INTRODUCTION 11 



intestinal mucous membrane, and whether transformed into the 

 serum-proteins by these cells, or passed on into the blood as amino- 

 acids and other final products of hydrolysis, is, for the present, 

 an open question. The evidence on the whole would make it 

 appear probable that the products of the digestive hydrolysis of 

 proteins enter the blood unchanged, and are taken up by the 

 tissue cells at a rate not appreciably different from the rate at 

 which they are absorbed by the blood from the intestine. In the 

 somewhat analogous condition in plants during germination, 

 proteins, stored as food material in the seed, undergo hydrolytic 

 changes, and circulate in the sap in the form of amino- acids 

 practically identical with those formed during digestion in 

 animals. In the plant nitrogen is supplied to the cells in the 

 form of these cleavage products during the period of active 

 growth, and it is from these that the vegetable proteins are 

 built up. 



Whether in the form of synthesized serum-proteins, or, what is 

 more likely, in the form of the unchanged products of digestive 

 hydrolysis, the protein of the food finds its way into the blood. 

 The evidence available at present would point to the conclusion 

 that the amino- acids, on passing through the cells of the intestinal 

 mucous membrane, are to a great extent, robbed of their nitrogen, 

 which is split off from them in the form of ammonia. This 

 ammonia, which Nencki found to be present during digestion up 

 to four times the normal amount in the blood of the portal vein, 

 is converted in the liver into urea and eliminated as such in the 

 urine. This fate, however, only befalls those units or fragments 

 of the protein of the food that are not required for the building up 

 or repair of tissue protein, or that are not required by the body 

 at that particular time. The units that are then in demand, and 

 which may be quite different from those needed for repair at a 

 different period, are made use of by the tissue cells for their 

 growth and nutrition, and for the supply of the necessary com- 

 plexes of molecules which are broken down during the ordinary 

 disintegration of living tissue ever taking place. 



It has been pointed out that this seemingly wasteful method of 

 making use of precious protein material was open to a different 

 interpretation to that which appears on the surface, and to that 

 put forward by the advocates of the superiority of a low protein 

 dietary. Evidence has been adduced to show that unless the 

 proper and suitable building-stones were furnished in the final 



