OF THE ALIMENTARY CANAL 123 



readers may recollect a Cleavage Theory, suggesting 

 that half of these peptones were further acted on by 

 the trypsin of the pancreatic juice and broken down 

 into two aminoacids called leucin and tyrosin, whose 

 fate was in doubt. The modern view is very different. 

 The researches of Fischer, Kossel, and others have 

 thrown a flood of light on the composition of the 

 protein molecule. We now know that protein con- 

 sists of a complicated chain of the bodies called 

 aminoacids (that is, organic acids of which a hydrogen 

 has been replaced by the NH^ group). These may 

 be classified as monoamines (as leucin, glycin), 

 diamines (as arginin, lysin), and aromatic amines 

 (as tyrosin, tryptophan). By the trypsin of the 

 pancreatic juice proteins are resolved into their 

 various components, and consequently a mixture, in 

 differing proportions, of these aminoacids is found 

 in the intestine. Some peptone appears to resist 

 further disintegration by the pancreatic ferment, 

 but there is a ferment in both the pancreatic and 

 intestinal juices called erepsin, which completes the 

 action of the gastric and pancreatic juices by con- 

 verting all the peptones into aminoacids. 



Neither albumin nor peptone can be absorbed by 

 the intestine. They must first be converted into 

 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 



