48 STUDIES IN DIGESTION 



aminoacids. These are the actual substances which 

 traverse the intestinal wall and enter the blood- 

 stream. They do not circulate as serum albumin, 

 but as aminoacids, and are taken up by the tissue 

 proteins according to their needs. Should these 

 require more of the aromatic amines, they will 

 abstract tyrosin or tryptophan from the blood, and 

 so on. Any aminoacids that are in excess of the 

 requirements of the body are broken down by the 

 liver to urea, and excreted by the kidney. This 

 constitutes the so-called exogenous origin of urea. 



The evidence for these fundamental changes in 

 our view of the absorption of proteins may be 

 summarized briefly as follows : We now know that 



(). Aminoacids are abundantly formed in the 

 intestine. 



(b). Feeding on aminoacids will sustain life. Gelatin 

 will not sustain life, because it lacks the aromatic 

 amines, but if it is given with tyrosin and tryptophan, 

 the animal lives. 



(c). During protein absorption, it is not the pro- 

 teins (serum albumin and globulin) which increase in 

 the blood, but the nitrogenous constituents of the 

 plasma which are not coagulated by heat. 



Carlyle said that an error is never proved to be an 

 error until it is shown how the error arose, and this 

 is possible in regard to the older theory, that peptones 

 were converted by the intestinal epithelium into 

 albumin. The disappearance of the peptone in 

 contact with the intestinal wall was taken to indicate 

 a conversion into albumin, because the nature and 

 function of the ferment erepsin were not then known. 



