DIGESTION 29 



peptone, or even at a still earlier stage. The only posi- 

 tive evidence as to the formation of leucine and 

 tyrosine in natural digestion, rests on the amounts 

 found in the intestinal contents during protein diges- 

 tion." It is then stated that in general the quantities 

 of amino acids present during digestion are small. 



In 'the few years since the above was written the 

 advances made in the chemistry of the proteins and of 

 digestion have made necessary a radical revision of our 

 ideas of the nature and extent of the alimentary treat- 

 ment of protein. No longer tenable is the view that 

 digestion stops with the transformation of insoluble 

 and non-diffusible substances into compounds soluble 

 and diffusible, nor can the idea be accepted of a dis- 

 tinction between directly assimilable and non-assimi- 

 lable proteins. The change to "peptone" is now held to 

 be merely an intermediate stage in digestion, not the 

 end, as was once assumed. According to the latest con- 

 ception of protein digestion a profound disintegration 

 occurs, the ultimate products formed being a variety of 

 polypeptides and amino acids. Digestion, in accord- 

 ance with this idea, consists in a series of hydrolytic 

 cleavages which are induced through the agencies of 

 the enzymes present in the gastro-enteric tract. The 

 products formed by these enzymes undoubtedly are 

 identical with those produced outside the body by 

 means of the action of acids. Amino acids therefore 

 must be looked upon as the ultimate nitrogenous food- 

 stuffs it is to these substances that the organism must 

 look for its essential requirement of nitrogen. 



