WHY DO WE BREATHE? automatic process. 327 can be given. We know that the chest expands and contracts with beautiful rhythm, and, mostly, as an involuntary, We know that our attention is not required, that no effort is needed, and indeed that no effort of ours can prevent the regular alternation of inspiration and expiration. We can by an effort accelerate or retard these motions, but we cannot prevent them. The process, then, clearly depends on a stimulus given to the involuntary part of the nervous system: it is called into action by nervous stimulus, and physiologists have vainly endeavoured to discover the nervous apparatus which is involved, and the rationale of its action. The pressure of carbonic acid in the air-cells, or of venous blood in the capillaries, may act as a stimulus to the pneumogastric nerve; but what is the rationale of making a new-born child draw breath, by whipping its back and continuation? Generally, the stimulus of the cold air on the child's face suffices to make it draw breath, which it expires again in a well-known cry, to mothers' ears most musical; but this stimulus is often insufficient, and the doctor or nurse initiates the little stranger into that experience of "external local applications which, in later years, will also be freely used as a stimulus to virtue or learning. The fact we know; but why such "local applications" excite the respiratory activity, we do not know, for we do not know the nervous apparatus which regulates the actions of respiration. It is probable that the researches of physiologists will, ere long, clear up this point, as they have cleared up so many others; meanwhile we must content ourselves with vague answers to our ques. tion, Why do we breathe? CHAPTER VII. WHY WE ARE WARM, AND HOW WE KEEP SO. Animals have a temperature of their own-This not owing to a "Vital Principle" -Description of the various instruments employed in measuring animal heatMistake of supposing that there are cold-blooded and warm-blooded animalsMan preserves his temperature in all climates and all seasons-His power of enduring great heat-The cooling apparatus of his organism-Perspiration carries off superfluous heat-Difference between feeling hot and being hot-Influence of age, sex, and food on our temperature--Young animals cannot well resist cold-The notion of "hardening" infants erroneous-Is food warmth ?— Influence of the seasons-Effect of a cold wind-Why is the east wind felt to be cold? Where is cold dangerous?-Can cold water be drunk with safety when we are hot?-The theory of animal heat-Respiration is not the cause of animal heat-Heat in dead bodies-Relation between respiration and animal heat in various classes of animals-Hybernating animals-Frogs and tritons in winter and summer. A BIRD-CAGE hangs above a small aquarium: in the cage there is a bird; in the glass tank, seaweeds, zoophytes, molluscs, and fish. The atmosphere of the apartment varies with those variations of temperature which accompany the earth's daily rotation and annual movement. The summer sunlight streams in through the windows; the icy north wind rushes through the crevices; the shadows of night, and the evaporations of morning, bring with them perpetual risings and fallings of the temperature of that room; and with these risings and fallings there are corresponding fluctuations in the temperature of the glass and water of the tank, the brass and woodwork of the cage. This is according to the law by which an equilibrium of temperature is always estab DOES LIFE SUSPEND PHYSICAL LAWS? 329 lished among inorganic bodies. The warmer atmosphere rapidly warms the glass and water-the cooler atmosphere rapidly cools them; it is true that the water will always be somewhat colder than the atmosphere, because it loses heat in evaporation, but nevertheless, as the external temperature rises and falls, that of the water also rises and falls. While these changes, so familiar and so easy of expla nation, have been taking place, the bird has been neither colder nor warmer; throughout the fluctuations of external temperature it has preserved almost uniformly the very high degree of warmth which, as a bird, belongs to it. Neither the beams of an August sun, nor the nipping east wind of December, have raised or lowered its normal heat at any time more than one or two degrees. You may perhaps imagine that it has been kept warm through the winter by its envelope of feathers, but this is true only to a very slight extent; strip it of its feathers, and you will still find its heat greatly above that of the air; whereas, if a heated substance be enveloped in feathers, and left exposed to the air, it will soon become as cold as the air. Driven from this explanation, you will ask, How is it that the bird is enabled to preserve a steady temperature of a high degree amid unsteady influences from without? The answer is obviously to be sought in the organism and its processes, not in any external influence; and a certain Philosophy, somewhat rash and ready, fond of phrases and impatient of proof, will assure you that the bird, as an organised body, is absolved from the law of equilibrium which rules all inorganic bodies, because the bird is endowed with a "vital principle which suspends the action of physical laws." This explanation, which to many has seemed satisfactory, labours under two disadvantages-first, that it invokes the operation of a "vital principle," of which we can form no definite conception; and secondly, that the assumed suspen 66 sion of physical laws is a pure figment. The organism, living or dead, radiates heat with equal facility; but when living, it produces heat to compensate the loss; and when dead, it no longer produces heat, so that it speedily becomes as cold as the external air. The processes of Life do not suspend" the operation of physical laws, although, by the introduction of more complex conditions, they bring about results which, superficially considered, look like a suspension of those laws. A close analysis always detects the physical laws. No one thinks of attributing to a spiritlamp, when lighted under a vessel of water, the power of suspending the equilibrium of temperature, because it keeps the water boiling, in spite of the constant loss of heat by evaporation. Without the lamp, the boiling water would speedily cool below the temperature of the air; with the lamp, it may be kept indefinitely at the boiling-point, it fresh water be from time to time added to replace what has evaporated. There is no "lamp-principle" suspending physical laws. Nor is there any such mysterious agent in Animal Heat. Just as the temperature of the water is kept constant by the continual reproduction of heat equalling the amount lost, so is the temperature of the bird kept constant by a continual reproduction of heat within; and although the vital processes by which that reproduction is effected are very far from exhibiting the simplicity of the spirit-lamp, and are indeed still involved in great obscurity, yet we know that physical laws are in no sense suspended thereby, and that the living animal has the tendency to es tablish an equilibrium between its temperature and that of the objects surrounding it. We have only to extend our investigations and examine the temperature of the other organised bodies-seaweeds, zoophytes, molluscs, and fish-during these changes which seem not to have affected the bird, to find that this myste HOW TEMPERATURE IS MEASURED. 331 rious "vital principle" suddenly fails altogether. It here abdicates its autocratic power. It suspends no laws; on the contrary, it permits equilibrium to be established unopposed. The seaweeds are, within a very slight fraction, as cold as the water, and get warmer as the water warms. The zoophytes have no appreciable superiority of temperature. The fish are only two or three degrees warmer. Either we must give up the explanation which the vital principle seemed to afford, or we must deny that the coldblooded animals, as they are called, have any vital principle at all. In vain will a refuge be sought in the greater cooling agency of water over that of air; for although something must be allowed for this, we cannot by it account for the enormous disproportion between the temperature of the fish and the bird; and for these reasons: The Bonito, equally subject to this cooling agency of water, preserves a constant temperature of 20° above the sea; and the temperature of the Narwhal is nearly that of man-namely, 96° Fahrenheit. Moreover, while some marine animals are thus independent of the temperature of water, serpents, lizards, and frogs are dependent on the temperature of the air. Before entering further into this subject, it will be well to glance at the instruments by which physiologists measure the delicate differences in amount of heat which different animals and plants produce. Of the various thermometers we need say nothing. But there are thermo-electrical instruments which need explanation. Fig. 27 represents a thermo-electric couple. Its construction rests on the fact that two metals soldered together develop electric currents when their solders are maintained at different F C M Fig. 27. temperatures. Two bars of copper (c, c) are soldered to a bar |