'I III BI0CH1 Mil \l. I'I;im i SSI g OF DIG! 81 



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

 being organic material such ;is epithelial cells, inorganic salts, pigm 

 etc. The origin of cholesterol is partly endogenous and partly 

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

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

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



cholesterol esters in the blood after feeding with \>»><\ rich in this Bub- 

 stance has been shown, particularly in rabbits. 



That the bile should be the pathway through which cholesterol is 

 excreted depends oo doubl on the fact thai it contains bile salts, which 

 along with their other properties have a remarkable solvent action on 

 cholesterol. This solvenl property depends on the cholic acid pari 

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



• «... 



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

 suggested thai 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 thai the cholesti 

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

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

 tula bile. This evidence is, however, nol very strong, for tl _ iter 

 excretion of cholesterol under conditions where the circulation <>f bile 

 is going on may be explained as due to the ]"■ of bile salts, which 



serve to carry the cholesterol ou1 of the blood. 



Many problems remain to be elucidated in connection with the metabolic 



history of cholesterol. That son f it is absorbed when cholesterol is 



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

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



cholesterol in the feces of herbivorous animals is of the same vai 



that presenl in those of carnivorous animals and not the phytosterol 

 which is presenl in plants; and 'J thai the universal pr< of 



cholesterol in cells indicates thai it must he manufactured tin 



The Bile Pigments 



The pigments of bile are bilirubin ami biliverdin. The latter is pi 

 duced from tin- former by oxidation. If the oxidation he carried a 

 stage further, a blue pigmenl called bilicyanin is formed. This p 

 of oxidation can he observed in the ring tesl for bile pigment with 

 fuming nitric acid. When bilirubin is reduced, urobilin, 01 the 



pigments in urine, is formed. Bilirubin must therefore b< 

 as the mother substance of all those pigments, and it is ><{ 1 • in 



connection with its derivation to know that it lias the sami lla 



