ABSORPTION 



221 



portal blood or in the lymph of the thoracic duct, it was assumed that the 

 epithelium after absorbing, also synthesized them into some form of coagu- 

 lable protein (plasma-albumin) which is readily assimilable by the blood. 

 The plasma-albumin thus supposed to be formed by the epithelial cells was 

 believed to pass into the interior of the villus, thence into the blood by 

 which it was carried to the liver. After passing through this organ it 

 entered the blood of the general circulation and became a part of the com- 

 mon store of protein, out of which each tissue cell constructed the particular 

 protein characteristic of it. That such a conversion of peptones to plasma- 

 albumin would be necessary in case of their absorption would appear 

 from the fact that the introduction of peptones even in small amounts into 

 the blood is 'followed by their elimination unchanged in the urine. When 

 injected into the blood in large amounts, they act as toxic agents, giving 

 rise to a fall of blood-pressure, a diminished coagulability of the blood, 

 coma, and death. 



This general view as to the absorption of peptones is no longer enter- 

 tained. There are reasons, however, for believing that the change which 

 proteins undergo in digestion, is more far-reaching and complete, and that 

 the peptones in turn are disintegrated and reduced to even simpler bodies 

 such as polypeptids, peptids and amino-acids (see page 191). The extent 

 to which this disintegration proceeds will doubtless depend, to some extent, 

 on the quantity and variety of proteins consumed. 



Since recent investigations make it highly probable that the final stage 

 in the digestion of protein is the formation of amino-acids it has come to 

 be believed that the problem of absorption is to be transferred to these 

 fragmentary bodies rather than to the peptones. Assuming this to be true, 

 the question at once arises as to what change, if any, they undergo during 

 their passage through the intestinal epithelium. The difficulty of detect- 

 ing the presence of amino-acids in the blood led to the assumption that after 

 their absorption they were synthesized, just as the peptone molecules were 

 supposed to be, and a protein molecule constructed similar to, if not identical, 

 with plasma-albumin. 



The recent investigation of Abel, carried out with a new. form of 

 diffusion apparatus, renders it highly probable that amino-acids are 

 present in the circulating blood in readily determinable quantities. This 

 has led to the belief that the amino-acids, in part, at least, are absorbed as 

 such, pass through the intestinal epithelium, the portal blood and the liver 

 unchanged, and enter the blood of the general circulation to be carried 

 by it directly to the tissues where they are at once utilized by the tissue 

 cells or stored for future use. This view renders it much easier to Bunder- 

 stand, how out of the different proteins of the foods, varying widely in their 

 composition, the specific proteins of the tissues are constructed. It is only 

 necessary to assume that the tissue cells select from the variety of amino- 

 acids presented to them, only those which are necessary to the formation 

 of the protein by which they are characterized. The ^ plasma-albumin, 

 whatever its origin, might then be regarded as a protein surplus to be 

 called on if the protein ingested should be insunicient. It may be, however, 

 that future investigations will ascribe other functions to it. 



Under whatever form the protein material is absorbed there is every reasc 

 to believe that it is carried by the portal circulation to the liver, through 



