DIGESTION 



mucin 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, on boiling with dilute acid, a carbo- 

 hydrate group (viz., glucosamine, C 6 H u O 5 Nll2, corresponding to 

 dextrose in which OH is replaced by NH 2 ). It is relatively rich in phos- 

 phorus, 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 bili- 

 verdin 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 interchangeable, but there 

 is no question that bilirubin is the pigment which is formed by the 

 liver-cells. 



Bilirubin (C 3 2H 36 N 4 O 6 ) can be prepared from powdered red gall- 

 stones by dissolving the chalk with hydrochloric acid, and extracting 

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

 solution, on evaporation, or from hot dimethyl anilin, beautiful rhombic 

 tables or prisms of bilirubin separate out. 



Biliverdin (C 32 H 36 N 4 O 8 ) 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 

 from bilirubin by oxidation. By the aid of active oxidizing agents, 



