CHAPTER IV.THE TWOFOLD CONDITIONING OF VITAL PHENOMENA. IRRITABILITY.
Appearance of internal activity of the living being—Vital phenomena regarded as a reaction of the ambient world.—§ 1. Extrinsic conditions—The optimum law.—§ 2. Intrinsic conditions—The structure of organs and apparatus—How experiment attacks the phenomena of life. Generalization of the law of inertia—Irritability.
Instability. Mutability. The Appearance of Internal Activity of the Living Being.—One of the most remarkable characteristics of the living being is its instability. It is in a state of continual change. The simplest of the elementary beings, the plastid, grows and goes on growing and becoming more complex, until it reaches a stage at which it divides, and thus rejuvenated it commences the upward march which leads it once again to the same segmentation. Its evolution is thus betrayed by its growth, by the variations of form which correspond to it, and by its division.
If it be a question of beings higher in organization than the cellular element the evolutionary character of this mutability becomes more obvious. The being is formed, it grows; then in most cases, after having passed through the stages of youth and adult age, it grows old, declines and dies, and isdisorganized after having gone through what we may call an ideal trajectory. This march in a fixed direction with its points of departure, its degrees, and its termination, is a repetition of the path that the ancestors of the living being have already followed.
Here, then, is a characteristic fact of vitality, or rather there are two facts. The one consists in this morphological and organic evolution, the negation of immutability, the negation of the indefinite maintenance of a permanent state or form which is regarded, on the contrary, as the condition of inert, fixed stable bodies, eternally at rest. The other consists in the repetition, realized by this evolution, of the similar evolution of its ancestors; this is a fact of heredity. Finally, evolution is always in a cycle—that is to say, that it comes to an end which brings the course of things to their point of departure.
This kind of internal activity of the living being is so striking, that not only does it serve us to differentiate the living being from the inert body, but it gives rise to the illusion of a kind of internal demon, vital force, manifested by the more or less apparent acts of the life of relation, of the motricity, of the displacement, or by the less obvious acts of vegetative life.
Vital Phenomena regarded as a Reaction of the Ambient World. Their Twofold Conditioning.—In reality, as the doctrine of energetics teaches us, the phenomena of vitality are not the effect of a purely internal activity. They are a reaction of the environment. “The idea of life,” says Auguste Comte, “constantly assumes the necessary correlation of two indispensable elements:—an appropriate organism and a suitable environment. It is from the reciprocalaction of these two elements that all vital phenomena inevitably result.” The environment furnishes the living being with three things:—its matter, its energy, and the exciting forces of its vitality. All vital manifestation results from the conflict of two factors: the extrinsic factor which provokes its appearance; the intrinsic factor, the very organization of the living body, which determines its form. Bichat and Cuvier saw in the phenomena of life the exclusive intervention of a principle of action entirely internal, checked rather than aided by the universal forces of nature. The exact opposite is true. The protozoan finds the stimuli of its vitality in the aquatic medium which is its habitat. The really living particles of the metazoan—that is to say, its cells, its anatomical elements—meet these stimuli in the lymph, in the interstitial liquids which bathe them and which form their real external environment.
Auguste Comte thoroughly understood this truth, and has clearly expressed it in the passage we have just quoted. Claude Bernard has fully developed it and given it its classical form.
In order to manifest the phenomena of vitality, the elementary being, the protoplasmic being, requires from the external world certain favourable conditions; these it finds there, and they may be called the stimuli, or extrinsic conditions of vitality. This being possesses no initiative or spontaneity in itself, it has only a faculty of entering into action when an external stimulus provokes it. This subjection of the living matter is calledirritability. The term expresses that life is not solely an internal attribute, but an internal principle of action.
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 ratheroxygen,heat, and a certainchemical constitutionof 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, themethod 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 ofthe vital medium were the conditions of thejuste 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 beoptimaproportions.
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 thepabulum vitæ. But the discovery of the beings called by Pasteuranaerobiaappears 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 thebacteria of tetanusand theseptic vibrion; or on those to which oxygen seems to be a poison, such as thebutyricand thebutylic 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 simplyaerobia. 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 sameway for the chemical composition of the vital medium—and for the other ambient physical conditions, such as atmospheric pressure.
It is therefore a law ofuniversalscope, 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.
Law of the Constitution of Organs and Apparatus.—If we consider more highly organized beings, theinfluence 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, thelaw of the construction of organismsor of thebringing to perfection of an organismis the same as the law of cellular life. It is otherwise suggestive as the law ofdivision of physiological labourformerly 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 ofencystment, the vibrio, the tardigrade, the cold-blooded animals, and perennial plants.
The ambient world therefore furnishes to the animal and to the vegetable, whole or fragmentary, those materials of its organization which are at the same time the stimuli of its vitality. That is to say, the vital mechanism would be a dormant and inert mechanism if nothing in the surrounding medium could provoke it to action or give it a check. It would be a kind of steam engine without coal and fire.
Living matter, in other words, does not possess real spontaneity. As I have shown elsewhere, the law of inertia which it is supposed it obeys with inert bodies is not special to them. It is applied to the living bodies whose apparent spontaneity is only an illusion contradicted by physiology as a whole. All the vital manifestations are responses to a stimulus of acts provoked, and not of spontaneous acts.
Generalization of the Law of Inertia in Living Bodies. Irritability.—In fact, vulgar prejudice opposesthis view. The opinion of the average man distrusts it. It applies the law of inertia only to inert matter. This is because the vital response does not always immediately succeed the external stimulus, and is not always proportional to it. But it is sufficient to have seen the flywheel of a steam engine to understand that the restitution of a mechanical force cannot be instantaneous. It is sufficient to have had a finger on the trigger of a firearm to know that there is no necessary proportion between the intensity of the stimulus and the magnitude of the force produced. Things happen in the living just as in the inert machine.
The faculty of entering into action when provoked by an external stimulus has received, as we have said, the name ofirritability. The word is not used of inert matter. However, the condition of the latter is the same. But there is no need to affirm its irritability, because no one denies it. We know perfectly well that brute matter is inert, that all the manifestations of activity of which it is the theatre are provoked. Inertia is for it the equivalent of irritability in living matter. But while it is not necessary to introduce this idea into the physical sciences, where it has reigned since the days of Galileo, it was, on the contrary, necessary to affirm it in biology, precisely because it was in biology that the opposing doctrine of vital spontaneity ruled supreme.
Such was the view held by Claude Bernard. He never varied on this point.Irritability, said he, is the property possessed “by every anatomical element (that is to say, the protoplasm which enters into its constitution) of being stimulated into activity and of reacting in a certain manner under the influence ofthe external stimuli.” He could not claim that this was a distinguishing characteristic between living bodies and brute bodies, and that all the less because he always tried to efface on this point the distinctions which were current in his time, and which were established by Bichat and Cuvier. And so also Le Dantec does not seem to have thoroughly grasped the ideas of the celebrated physiologist on this point when he asserts, as if he were thereby contradicting the opinion of Claude Bernard and his school, that irritability is not something peculiar to living bodies.[17]