§ 1 Food, a Source of Energy and Matter
Definitions of Food.—Before the introduction into physiology of the notion of energy, no one had succeeded in giving an exact idea and a precise definition of food and alimentation. Every physiologist and medical man who attempted it had failed, and this for various reasons.
The general cause of this failure was that most definitions, popular or technical, interposed the condition that the food must be introduced into the digestive apparatus. “It is,” said they, “a substance which when introduced into the digestive tube undergoes, etc., etc.” But plants draw food from the soil, and they possess no digestive apparatus; many animals have no intestinal tube; and in the case of certain rotifera, the females possess a digestive apparatus, while the males have none. Nevertheless all animals feed.
On the other hand, there are other substances than those which use the digestive tract for the purpose of entering the organism, and which are eminently useful or necessary to the maintenance of life. In particular we may mention oxygen.
The distinctive feature of food is its utility—when conveniently introduced or employed—to the living being. Claude Bernard’s definition is this:—A substance taken in the external medium “necessary for the maintenance of the phenomena of the healthy organism and for the reparation of the losses it constantly suffers.” “A substance which supplies an element necessary for the constitution of the organism, or which diminishes its disintegration” (stored-up food); this is the definition of C. Voit, the German physiologist. M. Duclaux says, in his turn, but in far too general terms, that it is a substance which contributes to assure the sound functional activity of any of the organs of the living being. None of these ways of describing food gives a complete idea.
Food, the Source of Energy and Matter.—The intervention of the notion of energy enables us more completely to understand the true nature of food. We must, in fact, have recourse to the energetic conception if we desire to take into account all that the organism requires from food. It not only requires matter, but also, and most important of all, energy.
Investigators so far concentrated their thoughts exclusively on the necessity of a supply of matter—that is to say, they only looked upon one side of the problem. The living body presents, at each of its points, an uninterrupted series of disintegrations and reconstitutions, the materials being supplied from without by alimentation, and rejected by excretion. Cuvier gave to this unceasing circulation of ambient matter throughout the vital world the name of vital vortex, and he rightly saw in it the characteristic of nutrition, and the distinctive feature of life.
This idea of the cycle of matter has been completed in our own time by that of the cycle of energy. All the phenomena of the universe, and therefore those of life, are conceived of as energetic transformations. We now look at them in their relationship instead of considering them individually as of old. Each has an antecedent and a consequent unity with which it is connected in magnitude by the law of equivalents taught us by contemporary physics. And thus we may conceive of their succession as the cycle of a kind of indestructible agent, which changes only apparently, or assumes another form as it passes from one to the other, but its magnitude remains unaltered. This is energy. Thus, in the living being there is not only a circulation of matter, but also a circulation of energy.
The most general result of research in physiological chemistry from the time of Lavoisier down to our own day has been to teach us that the antecedent of the vital phenomenon is always a chemical phenomenon. The vital energies are derived from the potential chemical energy accumulated in the immediate constituent principles of the organism. In the same way the consequent phenomenon of the vital phenomenon is in general a thermal phenomenon. The final form of vital energy is thermal energy. These three assertions as to the nature, the origin, and the final form of vital phenomena constitute the three fundamental principles, the three laws, of biological energetics.
Food, a Source of Heat. It is not quâ source of heat that food is the source of vital energy.—The place of vital energy in the cycle of universal energy is completely determined. It lies between the chemical energy which is its generating form and the thermal energy which is its form of disappearance, of breakdown, the “degraded form,” as the physicists say. Hence we have a result which can be immediately applied in the theory of food—namely, that heat is in the dynamical order an excretum of the animal life rejected by the living being, just as in the substantial order, urea, carbonic acid and water, are the materials used up and again rejected by it. We therefore must not think of the transformation in the animal organism of heat into vital energy, as certain physiologists always do. Nor must we think, with Béclard, of its transformation into muscular movement; or, as others have maintained, into animal electricity. This is not only an error of doctrine but an error of fact. It proceeds from a false interpretation of the principle of the mechanical equivalent of heat and a misunderstanding of Carnot’s principle. Thermal energy does not repeat the course of the energetic flux in the animal organism. The heat is not transformed into anything. It is simply dissipated.
