The bile of man and carnivorous animals is of a deep orange-red color, turning to greenish-brown by decomposition of its coloring matter. In herbivorous animals it has some shade of green when quite fresh, but turns to a muddy brown after a time. It is transparent, and more or less viscid according to the length of time it has remained in the gall bladder. It has a strong, bitter taste, a peculiar aromatic odor, and after remaining for some time in the gall bladder it has an alkaline reaction. Its specific gravity is about 1005 when taken from the bile ducts directly, but it may rise to 1030 after prolonged stay in the gall bladder, owing to the addition of mucus and the absorption of some of its fluid.

The following table gives approximately the proportions of the chief constituents of bile: -

Water

85.0 per cent

Bile salts,............

.... IO.O "

Coloring matter and mucus

.... 3.0 "

Fats,..............

.... I.O "

Cholesterin

.... 0.3 "

Inorganic salts

.... 0.7 "

100.0

Bile contains no structural elements nor any trace of albuminous bodies.

1. The bile acids are two compound acids, glycocholic and taurocholic, which exist in the bile in combination with sodium. The amount of each varies in different animals and at different times in the same animal. The bile of the dog and other car-nivora contains only taurocholate of soda. In the ox the glyco-cholate of soda is greatly in excess. In man both are present, the proportion being variable, but the glycocholate greatly preponderates.

To separate these acids, bile is evaporated to one-fourth its volume, rubbed to a paste with animal charcoal to remove the pigments, and carefully dried at ioo° C. The black cake is extracted with absolute alcohol, which dissolves the bile salts. From the strong alcoholic solution after partial evaporation the bile salts can be precipitated by ether. They first appear as an emulsion, and then form glistening crystals which are soluble in water or alcohol, but insoluble in ether.

From the solution of the two salts the glycocholic acid may be precipitated by neutral lead acetate, as lead glycocholate, from which the lead may be removed by sulphuretted hydrogen, and the acid precipitated from its alcoholic solution by the addition of water. The taurocholic acid may be obtained subsequently by treating with basic lead acetate.

Glycocholic acid, when boiled with weak acids, alkalies, or baryta water, takes up an atom of water, and splits into cholic acid and glycin (amido-acetic acid). (See p. 74).

Taurocholic acid, under similar treatment, splits into cholic acid and taurin (amido-ethyl-sulphonic acid. (See p. 73).

Cholic acid occurs free in the intestines, the bile salts being split up in digestion, and taurocholic and glycocholic acids decomposed.

The non-nitrogenous cholic acid is in a great measure eliminated with the faeces, while the taurin and glycin are reabsorbed into the blood with some of the other constituents of the bile, and are again probably utilized in the economy.

No traces of these bile acids can be detected in the blood, and there is no accumulation of them in the body after the removal of the liver; hence, it has been concluded that they are manufactured in the liver.

2. The greater proportion of the mucus contained in the bile is produced in the gall bladder, and there added to the bile. Some mucus comes from the mucous glands in the bile ducts, but, unless the bile has remained in the gall bladder, there is but an insignificant amount of mucus present, as is seen when a fistula is made from the hepatic duct. The mucus passes in an unchanged state through the intestine, and is evacuated with the faeces.

3. The bile pigment of man and carnivora is chiefly the reddish form called bilirubin. It is insoluble in water but soluble in chloroform. It can be obtained in rhombic crystals, and is easily converted by oxidation into a green pigment, biliverdin, which is the principal coloring matter in the bile of many animals, and is not soluble in chloroform, but readily so in alcohol.

Bilirubin is supposed to be identical with haematoidin, a deeply colored material found by Virchow in old extravasations of blood within the body, and hence the bile pigment is said to be derived from the coloring matter of the blood. Probably the haemoglobin of some red corpuscles which have been broken up in the spleen is converted into bile pigment by the liver.

Under the influence of decomposition bilirubin undergoes a change, taking up water and forming hydro-bilirubin; this occurs in the intestine, and the bilirubin is thus eliminated as the coloring matter of the faeces (stercobilin), which is probably identical with the urobilin of the urine.

4. Fatty matters, the principal of which are lecithin, palmitin, stearin, olein, and their soda soaps.

5. Cholesterin (C26HuO) is an alcohol, and crystallizes in clear rhombic plates, insoluble in water but held in solution by the presence of the bile salts. It can be obtained from gall stones, the pale variety of which are almost entirely composed of it. The cholesterin leaves the intestines with the faeces.

6. Among the inorganic salts are sodium and potassium chloride, calcium phosphate, some magnesia, and a considerable quantity of iron.