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THE ABSORPTION OF THE FOODSTUFFS 795 



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 

 equilibrium on a diet containing fat, starch, and a pancreatic digest of 

 protein which contained no substances giving 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. 



THE FATE OF THE AMINO-ACIDS AFTER ABSORPTION BY THE 

 INTESTINAL EPITHELIUM. During a condition of starvation the normal 

 protein requirements of the body, or rather of the active tissues, are met 

 at the expense of the less active tissues. The protein characteristic of any 

 tissue can be taken down, removed to another part of the body, and built 

 up into the protein characteristic of some other active tissue. It is difficult 

 to conceive that such a transference and transformation could occur in any 

 other way than by a more or less thorough, disintegration of the protein 

 molecule at one place and its synthesis at the other, and we know from the 

 researches of Hedin and others that every tissue contains intracellular 

 ferments which are capable of effecting the disintegration of the protein 

 molecule, and are responsible for the autolytic degeneration of tissues after 

 death. If therefore the normal interchange of protein between the tissues 

 is accomplished, as we know it to be in plants, by the disintegration of the 

 proteins into their constituent amino-acids and their subsequent reintegra- 

 tion, there is no a priori reason to believe that the blood carries the proteins 

 from the alimentary canal to the tissues in any other form than that of 

 amino-acids. The experimental proof of this conclusion was hardly possible 

 before the invention of a reliable method for the detection of small quantities 

 of amino-acids in the blood and tissues. This is rendered possible by van 

 Slyke's method in which, after the separation of coagulable proteins by 

 alcohol, the amino-acids are determined by measuring the nitrogen evolved 

 on addition of nitrous acid. Van Slyke has shown that the blood always 

 contains a certain amount of amino-acids even during fasting. After a 