The Part played by Animal Heat as a Condition of Physiological Manifestations.—Does this mean that heat is useless to life in the very beings in which it is most abundantly produced—i.e., in man and in the warm-blooded vertebrates? So far from this being so, it is necessary to life. But its utility has a peculiar character which must neither be misunderstood nor exaggerated. It is not transformed into chemical or vital reactions, but merely creates for them a favourable condition.
According to the first principle of energetics, for the vital fact to be derived from the thermal fact, the heat must be preliminarily transformed into chemical energy, since chemical energy is necessarily an antecedent and generating form of vital energy. Now this regressive transformation is impossible according to the current theories of general physics. The part played by heat in the act of chemical combination is that of a primer to the reaction. It consists in placing the reacting bodies, by changing their state or by modifying their temperature, in the condition in which they ought to be for the chemical forces to come into play. For example, in the combination of hydrogen and oxygen by setting light to an explosive mixture, heat only acts as a primer to the phenomenon, because the two gases which are passive at ordinary temperatures, require to be raised to 400° C. before chemical affinity comes into play. And so it is with the reactions which go on in the organism. They have a maximum temperature, and the part played by animal heat is to furnish them with it.
It follows that heat intervenes in animal life in two capacities—first and foremost as excretum, or end of the vital phenomenon, of physiological work; and on the other hand, as a condition or primer of the chemical reactions of the organism; and generally, as a favourable condition for the appearance of the physiological manifestations of living matter. Thus, it is not dissipated in sheer waste.
I was led to adopt these views some years ago from certain experiments on the rôle played in food by alcohol. I did not then know that they had already been expressed by one of the masters of contemporary physiology, M. A. Chauveau, and that they were related in his mind to a series of conceptions and of researches of great interest, in the development of which I have since then taken a share.
Two Forms of Energy supplied to Animals by Food.—To say that food is simultaneously a supply of energy and a supply of matter, is really to express in a single sentence the fundamental conception of biology, in virtue of which life brings into play no substratum or characteristic dynamism. According to this, the living being appears to us as the seat of an incessant circulation of matter and energy, starting from the external world and returning to it. All food is nothing but this matter and this energy. All its characteristics, our views as to its rôle, its evolution, all the rules of alimentation are simple consequences of this principle, interpreted by the light of energetics.
And first of all, let us ask what forms of energy are afforded by food? It is easy to see that there are two—food is essentially a source of chemical energy; and secondarily and accessorily, it is a source of heat. Chemical energy is the only energy, according to the second law of energetics, which may be transformed into vital energy. It is true at any rate for animals; for in plants it is otherwise. There the vital cycle has neither the same point of departure nor the same final position. The circulation of energy does not take place in the same manner.
On the other hand, and this we are taught by the third law, energy brought into play in vital phenomena is finally liberated and restored to the physical world in the form of heat. We have just said that this release of heat is employed in raising the temperature of the living being. It is animal heat.
Thus there are two forms of energy supplied by food, chemical and thermal.
It must be added that these are not the only forms, but the principal, and by far the most important. It is not absolutely true that heat is the only outcome of the vital cycle. It is only so in the subject in repose, contented to live idly without doing external mechanical work, without lifting a tool or a weight, even that of its own body. And again, speaking in this way, we neglect all the movements and all the mechanical work which is done without exercise of the volition, by the beating of the heart and of the arteries, the movements of respiration, and the contractions of the digestive tube.
Mechanical work is, in fact, another possible termination of the cycle of energy. But there is no longer anything necessary or inevitable in this, since motion and the use of force are in a certain measure subordinated to the capricious volition of the animal.11
At other times, again, it is an electrical phenomenon which terminates the vital cycle, and it is, in fact, in this way that things happen in the functional activity of the nerves and muscles in all animals, and in the functional activity of the electrical organ in fish, such as the ray and the torpedo. Finally, the termination may be a photic phenomenon, and this is what happens in phosphorescent animals.
It is idle to diminish the power of these principles by proceeding to enumerate the whole of the exceptions to their validity. We know perfectly well that there are no absolute principles in nature. Let us say, then, that the energy which temporarily animates the living being is furnished to it by the external world under the exclusive form of potential chemical energy; but that, if there is only one door of entry, there are two exits. It may return to the external world in the principal form of thermal energy and in the accessory form of mechanical energy.