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 difficulties 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

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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 may be 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 may 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 cer

tainly 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 fluid which, in general, appears to be merely the superfluous drainage, as it were, of

the blood-vessels; though at intervals, as we shall see, the lacteals make substantial additions of new matter. It is very probable that the multitudinous lymphatic glands may effect some change in the fluid which traverses them, or may add to the number of corpuscles in the lymph.

Nothing certain is known of the functions of certain bodies which are sometimes called ductless glands, but have quite a different structure from ordinary secreting glands; and indeed do not resemble each other in structure. These are, the thyroid gland, which lies in the part of the throat below the larynx, and is that organ which, when enlarged by disease, gives rise to " Derbyshire neck " or "goître ; " the thymus gland, situated at the base of the heart, largest in infants, and gradually disappearing in adult, or old, persons; and the supra-renal capsules, which lie above the kidneys.

28. We are as much in the dark respecting the office of the large viscus called the spleen, which lies upon the left side of the stomach in the abdominal cavity (Fig. 38). It is an elongated flattened red body, abundantly supplied with blood by an artery called the splenic artery, which proceeds almost directly from the aorta. The blood which has traversed the spleen is collected by the splenic vein, and is carried by it to the vena porta, and so to the liver.

A section of the spleen shows a dark red spongy mass dotted over with minute whitish spots. Each of these last is the section of one of the spheroidal bodies called corpuscles of the spleen, which are scattered through its substance, and consist of a solid aggregation of minute bodies, like the white corpuscles of the blood, traversed by a capillary network, which is fed by a small twig of the splenic artery. The dark red part of the spleen, in which these corpuscles are embedded, is composed of fibrous and elastic tissue supporting a very spongy vascular network.

The elasticity of the splenic tissue allows the organ to be readily distended, and enables it to return to its former size after distension. It appears to change its dimensions with the state of the abdominal viscera, attaining its largest size about six hours after a full meal, and falling to its minimum bulk six or seven hours later, if no further supply of food be taken.

The blood of the splenic vein is found to contain proportionally fewer red corpuscles, but more colourless corpuscles and more fibrin, than that in the splenic artery; and it has been supposed that the spleen is one of those parts of the economy in which the colourless corpuscles of the blood are especially produced.

The spleen (Spl.) with the splenic artery (SpA.). Below this is seen the splenic vein running to help to form the vena portæ (V.P.). Ao. the aorta; D. a pillar of the diaphragm; P.D. the pancreatic duct exposed by dissection in the substance of the pancreas; Dm. the duodenum; B.D. the biliary duct uniting with the pancreatic duct into the common duct, x; y, the intestinal vessels.

29. It has been seen that heat is being constantly given off from the integument and from the air-passages; and everything that passes from the body carries away with it, in like manner, a certain quantity of heat. Furthermore, the surface of the body is much more exposed to cold than its interior. Nevertheless, the temperature of the body is maintained very evenly, at all times and in all parts, within the range of two degrees on either side of 99° Fahrenheit.

This is the result of three conditions :-The first, that heat is constantly being generated in the body; the second, that it is as constantly being distributed through