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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 990 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 ; and hence, whenever proteid substances (see Lesson VI., $ 4) or fats, or amyloidal matters, are being converted into the more highly oxidated waste products,-urea, carbonic acid, and water, -heat is necessarily evolved. But these processes are taking place in all parts of the body by which vital activity is manifested ; and hence every capillary vessel and every extravascular 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.
30. 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 arrangements by which the heat is distributed and regulated.
Whatever oxidation occurs in any part, raises 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, &c., is hurried away before it has time to get thoroughly cooled into the deeper organs; and in then it becomes warm by contact, as well as by the oxidating processes in which it takes a part. 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.
31. If such a system were entirely composed of closed pipes, the temperature of the water might be taised 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 teinperature 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 greater number of them are enclosed 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 by the state of the small vessels, inasmuch as exudation from these takes place more readily when the walls of the veins and arteries are relaxed, and the blood distends them and the capillaries. But the condition of the walls of the vessels depends upon the nerves by which they are supplied ; and it so happens that cold so affects these nerves in such a manner as to give rise to contraction of the small vessels, while moderate warmth has the reverse effect.
Thus the supply of blood to the surface is lessened, and loss of heat is thereby checked, when the external temperature is low ; while, when the external temperature is high, the supply of blood to the surface is increased, the fluid exuded from the vessels pours out by the sweat-glands, and the evaporation of this fluid checks the rise in the temperature of the superficial blood.
Hence it is that, so long as the surface of the body perspires freely, and the air-passages are abundantly moist, man may remain with impunity, for a considerable time, in an oven in which meat is being cooked. The heat of the air is expended in converting this superabundant perspiration into vapour, and the temperature of the man's blood is hardly raised.