344 STUDIES IN ADVANCED PHYSIOLOGY. 



small amounts, however, it is found in all animal and plant 

 cells. This cholesterin is, therefore, probably not formed 

 in the liver, but is merely eliminated by the liver cells from 

 the blood, and poured into the bile secretion to be removed 

 from the body. As far as we know now it is a pure excre- 

 tion, the accumulation of which in the blood would prove 

 dangerous. It is sometimes precipitated in the bile-ducts 

 or gall-bladder and so gives rise to the formation of gall 

 stones, which may in many cases consist of pure cholesterin, 

 the removal of which from the gall-bladder or bile-ducts 

 is frequently a matter of the severest pain, and oftentimes 

 results fatally. In the second place these bile acids seem 

 to facilitate the absorption of fats from the intestine, and to 

 help materially in the emulsion of the fats, a point to be 

 discussed later. The exceedingly bitter taste of bile is due 

 to these organic bile salts. 



The Bile Pigments. The bile owes its color to the pres- 

 ence of small amounts of substances called bile pigments. 

 These pigments, according to the animals from which they 

 are taken, are of two kinds, the golden bilirubin or the 

 greenish biliverdin. The bilirubin is present in fresh human 

 bile and occurs normally in the bile of most carnivora. On 

 the other hand the bile pigments in the case of most herbi- 

 vorous animals is the greenish biliverdin. Biliverdin and 

 bilirubin are practically the same, one being only a slight 

 oxidation product of the other, and it is very easy by this 

 means to change one into the other. This fact is made use 

 of in detecting the presence of bile pigments. If to some 

 human bile in a test-tube a little fuming nitric acid be 

 added, the acid being heavier will sink to the bottom, the 

 bile being in contact with it above. At this point of con- 

 tact the bilirubin undergoes a succession of color changes, 

 through green, blue and violet, to yellow. This play of 

 colors is due to the successive stages of oxidation of the 

 bile pigments. The bilirubin is in the first stage changed 

 to green, a change due to the formation of biliverdin. In 



