MULTICELLULAR ANIMALS 163 



action of the rennin is confined to the coagulation of the milk; the 

 digestion of its proteins is accomplished by the HCl and pepsin. 

 Fats are unchanged by digestive events in the mouth and stomach. 

 Digestion in the Intestine. In the small intestine the food, 

 liquefied in the mouth and stomach (chyme), encounters secretions 

 from three sources; the pancreatic juice, received from the pan- 

 creatic duct, the INTESTINAL JUICE, sccrctcd by glands in the walls of 

 the intestine, and bile, secreted by the liver. In the pancreatic secre- 

 tion are three enzymes; one, amylase, acts on the starches; the 

 second, trypsin, acts on the proteins; the third, lipase, acts on the 

 fats. The fats are changed chemically (saponified) by the action of 

 the lipase and also physically by the action of the enzyme and the 

 bile. The physical change in the fats occurs first and breaks them 

 up into exceedingly fine droplets, a process known as emulsifica- 

 tion. The fats are converted to glycerin and fatty acids; they then 

 pass into a set of fine tubules of the lymph system located in the 

 wall of the intestine and from there drain into a large vein of the 

 main circulation through the thoracic duct. The secretions of the 

 intestinal glands contain three chief enzymes that complete the sim- 

 plification of the starches and sugars and one enzyme, erepsin, that 

 completes the conversion of the proteins, proteoses, and peptones 

 into their constituent amino-acids. Thus the amino-acids, first con- 

 structed by plant metabolism from inorganic materials and built up 

 into proteins, are again resolved into amino-acids. The amino-acids 

 are readily absorbed into the fine vessels of the venous system and 

 are thence available for reconstruction into human proteins in the 

 several tissues of the body. A summary of the digestive processes in 

 the human digestive tract is given: 



