30 APPLIED PHYSIOLOGY 



in the stomach, and its molecules are then split up in 

 the intestine into finer fragments, nearly all of which are 

 amido-acids. It is in this form that proteins are absorbed, 

 and out of these fragments the specific body proteins are 

 again built up. It is not yet determined where this 

 reconstitution takes place. There is much reason to 

 believe, however, that the epithelial cells of the intestine 

 are mainly responsible for the rebuilding, and that the 

 proteins of the blood are the material that they produce. 

 On this view all the proteins of the food are ultimately 

 converted into serum proteins, which are in turn taken 

 to pieces by the cells of the tissues, and from the products 

 of their disintegration the special protein peculiar to any 

 particular tissue is finally formed. On the other hand, 

 there are some who believe that the amido-acids derived 

 from the original food-proteins are conveyed to the 

 tissues direct, and that it is from them that the specific 

 tissue-proteins are reconstituted. Whichever view be 

 correct, it is easy to understand how the initial picking 

 to pieces of the protein molecule can enable the body out 

 of the manifold forms of food-protein to produce for 

 itself any kind of tissue-protein required. It must be 

 remembered, however, that there are many different 

 kinds of protein contained in the food, and that the 

 nature of the amido-acids or ultimate fragments of which 

 these are composed is very variable; indeed, at least 

 twenty different kinds of amido-acids are already known. 

 One ought not, therefore, to speak of the food containing 

 so much ' protein,' as if the latter were always one 

 definite chemical compound. It may well prove to be 

 the case that it is not a matter of indifference to a patient 



