§ 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.
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 opposes this 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 of irritability. 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 of the 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