140 PHYSIOLOGY OF ALIMENTATION. 



on the other, will occur to every one. The mucous mem- 

 brane of the alimentary tract is, by virtue of its contained 

 antiproteinase, "immune" against the "toxic" proteinases 

 which daily pass over it. 



A second reason why the tissues of the alimentary tract 

 as well as the tissues of the body in general, are not acted 

 upon by the ferments present in them may reside in the 

 chemical constitution of the substances making up the cells 

 themselves. As ABDERHALDEN I has pointed out, the different 

 proteins contained in an organ are by no means acted upon 

 by the proteinases with the same ease. The substances mak- 

 ing up the connective tissues, such as elastin and spongin for 

 example, are acted upon scarcely at all by a pepsin-hydro- 

 chloric acid mixture or trypsin. This property is connected 

 with the chemical constitution of the substances concerned, 

 which contain those amino-acids in largest amounts whose 

 presence offers the greatest resistance to hydrolysis of the 

 protein molecule. It might well be possible, therefore, that 

 living cells are endowed with properties which enable them to 

 so modify the proteins absorbed by them as to render them in- 

 capable of being acted upon by the ferments contained in the 

 cells. Only a very slight change in the chemical constitution 

 of a compound will make it impossible for a ferment to attack 

 it, for, as is well known, the ferments are dependent in a 

 most limited way upon the stereochemical construction of 

 the molecules upon which they are capable of acting. 



9. On the Reversible Action of the Proteinases. The 

 question of the means by which the simple digestion-prod- 

 ucts of the proteins are again built up into the complicated 

 albumins, globulins, etc., which we find in the organism has 

 within the last few years been much debated. Of first in- 

 terest in this connection are the researches of EMIL FisCHER, 2 



' ABDERHALDEN: Lehrbuch d. physiol. Chemie, Berlin, 1906, p. 510. 



2 EMIL FISCHER: Berichte d. d. chem. Gesellsch., 1906, XXIX, p. 530, 

 where references to the individual papers of the author will be 

 found. 



