448 ABSORPTION 



the alimentary canal, they behave in a very different manner from 

 the same proteins when given by the mouth. One notable differ- 

 ence is that the parenterally administered proteins give rise in 

 general to the formation of antibodies -e.g., specific precipitins^ 

 (p. 31). This is not the case when they are administered per os, 

 unless, like raw egg-white, which, as already mentioned (p. 404), 

 evokes no secretion of gastric juice, they remain long undigested 

 in the alimentary canal, when an amount sufficient to cause the 

 production of precipitins may eventually be absorbed unaltered. 

 This has also been shown by means of the anaphylactic reaction. 

 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 

 not so much in the kinds as in the relative quantities 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 decomposi- 

 tion need not be the same for all the food proteins, although all 

 must be decomposed, for even among the proteins the products of 

 whose hydrolysis do not exhibit qualitative differences, no two 

 have hitherto been discovered which show the same quantitative 

 relations among the ' building-stones. ' 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 construction; 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 proteo- 

 lysis 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. But it would seem that it 

 must be very complete. Even a body so simple in comparison with 

 the proteins as the tripeptide (p. 2) alanyl-glycyl-tyrosin, con- 



