ABSORPTION 419 



eventually be absorbed unaltered. Secondly, they are not, as a 

 rule, utilized in the metabolism of the body, or are utilized very 

 incompletely. Egg-albumin, for instance, when injected into 

 the blood, is excreted in the urine. It has been previously 

 pointed out that the various proteins differ remarkably in the 

 kind and quantity of the amino- and diamino-acids which can 

 be obtained from them (p. 2). This is unquestionably one 

 important reason why the food proteins are for the most part, 

 at any rate so thoroughly hydrolysed before absorption. 

 Another may be that it is easier for the intestine to take up the 

 small molecules of the decomposition products than the large 

 colloid aggregates of the original protein solutions. 



So far as the first reason is concerned, the degree of decom- 

 position need not be the same for all the food proteins. A 

 new house has to be built from the materials of an old one. 

 How far the work of demolition must be carried will depend 

 upon the difference between the plans of the two houses. 

 Sometimes the main part of the old building may be saved, 

 and only the wings require reconstruction. In like manner it 

 is conceivable that the central group or nucleus of the molecule of 

 a given food protein may be identical with that of a given body 

 protein, and that only the side chains may be so different that 

 they must be broken up and reconstructed. Or, again, the whole 

 architectural plan of the new house may be so distinct from that 

 of the old that the only feasible method is to completely demolish 

 the latter, and then to use the individual bricks in the new con- 

 struction ; just as a protein in the food may differ so radically 

 from a tissue protein into which it is to be transformed that it 

 must be decomposed into the simplest products of proteolysis 

 before the reconstruction of the molecule can begin. It is not 

 known what the minimum degree of hydrolysis is which will 

 permit of effective absorption and utilization. There can be no 

 doubt that by far the greater part of the proteins of the food is 

 first changed into proteoses and peptones. But proteose and 

 peptone are absent from the blood, and, indeed, when injected 

 into the blood, they are excreted in the urine. When injected 

 in larger amount, they pass also into the lymph, from which they 

 gradually reach the blood again, and are eventually, as before, 

 eliminated by the kidneys. The clear inference is that if they 

 are absorbed as such from the alimentary canal, they must be 

 changed in their passage through its walls. The fact that a 

 portion of the peptone and albumose is decomposed into amino- 

 acids, etc., in the lumen of the intestine has been already alluded 

 to. It would seem that a further portion of the remaining 

 peptone and albumose probably the whole is similarly de- 

 composed by the action of erepsin in the intestinal wall. It 



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