342 FOOD AND DIGESTION 



urobilin, and of the feces, stercobilin. It is probable they are, all of them, 

 varieties of the same pigment, or derived from the same source. Cholesterin, 

 C 27 H 45 OH, and lecithin, C 42 H 84 NPO 9 are constant constituents of bile. Iron 

 is found among the salts of the ash. 



FIG. 273. Crystalline Scales of Cholesterin. 



The Role of Bile in Intestinal Digestion. Though it is not a true 

 digestive fluid, in that it has no ferment and digests nothing itself, yet it must 

 be regarded as an important aid to digestion for the following reasons: (a) Bile 

 assists in emulsifying the fats of the food, and thus renders them capable of 

 passing into the lacteals by absorption. For it has appeared in some experi- 

 ments in which the common bile-duct was tied, that, although the process 

 of digestion in the stomach was unaffected, chyle was no longer well formed; 

 the contents of the lacteals consisting of clear, colorless fluid, instead of being 

 opaque and white, as they ordinarily are after feeding. It is, however, the 

 combined action of the bile with the pancreatic juice to which the emulsifica- 

 tion is due rather than to that of the bile alone. The bile itself has a very 

 feeble emulsifying power. If the theory be accepted that fats are absorbed 

 as fatty acids and soaps, in solution, the action of the bile becomes very im- 

 portant because solutions of bile salts have the power of dissolving the fatty 

 acids. The moistening of the mucous membrane of the intestines with bile, 

 for this very reason, facilitates absorption of fatty matters through it. 



(b) The bile, like the gastric fluid, has a certain but not very considerable 

 antiseptic power, and may serve to prevent the decomposition of food during 

 the time of its sojourn in the intestines. Experiments show that the contents 

 of the intestines are much more fetid after the common bile-duct has been 

 tied than at other times. Moreover, it is found that the mixture of bile with 

 a fermenting fluid stops the process of fermentation. 



Bile is also an excretive fluid carrying waste products thrown off by the 

 liver. The liver during fetal life is proportionately larger than it is after 

 birth, and the secretion of bile is active, although there is no food in the in- 

 testinal canal upon which it can exercise any digestive property. At birth, 



