ABSORPTION OF THE VARIOUS FOOD SUBSTANCES 443 



taining only three ' building-stones,' can only be changed into the 

 tripeptide alanyl-tyrosyl-glycin by first hydrolysing it into its three 

 components and then synthesizing these afresh. On the other 

 hand, in obtaining the tripeptide glycyl-tyrosyl-alanin from alanyl- 

 glycyl-tyrosin, the dipeptide group glycyl-tyrosin can remain un- 

 decomposed, and it is only necessary to split alanin off and link it 

 up to the dipeptide to obtain the desired glycyl-tyrosyl-alanin 

 (Abderhalden). There can be no doubt that by far the greater 

 part, if not the whole, of the proteins of the food is first changed into 

 proteoses and peptones. But proteose and peptone are absent 

 from the blood, and, indeed, when injected into the blood they 

 are excreted in the urine. When injected in larger amount, they 

 pass also into the lymph, from which they gradually reach the blood 

 again, and are eventually, as before, eliminated by the kidneys. 

 The clear inference is that if they are absorbed as such from the 

 alimentary canal, they must be changed in their passage through its 

 walls. The fact that a portion at any rate of the peptones and 

 proteoses is decomposed into amino-acids, etc., in the lumen 

 of the intestine has been already alluded to. It is certain that 

 this portion is a very large proportion of the whole, although the 

 question how much, if any, of the peptone passes as such from the 

 lumen into the mucosa must still be left undecided. It is true 

 that along with amino-acids peptones are always found in the 

 intestine during the digestion of protein, and the quantity of 

 amino-acids actually present in the lumen at any moment may 

 be small in proportion to the quantity of peptone. But this is 

 precisely what is to be expected if the peptone as such is incapable 

 of absorption. For the easily absorbed amino-acids will disappear 

 from the gut as fast as they are formed, leaving behind the peptone 

 for further hydrolysis. The fact that all the amino-acids which the 

 proteins are capable of yielding can be detected in the contents of 

 the intestine, including even those which appear late and, as it 

 were, reluctantly in artificial digestion, is a proof that the decom- 

 position of the protein goes fast and far in the alimentary canal. 

 If it is not complete, if some of the partially hydrolysed protein is 

 taken up by the mucous membrane in the form of peptones or 

 possibly even of proteoses, it would seem that this is similarly 

 decomposed by the action of erepsin in the intestinal wall. 



It has been stated that during the digestion of a protein meal the 

 mucosa of the stomach and intestine contains proteose and pep- 

 tone, while none is present in the muscular coat or in any other 

 organ. They rapidly disappear from a portion of the mucous mem- 

 brane kept at a temperature of about 40 C. outside of the body, and 

 their disappearance is due, not to their regeneration into serum 

 proteins, as was once supposed, but to their decomposition by the 

 erepsin. We must suppose that the serum and organ proteins are 



