The smaller branches of the hepatic duct, lined by an epithelium, which is continuous with that of the main duct, and thence with that of the intestines, into which the main duct opens, may be traced to the very surface of the lobules, where they seem to end abruptly. But, upon closer examination, it is found that they communicate with a network of minute passages passing between the hepatic cells, and traversing the lobule in the intervals left by the capillaries (Fig. 37, B). The bile manufactured by the hepatic cells finds its way first into these minute passages, and from them into the ducts.

21. The work of the liver, and this, as has been said, is carried out by the hepatic cells, may be considered as consisting of two kinds.

On the one hand, the hepatic cells are continually engaged in the manufacture of a complex fluid called bile, which they pour into the minute passages spoken of above, and thence into the branches of the hepatic duct; whence it flows through the duct itself into the intestines, or, when digestion is not going on and the opening of the duct into the intestine is closed, back to the gall-bladder. The materials for this bile are supplied to the hepatic cells by the blood; hence the secretion of the bile constitutes a loss to the blood.

22. The total quantity of bile secreted in the twentyfour hours varies, but probably amounts to not less than from two to three pounds. It is a golden yellow, slightly alkaline fluid, of extremely bitter taste, consisting of water with from 17 per cent. to half that quantity of solid matter in solution. The solids consist in the first place of a somewhat complex substance which may be separated out by crystallisation, as an apparently simple mass, but is in reality a mixture of two acids, in combination with soda; one called glycocholic, and consisting of carbon, hydrogen, nitrogen and oxygen, the other taurocholic, and containing, in addition to the other elements, a considerable quantity of sulphur. Besides the taurocholate and glycocholate of soda, or bile salts as the two are sometimes called, the bile contains a remarkable crystalline substance, very fatty-looking, but not really of a fatty nature, called cholesterin, one or more peculiar colouring matters

K

B

FIG. 37. A. Section of partially injected liver magnified. is introduced to mark the limits of a lobule. vein breaking up into capillaries, which run lobule, and join H.V., the intralobular branch of the hepatic vein. The

probably related to the hæmatin of the blood, and certain saline matters.

23. Of these constituents of the bile the essential substances, the bile acids and the colouring matter, are not discoverable in blood which enters the liver; they must therefore be formed in the hepatic cells. How they are exactly formed we do not at present clearly know. The material of which they are composed is brought to the hepatic cells by the blood, but the exact condition of that material—whether, for instance, the blood brings something very like the bile acids, and only needing a slight change to be converted into bile acids; or whether the hepatic cells manufacture the bile acids from the beginning, as it were, out of the common material which the blood brings to the liver as to all other tissues and organs---is not as yet quite determined. The saline matters and cholesterin, on the other hand, appear to be present in the blood of the portal vein, and may therefore, like the water, be simply taken up by the cells from the blood, and passed on to the bile ducts.

24. Thus the bile is a continual loss to the blood. But, besides forming bile, the hepatic cells are concerned in other labours, the result of which can hardly be considered either as a loss or as a gain, since these labours simply consist in manufacturing from the blood and storing up in the hepatic ceils substances which, sooner or later, are returned, generally in a changed condition, back into the blood.

As we shall presently see, the portal blood is, after a meal, heavily laden with substances, the result of the digestive changes in the alimentary canal. When these substances, carried along in the portal blood, reach the hepatic cells, in the meshes of the lobules, some of them appear to be taken up by those cells and to be stored up in them in a changed condition. In fact, the products of digestion passing along the portal veins suffer (in the liver) a further change, which has been called a secondary

outline of the liver cells are seen as a fine network of lines throughout the whole lobule.

B. Portion of lobule very highly magnified. a, liver cell with n, nucleus (two are often present); b, capillaries cut across; c, minute biliary passages between the.cells, injected with colouring matter.

digestion. Thus the liver produces a powerful effect on the quality of the blood passing through it, so that the blood in the hepatic vein is very different, especially after a meal, from the blood in the portal vein.

The changes thus effected by the hepatic cells are probably very numerous, but they have not been fully worked out, except in one particular case, which is very interesting and deserves special attention.

It is found that the liver of an animal which has been well and regularly fed, when examined immediately after death, contains a considerable quantity of a substance which is very closely allied to starch, consisting of carbon, hydrogen, and oxygen in certain proportions. This substance, which may by proper methods be extracted and preserved as a white powder, is in fact an animal starch, and is called glycogen. As we shall see, common starch is readily changed by certain agents into grape-sugar, dextrose or glucose, as it is sometimes called; and this glycogen is similarly converted with ease into grape-sugar. Indeed, if the liver of such an animal as the above, instead of being examined immediately after death, be left in the body, or be placed on one side after removal from the body for some hours before it is examined, a great deal of the glycogen will have disappeared, a quantity of grape-sugar having taken its place. There seems to be present in the liver some agent capable of converting the glycogen into grape-sugar, and this change is particularly apt to take place if the liver is kept at blood-heat or near that temperature.

Now if, instead of the liver of a well-fed animal, the liver of an animal which has been starved for several days be examined in the same way, very little glycogen indeed will be found in it, and when the liver is left exposed to warmth for some time very little grape-sugar is found. That is to say, the liver has, in the first case, formed the glycogen and stored it up in itself, out of the food brought to it by the portal blood in the second case, no food has been brought to the liver from the alimentary canal, no glycogen has been formed, and none stored up. If the liver in the first case be examined microscopically with certain precautions, the glycogen may be seen stored up in the hepatic cells; in the second case little or none can be seen.

The kind of food which best promotes the storing up of glycogen in the liver is one containing starch or sugar; but some glycogen will make its appearance even when an animal is fed on an exclusively proteid diet, though not nearly so much as when starch or sugar is given.

It would appear, then, that the hepatic cells can manufacture and store up in themselves the substance glycogen, being able to make it out of even proteid matter, but more easily making it out of sugar; for, as we shall see, all the starch which is eaten as food is converted into sugar in the alimentary canal, and reaches the liver as sugar.

There are reasons for thinking that the glycogen, thus deposited and stored up in the liver, is converted into sugar little by little as it is wanted, poured into the hepatic vein, and thus distributed over the body. So that we may regard this remarkable formation of glycogen in the liver as an act by which the blood, when it is overrich in sugar, as after a meal, stores it up or deposits it in the liver as glycogen ; and then, in the intervals between meals, the liver deals out the stored-up material as sugar back again in driblets to the blood. The loss to the blood, therefore, is temporary-no more a real loss than when a man deposits at his banker's some money which he has received until he has need to spend it.

This story of glycogen, important in itself, is also useful as indicating other possible effects of a similar nature which the hepatic cells may bring about on the blood, as it is passing in the meshes of the lobules of the liver from the veinlets of the portal to the veinlets of the hepatic vein.

25. We must next consider the chief sources of constant gain to the blood; and, in the first place, the sources of gain of matter.

The lungs and skin are, as has been seen, two of the principal channels by which the body loses liquid and gaseous matter, but they are also the sole means by which one of the most important of all substances for the maintenance of life, oxygen, is introduced into the blood. It has already been pointed out that the volume of the oxygen taken into the blood by the lungs is rather greater than that of the carbonic acid given out. The absolute weight of oxygen thus absorbed may be estimated at 10,000 grains (see Lesson VI. § 2).