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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 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, &c., 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 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 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 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, a 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.

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FIG. 39.-A DIAGRAM TO ILLUSTRATE THE STRUCTURE OF GLANDS. A. Typical structure of the mucous membrane. a, an upper, and b, a lower, layer of epithelium cells; c, the dermis with e, a blood-vessel, and ƒ, connective tissue corpuscles.

32. The chief intermittently active sources of loss to the blood are found among the glands proper, all of which are, in principle, narrow pouches of the mucous membranes, or of the integument of the body, lined by a continuation of the epithelium, or of the epidermis. In the glands of Lieberkühn, which exist in immense numbers in the walls of the small intestines, each gland is nothing more than a simple blind sac of the mucous membrane, shaped like a small test tube, with its closed end outwards, and its open end on the inner surface of the intestine (Fig. 39, 1). The sweat-glands of the skin, as we have already seen, are equally simple, blind, tube-like involutions of the integument, the ends of which become coiled up. The sebaceous glands, usually connected with the hair sacs, are shorter, and their blind ends are somewhat subdivided, so that the gland is divided into a narrow neck and a more dilated and sacculated end (Fig. 39, 5). The neck by which the gland communicates with the free surface is called its duct. More complicated glands are produced by the elongation of the duct into a long tube, and the division and subdivision of the blind end into multitudes of similar tubes, each of which ends in a dilatation (Fig. 39, 6). These dilatations, attached to their branched ducts, somewhat resemble a bunch of grapes. Glands of this kind are called racemose. The salivary glands and the pancreas are such glands.

Now, many of these glands, such as the salivary, and the pancreas (with the perspiratory, or sudoriparous glands, which it has been convenient to consider already), are only active when certain impressions on the nervous system give rise to a particular condition of the gland, or of its vessels, or of both.

Thus the sight or smell, or even the thought of food, will cause a flow of saliva into the mouth; the previously

B. The same, with only one layer of cells, a and b, the so-called basement membrane between the epithelium, a, and dermis, c.

1. A simple tubular gland.

2. A tubular gland bifid at its base. In this and succeeding figures the blood-vessels are omitted.

3. A simple saccular gland.

4. A divided saccular gland, with a duct, d.

5. A similar gland still more divided.

6. A racemose gland, part only being drawn.

quiescent gland suddenly pouring out its fluid secretion, as a result of a change in the condition of the nervous system. And, in animals, the salivary glands can be made to secrete abundantly, by irritating a nerve which supplies the gland and its vessels. How far this effect is the result of the mechanical influence of the nerve on the state of the circulation, by widening the small arteries (see p. 51) and so supplying the gland with more blood, and how far it is the result of a more direct influence of the nerve pon the state of the tissue of the gland itself, making the cells secrete, just as a nerve when stimulated makes a muscle contract, is not at present finally determined.

The liquids poured out by the intermittent glands are always very poor in solid constituents, and consist chiefly of water. Those poured on to the surface of the body are lost, but those which are received by the alimentary canal are doubtless in a great measure re-absorbed.

33. The great intermittent sources of gain of waste products to the blood are the muscles, every contraction of which is accompanied by a pouring of certain products into the blood. That much of this waste is carbonic acid is certain from the facts (a) that the blood which leaves a contracting muscle is always highly venous, far more so than that which leaves a quiescent muscle; (b) that mus cular exertion at once immensely increases the quantity of carbonic acid expired; but whether the amount of nitrogenous waste is increased under these circumstances, or not, is a point yet under discussion.

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