246 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



the proteins of connective tissues could not be synthesized at all from 

 serum-albumin, because it contains no glycocoll, and even their syn- 

 thesis, from serum-globulin would involve a great deal of wastage, 

 because, whereas serum-globulin contains only 3.5 per cent, of glyco- 

 coll, the connective-tissue proteins contain about twenty per cent., 

 so that not less than six molecules of serum-globulin would have to be 

 destroyed to build up one molecule of connective-tissue protein, and 

 the greater part of the remaining amino-acids in these six molecules 

 would not be needed. What, then, is to become of them? They might 

 be locally deaminized, but the predominant deaminizing tissue is that 

 of the liver. In order to reach the liver the rejected amino-acids would 

 generally have to travel thereto in the circulation. If, on the contrary, 

 the amino-acids rejected by the connective tissues are utilized by 

 another tissue, then in order to travel from the one tissue to another 

 they must again enter into the circulation. Whichever hypothesis we 

 adopt we are therefore compelled to revert to the presence of amino- 

 acids in the blood. 



The discovery that the amino-acid products of digestion are actually 

 absorbed into the blood-stream as such, and are absorbed from the 

 blood by the tissues, removes these complexities from our interpre- 

 tation of the process of protein synthesis. It seems most reasonable 

 to suppose that each tissue synthesizes its own individual proteins, 

 and that it is able to utilize for this purpose all of the amino-acids 

 which it absorbs for the reason that the characteristic composition of 

 each individual tissue-protein is already determined by the character- 

 istic admixture and proportion of the various amino-acids which that 

 tissue absorbs and holds in equilibrium with the blood, on the one 

 hand, and with the tissu -protein itself on the other. The individual 

 characteristics of the proteins of the various tissues are therefore 

 determined, in ultimate analysis, by the relative Permeability of the 

 tissue in question for various amino-acids, i. e., by the relative ease 

 with which the amino-acids traverse the boundaries which demarcate 

 the tissue. 



It is not unlikely that this mechanism of two-sided equilibrium is 

 limited in its powers and that it is for this reason that it is safeguarded 

 or assisted by a degree of preliminary selection by the Intestinal Epi- 

 thelium. It will be recollected that the experiments of London show 

 that if a protein differing very widely from animal tissue-protein be 

 administered, certain amino-acids are absorbed selectively. Thus 

 from Gliadin, tyrosine is absorbed much more rapidly than the glut- 

 amic acid which it contains in notable excess. The composition and 

 general nutritional standard of the tissues is therefore determined by 

 the following interrelated factors which are severally in equilibrium: 



(1) The selective absorptive activities of the intestinal epithelium. 



(2) The general average concentration of food-products in the blood, 

 i. e., the abundance of the dietary. (3) The deaminizing activity of 



