310 A MANUAL OF PHYSIOLOGY 



Water ... 982 parts in 1,000 



Solids : 



Mucin and pigments - 



Bile-salts 



Lecithin and soaps 



Cholesterin 



Inorganic salts - 



It will be observed that no proteids are enumerated in 

 this table ; bile contains none, and it is unlike all the other 

 digestive juices in this respect. 



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. The 

 mucin of human bile is a true mucin, but that of ox-bile is a nucleo- 

 albumin (p. 17). It may be removed by precipitation with alcohol 

 or dilute acetic acid. 



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

 only two of the pigments of that series appear to be present m 

 normal bile, bilirubin and biliverdin. In human bile as usually 

 obtained, the former, in herbivorous bile and that of some cold- 

 blooded animals, such as the frog, the latter, is the chief pigment. 

 But in fresh human bile biliverdin may be chiefly present, and 

 bilirubin can be extracted in large amount from the gallstones of 

 cattle; while in the placenta of the bitch biliverdin is present in 

 quantity, although, as in all carnivora, it is either absent from the 

 bile or exists in it in comparatively small amount. All these facts 

 show that the two pigments are readily interchangeable. 



Bilirubin is best prepared from powdered red gallstones 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 

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

 such as yellow nitric acid (which contains some nitrous acid), a 

 whole series of oxidation products of bilirubin is obtained, beginning 

 with biliverdin, and passing through bilicyanin, a blue pigment, to 

 choletelin, a yellow substance. It is possible that there are other 

 intermediate bodies. This is the foundation of Gmelirts test for bile- 

 pigments (see Practical Exercises, p. 381). The same substances are 

 produced, and in the same order, when a solution of bilirubin in 

 chloroform is treated with a dilute alcoholic solution of iodine. 



The positive pole of a galvanic current causes the same oxidative 

 changes, the same play of colours, while the reducing action of the 



