ALIMENTATION AND DIGESTION 121 



details of the somewhat complicated digestion of fats. The 

 change, like that of proteins into peptones and of starches 

 into sugar, involves the formation of a smaller molecule, 

 either of fatty acids or soaps, or both, and it is probably 

 in these forms that fats are received from the intestine by 

 the villi. 



The pancreatic juice thus contains a special enzyme for 

 each of the three great classes of nutrients proteins, fats, 

 and carbohydrates and thoroughly completes their diges- 

 tion after they have undergone the preparatory processes 

 effected by cooking, mastication, and gastric digestion. 

 Pancreatic juice is by far the most important of the digestive 

 juices in producing the chemical changes of digestion. In this 

 respect, also, we may say it is of primary importance in the 

 work of intestinal digestion, the -other two juices, the bile 

 and the intestinal juice, acting as aids in its work. 



28. The bile contains no enzymes of importance in diges- 

 tion. It is in fact partly an excretion, some of its con- 

 stituents being waste products which are poured into the 

 intestine only to be ultimately discharged from the rectum. 

 Other constituents of the bile play an important role in the 

 digestion and absorption of fats, as is shown by the fact that 

 if bile be prevented from entering the intestine, from forty 

 to sixty per cent of the fat eaten fails of absorption and is 

 discharged with the feces. It is probable that this is because 

 certain soaps formed in pancreatic digestion are not soluble 

 unless bile is present. When these soaps are not dissolved, 

 they are not only themselves not absorbed, but, by being 

 precipitated and adhering to other still undigested food, pre- 

 vent ready access of enzymes and so greatly retard digestion. 



29. The intestinal juice contains two kinds of enzymes, one 

 acting on protein, the other on carbohydrate material. The 

 former class, represented by the single enzyme erepsin, has 

 no action on the proteins of the food, but splits peptones and 

 other products of gastric and pancreatic digestion into very 



