which pours its contents into one of the ramifications of the hepatic vein. These ramifications, joining together, form larger and larger trunks, which at length reach the hinder margin of the liver, and finally open into the vena cava inferior, where it passes upwards in contact with that part of the organ.

Thus the blood with which the liver is supplied is a mixture of arterial and venous blood: the former brought by the hepatic artery directly from the aorta, the latter by the portal vein from the capillaries of the stomach, intestines, pancreas, and spleen.

FIG. 36.

Termination of bile duct at edge of lobule (somewhat diagrammatic). b, small bile duct, becoming still smaller at b', the low flat epithelium at last suddenly changing into the hepatic cells, 7, the channel of the bile duct being continued as small passages between the latter. c, capillary bloodvessel cut across.

In the lobules themselves all the meshes of the bloodvessels are occupied by the liver cells, or hepatic cells. These are many-sided minute bodies, each about Tooth of an inch in diameter, possessing a nucleus in its interior, and frequently having larger and smaller granules of fatty matter distributed through its substance (Fig. 37, a). It is in the liver cells that the active powers of the liver reside.

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

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A. Section of partially injected liver magnified. The artificial white line is introduced to mark the limits of a lobule. V.P. Branches of pcrtal vein breaking up into capillaries, which run towards the centre of the lobule, and join II.I., 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.

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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.