450 ABSORPTION 



built up from the products of this decomposition. But whether the 

 mucosa of the alimentary tract is especially a seat of the synthesis 

 is unknown (p. 573). On a priori grounds, it is at least equally 

 probable that it occurs in all the cells of the body, each one building 

 up for itself the particular kind of protein which it needs. The 

 direct way of testing the question would be to examine the blood 

 coming from the intestine during the absorption, of proteins, and to 

 determine quantitatively any changes which might have occurred in 

 the nitrogenous constituents. But the flow of blood through the 

 intestine is so great, the absorption of the digestive products so 

 gradual, and their removal from the blood by the tissues, in all 

 probability, so rapid, that there is no reason for surprise that till 

 lately the results of such determinations were ambiguous. Leathes, 

 however, showed some time ago that when peptone, proteose, or the 

 final products of tryptic digestion are introduced into a ligated 

 segment of a dog's small intestine, there is always, when absorption 

 occurs, an increase in the nitrogenous substances in the blood, in the 

 form of compounds which are not precipitated by tannic acid, and 

 therefore are neither native proteins nor proteose. Urea accounts 

 for about one-half of the increase ; the rest he considered to represent 

 probably amino-acids and similar substances. Quite recently it 

 has been conclusively demonstrated by improved methods that the 

 digestion of protein is associated with an increase of non-protein 

 nitrogen in the blood, due, there is every reason to believe, to amino- 

 bodies derived from the hydrolysed protein (Folin and Denis, Van 

 Slyke, Abel). This proves for the mammal what had been deduced 

 by Cohnheim for a much lower form from experiments made on the 

 intestines of certain octopods, which, when excised and suspended 

 in the oxygenated blood, will live for many hours. A solution of 

 peptone was introduced into the isolated intestine, and after twenty 

 hours the crystalline products, leucin, tyrosin, lysin, and arginin, 

 were found in the blood. In the intact animal none of these bodies 

 could be detected in the blood (Cohnheim). The inference was 

 that protein in these animals is absorbed in the form of amino-acids, 

 etc., which are then carried to the tissues and utilized there. In 

 the mammal the same thing appears to be true. For the increase 

 in the amino-acids during digestion of proteins occurs not only in 

 the portal blood, but in the blood of the general circulation. So 

 that, although a part of the absorbed amino-bodies may be removed 

 by the liver, a portion at least is available for the tissues in general. 

 That the tissues actually take up such decomposition products of 

 proteins is indicated by the fact that during and after the digestion 

 of protein in a loop of intestine the non-protein nitrogen of the 

 tissues is increased (Folin). It may be that some of the proteose 

 and peptone are regenerated by a shorter process, and without 

 having been further split up, but of this, too, -there is no definite 



