812 NUTRITION AND HEAT REGULATION. 



that the products of protein digestion are absorbed mainly into the 

 blood-vessels of the intestine, and therefore must pass through the 

 liver before reaching the general circulation. It will also be remem- 

 bered that we are as yet ignorant of the precise form in which these 

 products enter the portal blood. This deficiency in our knowledge 

 constitutes a serious obstacle to a satisfactory explanation of the 

 nutritional history of the protein. Three general views have been 

 advanced concerning the ultimate fate of the absorbed material. 

 In two of these theories it is assumed that the digested material is 

 synthesized into a new protein, before or after absorption, being 

 converted into what we might call a body protein characteristic of 

 the animal. Although it is not specifically stated, the assumption 

 seems to be that this body protein is the serum-albumin of the 

 animal's blood. 



Accepting this general assumption, one theory, advocated by 

 Pfliiger, supposes that before undergoing physiological oxidation 

 all of this absorbed material is built up into the living protoplasm 

 of the various tissues and then undergoes the characteristic metab- 

 olism (catabolism or disassimilation) of that tissue. 



The second theory, advanced by Voit, assumes that some of the 

 absorbed material is assimilated to form living protoplasm, so far 

 as this is necessary to replace the wastes of the tissue or to provide 

 new material for growth. The portion of the absorbed protein 

 that subserves this function is designated as tissue protein. It is 

 obvious that this function cannot be replaced by the non-protein 

 that is, the non-nitrogenous foodstuffs. The larger portion of the 

 absorbed material, however, after distribution to the tissues is 

 destroyed, with liberation of heat, under the influence of the activity 

 of the living cells, but without actually becoming transformed into 

 living matter. The cells act toward this material as the yeast 

 cells do toward the sugar that they decompose into alcohol and 

 carbon dioxid. The portion of the protein that undergoes this fate 

 is designated as the circulating protein, on the hypothesis that it 

 enters the circulating liquids of the body, the blood, and lymph. 



The third general point of view has not been formulated very 

 definitely, but represents perhaps the trend of modern investigation. 

 According to this theory, the split products of protein digestion, 

 the monamino- and diamino-bodies leucin, tyrosin, arginin, etc. 

 are not wholly built up into a new body protein. Some of the ma- 

 terial must be so synthesized, either in the intestine or in the tis- 

 sues, to provide material for the regeneration of the wastes of the 

 body, and it will be remembered that, as stated by Abderhalden 

 p. 729), there is some evidence that a portion of the protein mole- 

 cule during digestion is not broken up into the ultimate split prod- 



