Extrinsic Conditions.—By showing that every vital manifestation results from the conflict of two factors: the extrinsic or physico-chemical conditions which determine its appearance, and the intrinsic or organic conditions which regulate its form, Claude Bernard dealt a mortal blow at the old vitalist theories. For he has not only asserted the close dependence of the two kinds of factors, but he has shown them in action in most physiological phenomena. The study of the extrinsic or physico-chemical conditions necessary to vital manifestations teaches us our first truth—namely, that they are not infinitely varied as might be supposed. They present, on the contrary, a remarkable uniformity in their essential qualities. The fundamental conditions are the same for the animal or vegetable cells of every species. They are four in number:—moisture, the air, or rather oxygen, heat, and a certain chemical constitution of the medium, and the last condition, the enunciation of which seems vague, becomes more precise if we look at it a little closer. The chemical constitution of media favourable to life, the media of culture, obeys three general laws. It is the knowledge of these laws which formerly enabled Pasteur, Raulin, Cohn, and Balbiani to provide the media appropriate to the existence of certain relatively simple organisms, and thus to create an infinitely valuable method for the study of nutrition, etc.,—namely, the method of artificial cultures, numerous developments of which have been shown us by microbiology and physiology.
The Optimum Law.—It has been said, and it is more than a play on words, that the conditions of the vital medium were the conditions of the juste milieu. Water is wanted, there must not be too much or too little. Oxygen is necessary, and also in certain proportions. Heat is required, and for that, too, there is an optimum degree. Certain chemical compounds are needed and, in this respect too, there must also be optima proportions.
Water is a constituent element of the organisms. They contain fixed proportions for the same tissue, proportions varying from one tissue to another (between 2∕3 and 9∕10). The cell of a living tissue requires around it an aqueous atmosphere, formed by the different juices of the organism, the interstitial liquids, the blood, and the lymph. We are deceived by appearances when we distinguish between aerial, aquatic, and land-dwelling animals, and when we speak of the air, the water, and the land as their natural environment. If we go to the bottom of things, and fix our attention on the real living unities, on the cells of which the organism is composed, we shall find around them the juices, rich in water, which are their real environment. If these juices are diluted or concentrated the least in the world, life stops. The cell, the whole animal, falls into a state of latent life, or dies. “All living beings are aquatic,” said Claude Bernard. “Beings that live in the air are in reality wandering aquariums,” said another physiologist. “No moisture, no life,” wrote Preyer. The environment must contain water, but it must contain it in certain proportions. In the higher animals there is a mechanism which works automatically to keep at a constant level the quantity of water in the blood. Researches on the lavage of the blood (A. Dastre and Loye) have clearly shown this.
Oxygen is also necessary to life. It is the pabulum vitæ. But the discovery of the beings called by Pasteur anaerobia appears to contradict this statement. Pfeffer, the illustrious botanist, was certain, in 1897, that the dogma of the necessity of oxygen no longer held good. This is no longer tenable. In 1898 Beijerinck carried out most careful researches on anaerobia said to have been cultivated in a vacuum, such as the bacteria of tetanus and the septic vibrion; or on those to which oxygen seems to be a poison, such as the butyric and the butylic ferments, the anaerobia of putrefaction, the reducing spirilla of the sulphates. All use free oxygen. They consume very little it is true; they are micro-aerobia. The other organisms, on the contrary, need more. They are macro-aerobia or simply aerobia. Besides, if the so-called anaerobia take little or no free oxygen, it matters little. They take the oxygen in combination. It may be said with L. Errera that they have an affinity for oxygen, for they extract it from its combinations, and that “they are so well adapted to this mode of existence that life in the open air being too easy no longer suits them.” There are for the different animal species different optima of oxygen.
Living beings require a certain amount of heat. Life, which could not have existed on the globe when it was incandescent, will not be able to exist when it is frozen. For each organism and each function there is a maximum and a minimum of temperature compatible with activity. There is also an optimum. For instance, the optimum is 29° C for the germination of corn.
The condition of the optimum exists in the same way for the chemical composition of the vital medium—and for the other ambient physical conditions, such as atmospheric pressure.
It is therefore a law of universal scope, a regulating law, as it were, of life. Life is a function of extrinsic variables, water, air, heat, the chemical composition of the medium, and pressure. “Every vital phenomenon begins to be produced, starting from a certain stage of the variable (minimum), becomes more and more vigorous as it increases up to a determinate value (optimum), weakens if the variable continues to increase, and disappears when it has reached a certain limiting value (maximum).” This law, proved by Sachs, the German botanist, in 1860, apropos of the action of temperature on the germination of plants, by Paul Bert in 1875, apropos of the action of oxygen and of atmospheric pressure on animals, and already formulated at that time by Claude Bernard, was illustrated by Leo Errera in 1895. It is a law of moderation. It expresses La Fontaine’s “rien de trop” Terence’s “ne quid nimis,” the μηδὲν ἄγαν of Theognis, and the biblical phrase “omnia in mensura et numero et pondere.” L. Errera sees the profound cause of this optimum law in the properties of the living protoplasm, which are mean properties. It is semi-liquid. It is composed of albuminoid substances, which can stand no extremes either from the physical or from the chemical points of view.
§ 2. Intrinsic Conditions. The Law of the Constitution of Organs and Apparatus.
Law of the Constitution of Organs and Apparatus.—If we consider more highly organized beings, the influence of the intrinsic conditions appears quite as clearly. As we have seen, this is so that the requisite fundamental materials may be spent by each element in suitable proportions,—water, chemical compounds, air, and heat,—that organs may be added to organs, and that apparatus may be set to work in complex structures. Why a digestive apparatus? To prepare and introduce into the internal medium liquid materials which are necessary to life. Why a respiratory apparatus? To impart the vital gas necessary to the cells, and to expel the gaseous excrement, the carbonic acid which they reject. Why a circulatory apparatus? To transport and renew this medium throughout. The apparatus, the functional wheels, the vessels, the digestive and respiratory mechanisms do not exist for themselves, like the random sketches of an artistic nature. They exist for the innumerable anatomical elements which people the economy. They are arranged to assist and more rigorously to regulate cellular life with respect to the extrinsic conditions which it demands. They are, in the living body, as in civilized society, the manufactories and the workshops which provide for the different members of society dress, warmth, and food. In a word, the law of the construction of organisms or of the bringing to perfection of an organism is the same as the law of cellular life. It is otherwise suggestive as the law of division of physiological labour formerly enunciated by Henry Milne-Edwards; and in every case it has a more concrete significance. Finally, it brings the organic functional activity into relation with the conditions of the ambient medium.
How Experiment acts on the Phenomena of Life.— The two orders of conditions, the one provided by the being itself, the other by external agents, are equally indispensable—and therefore of equal importance or dignity. But they are not equally accessible to the experimentalist. It is not easy to exercise on the organization direct and measurable actions. On the contrary, the physical conditions are in the hands and at the discretion of the experimenter. By them he may reach the vital manifestations as they appear, stimulate or check them, defer or precipitate them. Thus, for instance, the physiologist suspends or re-establishes at his will full vital activity in a multitude of reviviscent or hibernating beings, such as grains, the infusoria capable of encystment, the vibrio, the tardigrade, the cold-blooded animals, and perennial plants.