THE DIGESTIVE JUICES 97 



up into substances with smaller molecules, and the ready diffusibility 

 of peptones led most physiologists to consider that protein was usually 

 absorbed as peptone, or as proteose and peptone. But proteose 

 and peptone are absent from the blood and lymph under all circum- 

 stances, even from the portal blood during the most active digestion. 

 It is fortunate that this is so, for proteose and peptone when intro- 

 duced into the blood produce poisonous effects ; the coagulability of 

 the blood is lessened, blood pressure falls, secretion ceases, and in 

 the dog O3 gramme of commercial peptone per kilogramme of body- 

 weight is often sufficient to produce death. 



This absence of 'peptone' (using the word to include the prote- 

 oses) did not, however, absolutely negative the idea that ' peptone ' 

 is the form in which proteins are absorbed, and the difficulty was met 

 by supposing that during absorption the products of proteolysis were 

 reconverted into native proteins (albumins and globulins). This 

 synthesis was further considered to be accomplished by the epithelial 

 cells that line the intestine. 



This view has now had its day, and the change of opinion that 

 has relegated it to the past is due (1) to our increased knowledge of 

 the power of trypsin and erepsin ; (2) to a more careful examination 

 of the intestinal contents, and of the blood during absorption. We 

 now know that in the intestine the proteins are, by the two enzymes 

 trypsin and erepsin, broken down beyond the peptone stage into 

 their final cleavage products, the ammo-acids, and that these pro- 

 bably pass into the blood as such, for the amount of non-protein 

 nitrogen in that fluid is increased during absorption. These amino- 

 acids are partly utilised by the cells of the body to repair their 

 waste, but partly and to a still greater extent converted by the liver 

 into the waste substance urea, which is finally excreted by the 

 kidneys. The view that the absorptive epithelium of the alimentary 

 tract has any special power in building up proteins from these simple 

 cleavage products has not been confirmed. If an animal is fed on 

 the cleavage products obtained from a pancreatic digest nitrogenous 

 equilibrium is still maintained. 



We thus see that the cells of the body possess the power of 

 rebuilding the proteins peculiar to themselves from the fragments 

 of the molecules of the food proteins. This accounts for the fact that 

 the animal tissues retain their chemical individuality in spite of the 

 great variations in the composition of the diet the animal takes. 



If a man wishes to build a new house, and to employ for the 

 purpose the bricks previously used in the building of another house, 

 he takes the old house to pieces and uses the bricks and stones most 



H 



