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

in absorption and promotes the forward movement of food, and more particularly of those insoluble materials which have to be evacuated per anum. This stimulation may amount to mild purging.

3. Moistening and Lubricating.-The bile adds to the ingesta an abundant supply of fluid and mucus, much of which passes along the intestine to moisten and lubricate the fæces and facilitate

their evacuation. In cases of jaundice, or when the bile is removed by a fistula, the fæces are hard and friable, and with difficulty expelled, owing to the deficient fluid and mucus, as well as to the weaker peristaltic movements.

4. As an Aid to Absorption.-The bile having some soap in solution has a close relationship to both watery and oily fluids, and possibly on this account, as well as owing to a peculiar power possessed by the bile salts, a membrane saturated with bile allows an emulsion of fat to pass through it much more readily than if the same membrane were kept moistened with water. This can be seen experimentally with filter paper.

5. As Excrement.-Although much of the bile is reabsorbed from the intestinal tract into the blood, and again used in the economy, some of its constituents pass off with the fæces, and are no doubt simply excrementitious matters that must be got rid of. Thus all the cholesterin, mucus, and coloring matter are normally eliminated, and a considerable quantity of the bile acids are split up, the cholic acid being found in the fæces.

6. Emulsification of Fats.-The bile has some share in forming an emulsion, but far less than the secretion of the pancreas; however, the mixed secretions are probably more efficacious than either separately, from the presence of the free fatty acids, which form soaps and aid in forming the emulsion.

7. As an Antiseptic, bile has been said to have some function to perform. Possibly it restricts the formation of certain of the bye products, such as the indol resulting from pancreatic digestion; but it is certainly not antiseptic, since bacteria abound and thrive in it and in the duodenum.

CHAPTER XI.

FUNCTIONS OF THE INTESTINAL MUCOUS MEMBRANE.

Two distinct varieties of gland are found in the small intestine. Those known as Brunner's glands are localized to the submucosa of the duodenum; they are insignificant in number when compared with the sound variety, called Lieberkühn's glands, which are distributed over the entire intestinal tract and are closely set in the mucous membrane.

Brunner's glands form, in some animals, a dense layer in the submucous tissue of the beginning of the duodenum; they are small, branched saccular glands resembling mucous glands in structure. Owing to their small size the secretion cannot be obtained in sufficient quantity to make satisfactory experiments in respect to its properties. It is said to dissolve albumin and to have a diastatic fermentative action, so that probably the secretion is analogous to that of the pancreas, as Brunner originally

supposed. The quantity of fluid secreted by these glands is so small that its existence is not taken into account in speaking of the intestinal juice, by which is meant the fluid poured out by the innumerable short tubes or follicles of the intestine. These Lieberkühn's follicles belong to a very simple form of

Section of the Mucous Membrane of small intestine, showing Lieberkühn's follicles (a) with their irregular epithelium, and the villi (6) passing out of view; (c) Muscularis mucosa; (d) Submucous tissue. (Cadiat.)

gland, each one being a single straight cavity in the mucous membrane hardly deep enough to deserve the name of a tube. In the small intestine they are set as closely as the villi permit. In the large intestine, where the villi are absent, they are more closely set and are also deeper (Fig. 78). They are bounded by