THE ABSORPTION OF THE FOOD-STUFFS 749 



hydration occurring in the intestine would involve such a loss of energy to the 

 body as to be unteleological. Neither of these theoretical objections is 

 justified in fact. We know from the researches of Fischer and others that 

 although the different proteins in our food present a marvellous qualitative 

 similitude, in that all of them yield on hydrolysis the same kinds of amino- 

 acids, there are great differences in the relative amounts of these amino-acids 

 contained in different proteins. Thus, in gelatin, glycine is contained in 

 considerable quantities, but is absent in many of the other proteins. 

 Caseinogen is distinguished by the large amount of leucine that it yields, 

 while gliadin, the chief protein of wheat flour, contains very large amounts 

 of glutamic acid. It is difficult to imagine how, for instance, muscle protein 

 could be formed from wheat protein, a process continually occurring in the 

 growing animal, unless we assumed that the protein molecule is first entirely 

 taken to pieces, and that its constituent molecules are then selected by the 

 growing cells of the body and built up in the order and proportions which are 

 characteristic of muscle protein. Moreover, when we measure the amount 

 of energy change involved in the hydrolysis of the proteins, we find it is rela- 

 tively small. There is not a loss of 5 per cent, of the total energy available 

 i.e. the heat of combustion of the products of pancreatic digestion would differ 

 from that of the original protein submitted to digestion by less than 5 per 

 cent. The energy of the protein as evolved in the body lies, not in the 

 coupling of the amino-acids with one another, or indeed in the coupling of 

 the nitrogen to the carbon, but, like that of the other food-stuffs, in the carbon 

 itself, and is derived from the combustion of the carbon of the molecule under 

 the influence of the oxidising processes of the body into carbon dioxide. 

 The experimental decision of this question was first attempted by 0. Loewi, 

 who found that it was possible to keep a dog in a state of nitrogenous equili- 

 brium on a diet containing fat, starch, and a pancreatic digest of protein 

 which contained no substances which would give the biuret test. These 

 results have been confirmed for carnivora by Henderson, by Liithje, by 

 Abderhalden and Rona, and by Henriques and Hansen. According to 

 Abderhalden, it is possible to keep an animal alive when the nitrogen in his 

 food is represented entirely by the end-products of pancreatic digestion. 

 The same result cannot be attained by the administration of the products 

 of acid hydrolysis of protein, but this may be due either to the racemisation of 

 the amino-acids under the action of the strong acid, or to the fact that the acid 

 splits up certain polypeptide groupings which are still contained in the trypsin 

 digest, and which possibly cannot be synthetised by the cells of the body. 



We are justified therefore in concluding that while a certain small 

 proportion of the proteins of the food may be absorbed unchanged, a much 

 larger proportion is taken up as proteoses and peptones or as amino-acids. 

 The proteoses and peptones are, however, rapidly changed in the 

 mucous membrane itself into amino-acids, which we may regard as the form 

 in which practically all the protein of the body is presented to the absorbing 

 mechanisms of the alimentary canal for absorption and for passing on 

 into the circulating fluids. 



