124 STUDIES IN CLINICAL PHYSIOLOGY 



so on. Any aminoacids that are in excess of the 

 requirements of the body are broken down by the 

 hver 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 



(a). Aminoacids are abundantly formed in the 

 intestine. 



(b). Feeding on aminoacids obtained by tryptic 

 digestion, though not by sulphuric acid disintegra- 

 tion of protein, 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. 



{d). Van Slyke has demonstrated a marked rise 

 in the quantity of aminoacids in the blood during 

 protein digestion, although it seems to last only a 

 very short time. These acids disappear from the 

 circulating blood within a few minutes of injection. 



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 waU was taken to indicate 

 a conversion into albumin, because the nature and 

 function of the ferment erepsin were not then known. 



