsin can liberate very many of the products 

 of acid proteolysis, and to a great extent destroy the protein 

 character of the substance on which they act. Further, the 

 ferment erepsin of the succus entericus is able to cause a more 

 complete breaking down of the peptones and albumoses that 

 have escaped the action of pepsin and trypsin, so that most 

 observers are unanimous in thinking that the hydrolysis of 

 proteins in digestion is complete. In the light of recent work it is 

 evident that the breaking down of the protein molecule of ingested 

 material, due to the action of the digestive enzymes, goes on to 

 the actual splitting up into organic radicles the amino-acids and 

 other crystalline derivatives. These derivatives of protein 

 digestion a large number of which have now been isolated and 

 many of them synthesized resemble in the main the units or 

 building-stones obtained as cleavage products in the acid hydro- 

 lysis of proteins in vitro. There is experimental proof that the 

 final products of tryptic digestion are able to take the place of 

 protein in the food. Otto Leowi maintained a dog in complete 

 health with an increase in body weight on the products of 

 pancreatic digestion, the chemical observations showing that no 

 protein was present in the food, and that the daily amount of 

 nitrogen metabolized was less than that ingested. Henriques 

 and Hansen have successfully carried out similar experiments 

 on rats, and have confirmed these results. From the building- 

 stones or cleavage products of protein digestion the body is, 

 therefore, capable of building up its bioplasm, and of making 

 good the wear and tear of everyday life ; in doing so, in all 

 probability, different sets of units or amino-acids are made use of 

 in the formation of the many different types of nitrogenous 

 material required by the several tissues of the body. That is, the 

 material out of which all proteins in the body are made is not 

 protein in any form, but the organic radicles derived from proteins 



