844 PHYSIOLOGY 



a mass of epithelial cells mixed with the products of the action of 

 erepsin on the introduced peptone solution. In no case was there 

 any diminution in the amount of uncoagulable nitrogen, i.e. there 

 was no formation of coagulable protein, while the processes of absorp- 

 tion had been brought by the desquamation entirely to a standstill. 



These additional experiments caused a complete revolution in 

 the attitude of physiologists towards the problem of protein absorp- 

 tion. All this evidence went to show that protein, however intro- 

 duced, whether as coagulated protein or as albumose and peptone, 

 underwent complete hydrolysis either in the gut or in the wall of 

 the gut before entering the blood stream. If this were the case, 

 it should be possible to feed an animal on a diet in which the necessary 

 protein had been replaced by the corresponding amount of ultimate 

 products of protein hydrolysis, i.e. by a mixture which would give 

 no biuret reaction. Such a possibility had previously been negatived 

 on theoretical grounds by Kiihne and by Bunge. It was thought 

 by these observers either that the animal body lacked the power of 

 synthesis of proteins from these crystalline products (hydration 

 products), or that any complete hydration occurring in the intestine 

 would involve such a loss of energy to the body as to be unteleo- 

 logical. Neither of these theoretical objections is justified in fact. 

 We know from the researches of Fischer and others that although 

 the different proteins in our food present a marvellous qualitative 

 similitude, in that all of them yield on hydrolysis the same kinds 

 of amino-acids, there are great differences in the relative amounts 

 of these amino-acids contained in different proteins. Thus, in 

 gelatin, glycine is contained in considerable quantities, but is 

 absent in many of the other proteins. Caseinogen is distinguished 

 by the large amount of leucine that it yields, while gliadin, the chief 

 protein of wheat flour, contains very large amounts of glutamic acid. 

 It is difficult to imagine how, for instance, muscle protein could be 

 formed from wheat protein, a process continually occurring in the 

 growing animal, unless we assumed that the protein molecule is first 

 entirely taken to pieces, and that its constituent molecules are then 

 selected by the growing cells of the body and built up in the order 

 and proportions which are characteristic of muscle protein. More- 

 over, when we measure the amount of energy change involved in 

 the hydrolysis of the proteins, we find it is relatively small. There 

 is not a loss of 5 per cent, of the total energy available, i.e. the heat 

 of combustion of the products of pancreatic digestion would differ 

 from that of the original protein submitted to digestion by less than 

 5 per cent. The energy of the protein as evolved in the body lies, not 

 in the coupling of the amino-acids with one another, or indeed in the 

 coupling of the nitrogen to the carbon, but, like that of the other 



