722 METABOLISM 



if we mix bile with fresh pancreatic juice, which by itself only slowly 

 digests fat, we shall find that the bile very materially increases the lipo- 

 lytic activity of the pancreatic juice. It has been found that the salts 

 of cholalic acid, the so-called bile salts, are the constituents of bile 

 that are responsible for this activation of lipase, this fact having been 

 demonstrated with bile salts prepared in such a way that there was no 

 possible chance of any other biliary constituent being present as an 

 impurity. It is important to remember, however, that lipase itself be- 

 comes slowly activated on standing, which explains why it should be 

 that bile added to pancreatic juice that has stood for some time, has a 

 less evident activating influence than bile added to fresh juice. It is 

 probable that the activating influence of bile salts is due to some physico- 

 chemical change induced in the digestion mixture. 



One may ask how it happens that, when bile and pancreatic juice are 

 both absent from the intestine, the fat which appears in the feces is not 

 neutral fat but fatty acid. The reason is that the neutral fat that has 

 escaped digestion in the small intestine becomes acted on by the intestinal 

 bacteria, particularly in the large intestine. Under these conditions, 

 however, the fatty acid that is split off is not absorbed, because the 

 epithelium of the lower parts on the intestinal tract can not perform this 

 function. 



Besides assisting the action of lipase, bile facilitates fat digestion in 

 other ways. Thus, by its containing alkali and mucin-like substances 

 it assists in the emulsification of fat. Although emulsification is no es- 

 sential part of fat absorption, yet it greatly facilitates the process by 

 breaking up the fat into small globules on which the lipase can act 

 much more efficiently. The alkali also combines with the fatty acids, 

 as they are liberated by the digestive process, to form water-soluble 

 soaps, which are readily absorbed by the epithelial cells. The bile salts 

 further assist in the solution of the fatty acids, and they lower the sur- 

 face tension of fluids in which they are contained and so bring the fat 

 and lipase into closer contact. 



ABSORPTION OF FATS 



After its digestion fat lies in contact with the intestinal border of the 

 epithelial cells as fatty acid and glycerol. The fatty acid is combined 

 either with alkali to form a water-soluble soap, or with bile salts to 

 form a compound, which is also soluble. The glycerol and the dissolved 

 fatty acids are separately absorbed into the epithelial cells of the in- 

 testine, in the protoplasm of which after the fatty acid has been set 

 free from the alkali or bile salt they become united or resynthetized 



