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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 portal vein breaking up into capillaries, which run towards the centre of the lobule, and join H.V., the intralobular branch of the hepatic vein. The
tend to show that they communicate with ininute passages left between the hepatic celis, and traversing the lobule in the intervals left by the capillaries (Fig. 37, B.). However this may be, any fluid separated from the blood by the lobules must really find its way into them.
In the lobules themselves all the meshes of the bloodvessels are occupied by the liver cells. These are manysided, minute bodies, each about joogth 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 are supposed to reside.
21. The nature of these active powers, so far as the liver is a source of loss to the blood which traverses it, is determined by ascertaining
a. The character of that fluid, the bile, which incessantly flows down the biliary duct, and which, if digestion is not going on, and the passage into the intestine is closed, flows back into and fills the gall-bladder.
b. The difference between the blood which enters the liver and that which leaves it.
22. a. 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 by crystallization, and has been called bilin. It is in reality a mixture of two acids, in combination with soda, i one called glycocholic, and consisting of carbon, hydrogen, nitrogen, and oxygen, the other tuurocholic, and containing in addition to the other elements a considerable quantity of sulphur. Besides the taurocholate and glycocholate of soda, or bile salts as they are sometimes called, the bile contains a remarkable crystalline substance, very satty
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 », nucleus (two
are often present); E, capillaries cut across; c, minute biliary passages between the cells, injected with colouring matter.
looking, but not really of a fatty nature, called cholesterin, one or more peculiar colouring matters probably related to the hæmatin of the blood, and certain saline matters.
b. Of these constituents of the bile the water, the cholesterin, and the saline matters, alone, are discoverable in the blood ; and, though doubtless some difference obtains between the blood which enters the liver and that which leaves it, in respect of the proportional quantity of these constituents, great practical diificulties lie in the way of the precise ascertainment of the amount of that difference. The blood of the hepatic vein, however, is certainly poorer in water than that of the portal vein.
23. As the essential constituents of bile, the bile acids and the colouring matter are not discoverable in the blood which enters the liver ; they must be formed at the expense of the tissue of that organ itself, or of some constituent of the blood passing through it.
24. 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).
How much is taken in by the skin of man is not certainly known, but in some of the lower animals, such as the frog, the skin plays a very important part in the performance of the respiratory function.
25. The blood leaving the liver by the hepatic vein not only contains proportionally less water and fibrin, but proportionally more corpuscles, especially colourless corpuscles, and, what is still more important, under certain circumstances at least, a larger quantity of liver-sugar, or glucose, than that brought to it by the portal veins and hepatic artery.
That the blood leaving the liver should contain proportionally less water and more corpuscles than that entering it, is no more than might be expected from the fact that the formation of the bile, which is separated from this blood, necessarily involves a loss of water and of some solid matters, while it does not abstract any of the corpuscles.
We do not know why less fibrin separates from the blood of the hepatic vein than from the blood brought to the liver. But the reason why there is always more sugar in the blood leaving the liver than in that entering it; and why, in fact, there may be plenty of sugar in the blood of the hepatic vein even when none whatever is brought to it by the hepatic artery, or portal vein, has been made out by careful and ingenious experimental research.
26. If an animal be fed upon purely animal food, the blood of the portal vein will contain no sugar, none having been absorbed by the walls of the alimentary canal, nor will that of the hepatic artery contain any, or, at any rate, more than the merest trace. Nevertheless, plenty will be found, at the same time, in the blood of the hepatic vein and in that of the vena cava, from the point at which it is joined by the hepatic vein, as far as the heart.
Secondly, if, from an animal so fed, the liver be extracted, and a current of cold water forced into the vena porta, it will flow out by the hepatic vein, carrying with it all the blood of the organ, and will, after a time, come out colourless, and devoid of sugar. Nevertheless, if the organ be left to itself at a moderate temperature, sugar will soon again become abundant in it.
Thirdly, from the liver, washed as above described, a substance may be extracted, by appropriate methods, which resembles starch or dextrine, in chemical composition, consisting as it does of carbon united with hydrogen and oxygen, the latter being in the same proportions as in water. This “ amyloid ” substance is the glycogen spoken of in § 20. It may be dried and kept for long periods without undergoing any change.
But, like the vegetable starch and dextrine, this animal amyloid, which must be formed in the liver, since it is certainly not contained either in the blood of the portal vein, or in that of the hepatic artery, is very readily changed by contact with certain matters, which act as ferments, into sugar.
Fourthly, it may be demonstrated that a ferment, competent to change the “ amyloid " glycogen into saccharine glucose,” exists under ordinary circumstances in the liver.
Putting all these circumstances together, the following explanation of the riddle of the appearance of sugar in the blood of the hepatic vein and vena cava, when neither it, nor any compound out of which it is easily formed, exists in the blood brought to the liver, appears to have much probability; though it may possibly require modification, in some respects, hereafter.
The liver forms glycogen out of the blood with which it is supplied. The same blood supplies the ferment which, at the temperature of the body, very speedily converts the comparatively little soluble glycogen into very soluble sugar ; and this sugar is dissolved and carried away by each intralobular vein to the hepatic vein, and thence to the vena cava.
Though after death a very considerable quantity of sugar accumulates in the hepatic vein, the amount which, at any given moment, can be detected during life is extremely small. This has led some physiologists to suppose that, in health, glycogen is not converted into sugar, but undergoes some other change. A very small quantity of sugar however, so small as to almost escape detection, thrown into the hepatic vein every instant, would amount to a considerable quantity in the twenty-four hours.
This formation of glycogen in the liver goes on in the total absence of starch or sugar from the food. It must, therefore, in such cases be formed at the expense of proteid material (see Lesson VI.). It appears, however, that the presence of starch or sugar in the food, though not essential, is very favourable to the production of glycogen in the liver.
27. The lymphatic system has been already mentioned as a feeder of the blood with a fuid which, in general, appears to be merely the superfluous drainage, as it were, oi