1032 ABSORPTION 



their amino-acids, from which the proteins of the body are then re- 

 constructed. It is also evident that these products of protein diges- 

 tion enter the portal radicles, because the composition of the lymph 

 obtained from the thoracic duct, is not appreciably altered during pro- 

 tein absorption. Moreover, it has been shown that the ligation of 

 this collecting channel does not interfere with the intake of proteins as 

 determined by the output of urea. 1 



These facts have led to the establishment of the hypothesis that 

 the amino-acids are reconstructed into the proteins of the blood while 

 they traverse the intestinal lining. But since this view is based upon 

 negative evidence, and is contradicted by the presence of amino-acids 

 in the blood, it cannot be retained in its original form. Instead, it 

 must be concluded that a true synthesis of the' ammo-acids by the 

 intestinal lining cells does not take place and that these bodies enter 

 the blood directly. From this medium they are then picked up by 

 the different cells either to replace the protein material which the latter 

 have lost, or to be excreted directly. 2 The acceptance of this view 

 makes it necessary for us to discard the assumption that the white 

 blood corpuscles play a part in the transfer of these bodies from the 

 lining cells into the blood-stream (Schafer). Consequently, it may be 

 concluded that the increase in the number of leukocytes after meals is 

 caused in all probability by changes in the circulation. 



The difficulties encountered in endeavoring to prove the presence 

 of amino-acids in the blood, are dependent upon the fact that their 

 absorption is effected very slowly and that they are diluted after that 

 by large quantities of blood, and carried with greatest speed to the 

 tissues. Consequently, they do not remain in the blood for any 

 length of tune, but are quickly acted upon by the tissue-cells. An 

 additional difficulty is presented by the fact that their chemical isola- 

 tion is seriously hampered by the presence in the blood of a large quan- 

 tity of coagulable proteins. 



In accordance with the above view, the amino-acids must be re- 

 garded as mere building stones which may be brought together selec- 

 tively to form the body-proteins. This is also true of the amino-acids 

 constituting the proteins of the food, because the differences which they 

 show are really due to the manner in which their molecules are com- 

 bined. As soon as the protein material has been split by the ac- 

 tion of the successive proteolytic enzymes, their amino constituents are 

 again united in the organism in accordance with the peculiar require- 

 ments of the tissue-cells. In this way, a large number of perfectly new 

 combinations may be produced. It must also be considered as an 

 established fact that the intestinal cells possess the power of splitting 

 the amino groups from those polypeptides which have been swept 

 into them. This deduction is based upon the fact that the intestinal 



1 Folin and Denis, Jour. Biol. Chem., xi, 1912, 493. 



2 Paton and Goodall, Jour, of Physiol., xxxiii, 1915, 20, also Burianand Schur, 

 Wiener, klin. Wochenschr., 1897. 



