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 faces, 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 faces.

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 hæmatoidin, 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 hæmoglobin 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 fæces (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 (CHO) 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 fæces.

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

Tests for Bile.-The most important constituents of the bile, viz., the bile acids and pigment, may be detected by appropriate tests, which are in common practical use:—

1. Pettenkofer's test for the bile acids.-To a fluid containing

either or both bile acids, or any solution of cholic acid, add some cane sugar, and then slowly, drop by drop, strong sulphuric acid. The solution turns to a cherry-red and then changes to purple. As other substances, such as albuminous bodies, give under this treatment a similar color, in order to make the reaction a trustworthy test for bile salts, the two characteristic absorption bands given by the spectroscope should also be observed.

The following is said to be a characteristic method: Rinse, out a porcelain capsule successively with the fluid to be tested with weak sulphuric acid, and with a weak solution of sugar, then heat to 70° C., when the capsule turns purple.

2. Gmelin's test for the bile pigments depends upon the fact that during the stages of oxidation the bilirubin undergoes a series of changes in color which follow the sequence of the familiar solar spectrum. Place a few drops of the fluid to be tested on a white surface (capsule or plate), and allow a drop of nitric acid, yellow with nitrous acid fumes which make it more oxidizing, to run into it; as they mingle together the rainbow-like play of color appears. This, when watched, will be found to consist of a series of changes to green, blue, violet, red and yellow.

This can also be observed by allowing the acid to trickle gently down the side of a test tube fixed in an inclined position so that it cannot be shaken : the play of color can then be seen starting from the point of junction of the two fluids.

METHOD OF SECRETION OF BILE.

The secretion of the liver varies less in the amount formed at different times than that of other digestive glands. Although the changes in the rate of its secretion are not so marked, they follow the same general rule as those of other glands connected with digestion, i. e., after food is taken there is a sudden rise, then a gradual fall, followed by a second rise in the rate of secretion. This is well seen in the case of the pancreas. Want of food is said to check the secretion of bile, but only does so in a slight degree, for the more important work of the liver is continuous, as is the activity of all glands whose duty it is to eliminate noxious substances or otherwise influence the composition of the blood. At the end of a period of fasting, the gall bladder is always found greatly distended, because the secretion has continued to flow into that receptacle, and there has been no call for its discharge into the duodenum.

The amount of bile produced by dogs is much influenced by their diet. It is very great when meat alone is consumed, less with vegetable, and very small with a diet of pure fat. As a general rule, bile is more abundantly produced in herbivorous than in carnivorous animals.

The rate of secretion is much influenced by the amount of

blood flowing through the organ, which probably explains the increase during digestion. Ligature of the portal vein causes arrest of the secretion, and death. After ligature of the hepatic artery the secretion continues, but soon diminishes from malnutrition of the tissue of the liver, which ultimately causes death if the entire vessel be tied.

These variations in the rate of secretion may depend on direct nervous influence, but no special secretory nerve mechanism has been discovered for the liver, and it is quite possible that the changes in the activity of the gland which accompany the different periods of digestion may be accounted for by changes in the intestinal blood supply, which give rise to corresponding differences in the amount of blood flowing through the portal vein.

The force with which the bile is secreted is very small. That is to say, the pressure in the ducts never exceeds that of the blood (as is the case in the salivary glands); but, on the contrary, when a pressure of about 16 mm. (.63 in.) mercury is attained, the evacuation of the bile ceases, and with a little increase of opposing force the fluid in the manometer retreats and finds its way into the blood. The low pressure which can be reached in the gall ducts does not imply any want of secretory power on the part of the liver cells, but merely that there exists a great facility of communication between the duct radicles and the blood vessels, probably through the medium of the lymphatics. This is made obvious by experiment, by which it can be shown that with a comparatively low pressure (200 mm.= nearly 8 in. of water for a guinea-pig) fluid can be forced into the circulation from the bile ducts.

This is seen also in stoppage of the bile ducts in the human subject, when some of the bile constituents continue to be formed, and pass into the blood, where their presence is demonstrated by the yellow color characteristic of jaundice. The ready evacuation of the bile is a matter of great importance for health, the least check to its free exit causing the secretion to be forced into the circulating blood instead of into the gall passages. Under normal circumstances, the large receptacle of the gall

bladder being always ready to receive the bile, ensures its easy exit from the ducts, but the forces which cause its flow are extremely weak. The smooth muscle in the walls of the duct seem rather for the purpose of regulating than aiding the flow.

When food from the stomach begins to flow into the duodenum, the muscular coat of the gall bladder contracts and sends a flow of bile into the intestine, which action is doubtless brought about by a reflex nerve impulse, for it is only when this part is stimulated that the bile flows freely from the bladder. The acid gastric contents seem to be the most efficacious stimulus.

In the human subject the quantity of bile secreted has been found to be about 600 c.c. (21 oz.) per diem in cases where there were biliary fistula. This would equal about 13 grms. per kilo of the body weight.

In the guinea-pig and rabbit, it has been estimated to be about 150 grms. per kilo of the body weight.

FUNCTIONS OF THE BILE.

1. Neutralizing and Precipitating Acid Peptones.-When the acid contents of the stomach are poured into the duodenum and meet with a gush of alkaline bile, a copious cheesy precipitate is formed which clings to the wall of the intestine. This precipitate consists partly of acid albumin (parapeptone) and peptones thrown down by the strong solution of bile salts, and partly of bile acids, the salts of which have been decomposed by the hydrochloric acid of the gastric juice. With the bile acids the pepsin is mechanically carried down. Thus, immediately on their entrance into the duodenum the peptic digestion of the gastric contents is suddenly stopped not only by the precipitation of the soluble peptones and the shrinking of the swollen parapeptone, but also by the removal of the pepsin itself from the fluid, and the neutralization of the gastric fluid by the alkaline bile.

By thus checking the action of the gastric ferment the bile prepares the chyme for the action of the pancreatic juice.

2. As a Stimulant, the bile is of considerable use, for it excites the muscles of the intestine to increased action, and thereby aids