DIGESTION. 101 



The coloring-matters which give the tints to the bile are biliverdin and 

 bilirubin, and are probably derived from the coloring-matter of the blood. 

 Their presence in any fluid can be recognized by adding to it nitric acid con- 

 taining nitrous acid, when a play of colors is observed, beginning with green, 

 blue, violet, red and yellow. 



The bile is both a secretion and an excretion; it is constantly being formed 

 and discharged by the hepatic ducts into the gall-bladder, in which it is stored 

 up during the intervals of digestion. As soon as food enters the intestines it is 

 poured out abundantly by the contraction of the walls of the gall-bladder. 



The amount secreted in twenty-four hours is about z\ pounds. 



Functions of the Bile : 



1. It assists in the digestion of the fats^and promotes their absorption. 



2. It tends to prevent putrefactive changes in the food. 



3. It stimulates the secretion of the intestinal glands, and excites the normal 

 peristaltic movement of the bowels. 



The digested food, the chyme, is a grayish, pultaceous mass, but as it passes 

 through the intestines it becomes yellow from admixture with the bile. It is 

 propelled onward by the peristaltic movement, the result of the contraction 

 of the circular and longitudinal muscle-fibers. 



During the passage of the digesting food through the intestinal canal the 

 nutritive products the amino-acids, the dextrose and levulose, the fatty 

 emulsions, the fatty acids and their soaps are absorbed into the blood, while 

 the undigested residue is carried onward by the peristaltic movements through 

 the ileo-cecal valve into the large intestine. 



Intestinal Fermentation. Owing to the favorable conditions for fer- 

 mentative and putrefactive processes e. g., heat, moisture, oxygen, micro- 

 organisms the food, when consumed in excessive quantity or when acted 

 upon by defective secretions, undergoes a series of decomposition changes 

 which are attended by the production of gases and various chemic compounds. 

 Grape-sugar and maltose are partially split into lactic acid, this into butyric 

 acid, carbon dioxid, and hydrogen. Fats are reduced to glycerol and fatty 

 acids; the glycerol, according to the organisms present, yields succinic and 

 other fatty acids, carbon dioxid, and hydrogen. 



The proteins, under the prolonged action of the pancreatic juice, are decom- 

 posed, and yield leucin and tyrosin; the former is split into valerianic acid, 

 ammonia, and carbon dioxid; the latter is split into indol, which is the ante- 

 cedent of indican in the urine. Skatol is another proteid derivative constantly 

 present in the fecal substance. 



