THE DIGESTION OF FOOD 107 



with two coloring matters, bilirubin and biliverdin. 

 Numerous other compounds are present in very small 

 proportions, such as fats, soaps, and mineral compounds, 

 but they appear to have no important relation to diges- 

 tion. If any ferment is present at all, it is only as a trace, 

 and therefore the bile is incapable of effecting decomposi- 

 tions of the proteins and carbohydrates, such as occur in 

 the mouth and stomach. 



152. Function of bile. Nevertheless, this liquid must 

 be regarded as having an important function, which it 

 exerts in two ways, (1) by preparing the chyme (partially 

 digested food from the stomach) for the action of the 

 pancreatic juice and (2) it acts in conjunction with the 

 pancreatic juice in preparing the fats for absorption. 



Pepsin, the stomach ferment, acts upon proteins only 

 in an acid medium. The opposite is true of the ferments 

 which the food meets in the intestines, for these require 

 an alkaline medium. The bile neutralizes the acidity of 

 the chyme, and so prepares the way for the pancreatic 

 juice to do its work. It is shown that when the entrance 

 of the bile into the intestines is prevented the fat of the 

 food largely passes off in the feces. 



Bile has very little, if any, direct digestive action, but 

 it may be said to cooperate with the pancreatic juice 

 in accomplishing the digestion of fats. It emulsifies fats, 

 especially in the presence of the pancreatic juice. When 

 the fats are split by a ferment in the pancreatic juice we 

 get as a result fatty acids which combine with the alka- 

 lies present to form soaps. Both the fatty acids and the 

 soaps are dissolved by the bile. In this way the fats are 

 prepared for absorption. In experiments by Vail on dogs, 

 cutting off the supply of bile reduced the absorption of fat 

 from 99 to 40 per cent. 



