DIGESTION 325 



molecule into an iron- containing part, which is retained in 

 the liver, and the iron-free biliary pigment. Their amount 

 is greatly increased when haemoglobin is set free or injected 

 into the blood. Old and breaking down red cells are 

 scavengered from the blood by the endothehal cells of the 

 hepatic capillaries, while free haemoglobin is taken up 

 directly by the liver cells. 



That the pigments are formed in the liver is shown by the 

 fact that, when the liver is excluded from the circulation, 

 the injection of haemoglobin is not followed by their 

 appearance in the blood. But the formation of haematoidin, 

 which is practically identical with bilirubin, apart from the 

 liver, indicates that other tissues have the power of splitting 

 hsematin into its iron-containing and iron-free portions. 



The liver has the property of excreting not only these 

 pigments formed b}' itself, but also other pigments. Thus, 

 the liver of the dog can excrete the characteristic pigment of 

 sheep's bile when this is injected into his blood. 



(3) Cholesterol is a monatomic alcohol — CogH^gOH — 

 which occurs free in small amounts in the bile. It is very 

 insoluble, and is kept in solution by the salts of the bile acids. 

 It readily crystallises in rhombic plates, generally with a 

 notch out of the corner. 



The significance of cholesterol in metabolism and its 

 source in the bile is not definitely known. It is a 

 constituent of all cells, and under various conditions its 

 amount in the blood plasma may be increased, e.g. 

 when the amount in the food is large. It then appears 

 in larger quantities in the bile, and it must therefore 

 be concluded that it is excreted by the liver. Possibly 

 it is derived, at least in part, from the stroma of the 

 erythrocytes. 



(4) Fats and Lecithin. — The true fats and the phosphorus- 

 containing lecithin are present in small amounts in the bile, 

 and apparently they are derived from the fats of the liver 

 cells. The fats may be increased in amount by the admini- 

 stration of fatty food. 



(5) Nucleo-protein and Mucin. — The bile owfi= its viscosity 

 to the presence of a muciu-like body, which, however, does 



