ABSORPTION OF PROTEINS 163 



in containing the same amino-acids cotnbined in different 

 proportions and in different ways. The individuality of a 

 protein is due to the arrangement of the amino-acids of 

 which it is composed. When animal proteins are being 

 built from plant proteins the change consists in a re- 

 arrangement of amino-acids. Assuming that the animal 

 body cannot to any extent synthesise amino-acids, it 

 might be expected that such rearrangement must first 

 involve breakdown of the food protein into its amino- 

 acids, and the ample provision of the means of effecting this 

 breakdown in the intestine would seem to confirm this view. 



But until recently proof of this was wanting. It was 

 difficult to detect amino-acids in the intestine owing, as 

 we now know, to their rapid absorption, and still more 

 difficult to detect them in the blood. 



Four views were held. 



1. That breakdown into amino-acids is not a necessary 

 prehminary to absorption. 



2. That amino-acid formation occurs only for the purpose 

 of absorption, being followed by immediate resynthesis 

 within the intestinal wall. 



3. That amino-acids are absorbed, and after absorption 

 are deaminised in the intestinal wall, ammonia and a non- 

 nitrogenous residue being carried into the circulation. 



4. That amino-acids are absorbed and circulate in the 

 blood. 



The first three theories may be dismissed, since they 

 have been disproved by the positive evidence in favour 

 of the fourth. This evidence is here presented. 



1. AbeVs Vividiffusion Method {Artificial Kidaeij) 



This is a device for separating amino-acids from circulating 

 blood. The blood is passed from the blood-vessel through 

 a tube whose walls are made of collodion. This is immersed 

 in a sahne solution isotonic with blood. The blood is 

 then returned to the circulation. The amino-acids readily 

 difiuse through the collodion, and can be estimated. 



