DIGESTION 



335 



gall-bladder water is absorbed from the bile and muciri added to 

 it, so that the specific gravity of bladder bile is as high as 1030 

 to 1040. The reaction is feebly alkaline to litmus. 



The composition of two specimens of human bile one from a 

 fistula, the other from the gall-bladder is shown in the following 

 table : 



The substance which renders bladder bile viscid, but which is 

 present in much smaller amount in bile from a fistula, and is probably 

 entirely absent from the fluid as it is secreted by the liver-cells, is 

 commonly termed ' mucin.' It has been shown, however, that in 

 many animals for example, the ox, dog, sheep, etc. the substance 

 is not a true mucin. It does not yield, like mucin, reducing sub- 

 stances on boiling with dilute acid, or only very small amounts of 

 these. It is relatively rich in phosphorus, and consists mainly, at 

 any rate of a phospho-protein (p. 2). The mucilaginous substance 

 of human bile consists largely of true mucin. 



Mucin is scarcely to be looked upon as an essential constituent of 

 bile ; it is not formed by the actual bile-secreting cells, but by 

 mucous glands in the walls and goblet-cells in the epithelial lining 

 of the larger bile-ducts, and especially of the gall-bladder. 



Bile-pigments. It has been said that these form a series, but 

 only two of the pigments of that series are present in normal bile, 

 bilirubin, and biliverdin. In human bile, the former, in herbivorous 

 bile and that of some cold-blooded animals, such as the frog, the 

 latter is the chief pigment. But bilirubin can be extracted in large 

 amount from the gall-stones of cattle ; while the placenta of the bitch 

 contains biliverdin in quantity, although, as in all carnivora, it is 

 either absent from the bile or exists in it in comparatively small 

 amount. These facts show that the two pigments are readily inter- 

 changeable. 



Bilirubin is best prepared from powdered red gall-stones by dis- 

 solving the chalk with hydrochloric acid, and extracting the residue 

 with chloroform, which takes up the pigment. From this solution, 

 on evaporation, beautiful rhombic tables or prisms of bilirubin 

 separate out ; and the crystals are finer when the solution also con- 

 tains cholesterin than when it is pure. 



Biliverdin can be obtained from the placenta of the bitch by 

 extraction with alcohol. It is insoluble in chloroform, and by means 

 of this property it may be separated from bilirubin when the two 

 happen to be present together in bile. Biliverdin can also be formed 



