THE DIGESTION OF THE NUTRIENTS 51 



Mineral Matter. — Mineral matter probably undergoes 

 no digestion other than simple solution. It probably is ab- 

 sorbed principally from the small intestine. 



Protein. — There is no action upon proteins in the mouth 

 except mastication. They are first acted upon in the stomach 

 by the pepsin of the gastric juice which, in acid solution, 

 breaks up some of the complex proteins into the simpler 

 and soluble proteoses and peptones. Rennin, in the gastric 

 juice, coagulates the casein of milk, which is then acted upon 

 by the pepsin. 



Passing from the stomach into the upper end of the small 

 intestine, the unchanged proteins, the proteoses, and the 

 peptones encounter the more active enzyme, trypsin, of 

 the pancreatic juice, and the enzyme, erepsin, of the pan- 

 creatic and of the intestinal juices. Trypsin, in alkaUne 

 solution, in the presence of the enterokinase of the intestinal 

 juice, attacks the remaining proteins, breaking them down 

 into proteoses, peptones, peptides, and the amino acids. 

 It also attacks some of the proteoses and peptones previously 

 formed by the peptic digestion, breaking them down still 

 further into peptids and amino acids. The proteoses, pep- 

 tones, and peptids are acted upon by the erepsin which, in 

 alkahne solution, breaks them down further into the amino 

 acids. Erepsin does not act upon unchanged protein, how- 

 ever, except in the cases of casein, fibrin, gelatin, and a few 

 others. The amino acids, as they are formed, are absorbed by 

 the villi of the small intestine into the capillaries and pass via 

 the portal vein through the liver into the general circulation. 

 Concerning the digestion and absorption of jiroteins, Un- 

 derhill ^ makes the following statement : " . . . demohtion 



1 " The Physiology of the Amino Acids," p. 34. 



