THE BIOCHEMICAL PROCESSES OF DIGESTION 495 



percentage of cholesterol in these varies from 20 to 90; the remainder 

 being organic material such as epithelial cells, inorganic salts, pigment, 

 etc. The origin of cholesterol is partly endogenous and partly exoge- 

 nous. In the former case it comes from the envelope of red blood cor- 

 puscles and from the nervous tissues, where it is present in considerable 

 amount. The latter source is, of course, the food. The increase in 

 cholesterol esters in the blood after feeding with food rich in this sub- 

 stance has been shown, particularly in rabbits. 



That the bile should be the pathway through which cholesterol is 

 excreted depends no doubt on the fact that it contains bile salts, which 

 along with their other properties have a remarkable solvent action on 

 cholesterol. This solvent property depends on the cholic acid part of 

 the .bile salts, which, as already remarked, is chemically very closely 

 related to cholesterol; indeed, the relationship is so close that some have 

 suggested that cholic acid is derived from cholesterol. This would mean 

 that the cholesterol of blood is excreted in two ways, as cholesterol and 

 as cholic acid. Other observers, however maintain that the cholesterol 

 is excreted mainly by the lining membrane of the gall bladder, and 

 that this explains why gall-bladder bile contains more of it than fis- 

 tula bile. This evidence is, however, not very strong, for the greater 

 excretion of cholesterol under conditions where tjie circulation of bile 

 is going on may be explained as due to the presence of bile salts, which 

 serve to carry the cholesterol out of the blood. 



Many problems remain to be elucidated in connection with the metabolic 

 history of cholesterol. That some of it is absorbed when cholesterol is 

 contained in the food might seem to indicate that its source is entirely 

 exogenous. Against this view, however, stand two facts: (1) that the 

 cholesterol in the feces of herbivorous animals is of the same variety as 

 that present in those of carnivorous animals and not the phytosterol 

 which is present in plants; and (2) that the universal presence of 

 cholesterol in cells indicates that it must be manufactured there. 



The Bile Pigments 



The pigments of bile are 'bilirubin and biliverdin. The latter is pro- 

 duced from the former by oxidation. If the oxidation be carried a 

 stage further, a blue pigment called bilicyanin is formed. This process 

 of oxidation can be observed in the ring test for bile pigment with 

 fuming nitric acid. When bilirubin is reduced, urobilin, one of the 

 pigments in urine, is formed. Bilirubin must therefore be considered 

 as the mother substance of all these pigments, and it is of interest in 

 connection with its derivation to know that it has the same formula 



