104 NUTRITION OF FARM ANIMALS 



bearing the same name but derived from different species or 

 different individuals of the same species, the proteins of one 

 animal often being toxic to another. 



The body proteins, then, are specific both as to composition 

 and structure and differ in both respects from those of the feed. 

 In order that the latter may nourish the organism they must be 

 converted into the specific proteins of the body. This is accom- 

 plished through their cleavage in the digestive tract into their 

 constituent " building stones " which the body may then 

 reassemble to form proteins constructed according to its own 

 specific pattern. Not only so, but the proteins of different 

 tissues or even cells must be regarded as specific. The body 

 proteins are built not after a single pattern but after numerous 

 ones. It is only by a very extensive, even if not complete 

 (139), breaking down of the structure of the feed proteins that 

 it becomes possible for the body to build up out of the fragments 

 the various proteins which it requires. "Its protein mole- 

 cules have a different architecture from those of the plant." 

 This fact throws an interesting light upon the coagulation of 

 the soluble casein of milk in the stomach. Although present 

 in soluble form, it is not a body protein and its coagulation 

 serves to retain it in the digestive tract and give the proteolytic 

 enzyms an opportunity to break it up into its constituent 

 amino acids. 



What is so strikingly true of the proteins is likewise true of 

 other nutrients. The digestive cleavages serve not merely, 

 or perhaps not chiefly, to render them soluble and diffusible 

 but to reduce the molecular complexes to forms which the body 

 cells can assimilate. The carbohydrates, e.g., are converted 

 into monosaccharids, even the somewhat larger molecules of 

 the disaccharids appearing to be too large or to have an un- 

 suitable molecular structure for the body cells to use. In 

 general the complex compounds of the feed are split up by the 

 enzyms of the digestive fluids into their constituent atomic 

 groupings or " building stones " which supply the material out 

 of which the body by selection and rearrangement builds up 

 the proteins, carbohydrates and fats peculiar to itself, and the 

 value of a feed depends upon the nature and amounts of the 

 cleavage products which it yields in digestion rather than upon 

 the specific substances which it contains. 



