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 esti-. mated 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.

26. 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 body, 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 body, situated at the base of the heart, largest in infants, and gradually disappearing in adult, or old persons; and the supra-renal bodies, which lie above the kidneys.

27. 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 white spots are embedded, is composed of a spongy framework of fibrous and, elastic tissue, frequently mixed with plain muscular fibres, and of peculiar

delicate vascular structures, which fill up the meshes of the framework, and through which the splenic blood flows. The elasticity of the splenic tissue allows the organ to be readily distended with blood, 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, than in the splenic artery; and it has been

The spleen (Spl) with the splenic artery (Sp A.). 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, the intestinal vessels.

; y,

supposed that the spleen is one of those parts of the economy in which, on the one hand, colourless corpuscles of the blood are produced, and, on the other, red corpuscles die and are broken up.

28. 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 in health maintained very evenly, at all times and in all parts, within the range of two degrees or even less 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 the body; the third, that it is subject to incessant regulation.

Heat is generated whenever oxidation takes place. As we have seen, the tissues all over the body, muscle, brainsubstance, gland cells and the like, are continually undergoing oxidation. The living substance of the tissue, built up out of the complex proteids, fats, and carbo-hydrates, and thus even still more complex than these, is, by means of the oxygen brought by the arterial blood, oxidised, and broken down into simpler more oxidised bodies, which are eventually reduced to urea, carbonic acid, and water. Wherever life is being manifested these oxidative changes are going on, more energetically in some places, in some tissues, and in some organs, than in others; and similar changes, though perhaps not to any very great extent, are taking place in the blood itself. Hence every capillary vessel and every extra-vascular islet of tissue is really a small fireplace in which heat is being evolved, in proportion to the activity of the chemical changes which are going on.

29. But as the vital activities of different parts of the body, and of the whole body, at different times, are very different; and as some parts of the body are so situated as to lose their heat by radiation and conduction much more easily than others, the temperature of the body would be very unequal in its different parts, and at different times, were it not for the arrangement by which the heat is distributed and regulated.

Whatever oxidation occurs in any part, raises the temperature of the blood which is in that part at the time, to a proportional extent. But this blood is swiftly hurried away into other regions of the body, and rapidly gives up its increased temperature to them. On the other hand,

the blood which, by being carried to the vessels in the skin on the surface of the body begins to have its temperature lowered by evaporation, radiation, and conduction, is hurried away, before it has time to get thoroughly cooled, into the deeper organs; and in them it becomes warm by contact, as well as by the oxidating processes there going on. Thus the blood-vessels and their contents might be compared to a system of hot-water pipes, through which the warm water is kept constantly circulating by a pump; while it is heated not by a great central boiler as usual, but by a multitude of minute gas jets, disposed beneath the pipes, not evenly, but more here and fewer there. It is obvious that, however much greater might be the heat applied to one part of the system of pipes than to another, the general temperature of the water would be even throughout, if it were kept moving with sufficient quickness by the pump.

30. If such a system were entirely composed of closed pipes, the temperature of the water might be raised to any extent by the gas jets. On the other hand, it might be kept down to any required degree by causing a larger, or smaller, portion of the pipes to be wetted with water, which should be able to evaporate freely-as, for example, by wrapping them in wet cloths. And the greater the quantity of water thus evaporated, the lower would be the temperature of the whole apparatus.

Now, the regulation of the temperature of the human body is effected on this principle. The vessels are closed pipes, but a great number of them are inclosed in the skin and in the mucous membrane of the air-passages, which are, in a physical sense, wet cloths freely exposed to the air. It is the evaporation from these which exercises a more important influence than any other condition upon the regulation of the temperature of the blood, and, consequently, of the body.

But, as a further nicety of adjustment, the wetness of the regulator is itself determined, through the aid of the nervous system, by the temperature of the body. The sweat-glands are so constituted that they are stimulated to activity by warmth and rendered inactive by cold. When the body is exposed to a high temperature (and the same occurs when a part only of the body is heated) the