242 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



increase was subsequently demonstrated by Abderhalden, who, by 

 employing an enormous volume, fifty liters of blood, succeeded in 

 demonstrating the presence therein of the amino-acids glycine, alanine, 

 leucine, valine, proline, aspartic acid, glutamic acid, arginine, histidine 

 and lysine. 



It has furthermore been shown by Abderhalden, Henriques and 

 others that animals may be maintained in perfect nitrogenous equilib- 

 rium for prolonged periods on a diet containing no other source of 

 nitrogen than amino-acids. It is necessary, however, to include in 

 this diet all of the amino acids-which contribute to the composition of 

 the various tissue-proteins. The omission of Tryptophane, for example, 

 leads to daily loss of weight and, in effect, to nitrogen-starvation. 

 Nitrogen equilibrium and even nitrogen retention, i. e., accretion of 

 tissue, was secured by Abderhalden in a dog to which a diet was ad- 

 ministered containing the following admixture of amino-acids as the 

 sole source of nitrogen: Glycocoll 5 grams, d-alanine 10 grams, 1-serine 

 3 grams, 1-cystine 2 grams, d-valine 5 grams, 1-leucine 10 grams, d-iso- 

 leucine 5 grams, 1-aspartic acid 5 grams, d-glutamic acid 15 grams, 

 1-phenylalanine 5 grams, 1-tryosine 5 grams, 1-lysine 5 grams, d-arginine 

 5 grams, 1-proline 10 grams, 1-histidine 5 grams, 1-tryptophane 5 grams. 

 The daily ration of amino acids therefore weighed 100 grams and con- 

 tained 13.87 grams of nitrogen. It approximately resembled in com- 

 position the mixture of products which results from the hydrolysis of 

 the proteins of muscular tissue. 



We have seen therefore: (1) That amino-acids are formed in impor- 

 tant proportion in the intestinal digestion of proteins. (2) That amino- 

 acids may be absorbed from the intestine and, (3) that amino-acids 

 suffice to supply the nitrogenous needs of the body. We may infer 

 that the absorption of amino-acids is a normal and probably the only 

 normal method whereby the materials for -the synthesis of proteins 

 are conveyed to the tissues. 



The difficulty of demonstrating the presence of amino-acids i,n the 

 blood after the absorption of the digestion-products of protein arises 

 from two sources: firstly the slowness of absorption and the rapidity 

 of circulation, which results in extreme dilution of the amino-acid 

 products which enter the portal venous system, and secondly the 

 rapidity with which the amino-acids in the blood are absorbed from it 

 by the tissues. The amino-acids are therefore present in the blood 

 even during the height of absorption only in very small concentrations 

 and, to add to the difficulty of the problem, the blood is a fluid which 

 is very rich in nitrogenous substances, proteins, which interfere to a 

 serious extent with the chemical manipulations which were formerly 

 necessary for the determination of small concentrations of amino-acids. 

 In recent years the development of our technical knowledge has 

 simplified and enhanced the accuracy of our methods of estimation 

 and, in particular, the development of the nitrous-acid method of 

 estimating amino-nitrogen immediately enabled us to detect with 



