The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

these actual movements, there is another way in which they lose energy; their bodies are warm. A hot piece of metal or stone will, in a short time, cool down to the temperature of the surrounding air, but a living body is always warm to the touch. A mouse or a man, like any warm object, loses heat continually to the cooler air which surrounds him, but he has internal sources of energy which compensate for his loss, so that except in extreme tropical heat his temperature is always higher than that of his surroundings.

The need of man and mouse for food is apparent in the greater part of their waking activities. On that we have already insisted. Less conspicuous, but in reality even more urgent, is the need for air. Normally, it is unnecessary toseek that. Always almost unconsciously, a mammal is inhaling and exhaling air. It hardly realizes the necessity of that until, under some exceptional conditions, the supply is cut off and suftfocation begins. But you can put a man out, just as you puta fire out, by stopping the airsupply. If we take the air a man breathes out and analyse it, we find that he has used some of the oxygen gas that normally air contains, and we find substances, water vapour and carbon dioxide, which are precisely what we would expect to be exhaled if the oxygen were used for disrupting the food in order to yield energy. A petrol motor, or a fire, does exactly the same; in both cases, besides fuel, there must be a supply of air, and in both cases oxygen is used up and carbon dioxide and water vapour are given off.

Finally we find various matters passing out of amammalian body, matters other than the mere undigested residue of his food. If our analogy with a petrol motor is sound, we may compare these matters to the more complex ingredients of exhaust gasesboth to the more complex products that result from the burning of the fuel, and to the fine particles of steel that wear away during the working of the engine itself. And, indeed, this is what our excreted substances represent.

Now is this analysis sound? Is a living man fundamentally a machine? ‘That is a question capable of experimental decision. We can measure the amount of food that a man or an animal consumes over a given period of time, and we can measure the energy yielded during the same period. If we burn an equal weight of similar food in a suitable apparatus and find out how much energy its combustion yields, and if this

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THE SCIENCE OF LIFE

CHAPTER 1

value is equal to the energy yielded by the experimental subject, then evidently the living organism so far as its energy-output is concerned is really and precisely a combustion engine.

Such experiments have been performed. A man has been shut up for a time in a small compartment, so constructed that the energy he gave out, as heat or otherwise, could be accurately measured. He was fed on a weighed amount of food of known composition and the energy actually yielded was compared with that which would have been obtained had the food been simply burnt. Such experiments have been made repeatedly both with man and animals. The following figures were obtained by Atwater, who worked with a human subject. A man was shut in a calorimeter (heat-energy measurer) for nineteen periods of twentyfour hours each: on the average he gave out 2,682 large calories in each period, and ate an amount of food which would yield if burnt 2,688 calories. (A large calorie —there are several sorts of calorie—is the amount of heat required to raise one litre of water from o° to 1° centigrade.) The agreement is good. The two sums differ by less than one fourth of one per cent., which is well within the margin of error of the methods employed. Numerous other experiments have yielded equally convincing results. We are, therefore, justified in concluding that man and the animals generally are fundamentally mechanisms, driven by the energy liberated in the oxidation of food. A mouse or a man works in much the same way as a petrol motor; its fuel is its food, and precisely after the fashion of a petrol motor, if it lacks either fuel or air it will cease to move, and slowly become cold. Moreover, the living engine needs an exhaust; as exhaled carbon dioxide or in other ways it must get rid of the products of this combustion or they will clog its system and impede its working. Indeed, the whole of

what we have called the fundamental round of Mr. Everyman is a mechanical process.

Naturally, we do not suggest that this demonstration of our fundamental mechanical nature explains everything ; as we have already pointed out, the living body has certain obvious properties that distinguish it from any man-made machine. It grows and reproduces its kind, and it is conscious. In later parts of this work we shall discuss the intensely interesting question how far it has been found possible to bring these higher and more intricate phenomena underlying