The instability of the vital powers, is the quicksand on which have sunk the calculations of all the Physicians of the last hundred years. The habitual variations of the living fluids, dependent on this instability, one would think should be no less an obstacle to the analyzes of the chemical physicians of the present age.
From this reasoning it is easy to perceive, that the science of organized bodies should be treated in a very different manner from that of inorganic bodies. To the former a different language almost is requisite; for the greater number of the words, which we transfer from the physical sciences, into those of the animal or vegetable economy, incessantly recall ideas, which are by no means consistent with their phenomena.
Had physiology been cultivated by men before physics, I am persuaded that many applications of the former would have been made to the latter; rivers would have been seen to flow from the tonic action of their banks, crystals to unite from the excitement, which they exercise upon their reciprocal sensibilities, and planets to move because they mutually irritate each other at vast distances. All this would appear unreasonable to us, who think of gravitation only in the consideration of these phenomena; and why should we not in fact be as ridiculous when we come with this same gravitation, with our affinities and chemical compositions, and with a language established upon their fundamental data to treat of a science, with which they have nothing whatsoever to do. Physiology would have made a much greater progress, if all those who studied it, had set aside the notions which are borrowed from the accessary sciences, as they are termed. But these sciences are not accessary; they are wholly strangers to physiology, and should be banished from it wholly.[18]
Physics and chemistry are related to each other in many points, because the same laws in a variety of instances preside over the phenomena of both of them; but an immense interval divides them from the science of organic bodies; because a very great difference exists between the laws which are proper to them, and those of life. To say that physiology is made up of the physics of animals, is to give a very inaccurate idea of it; as well might we say that astronomy is the physiology of the stars.
But the present digression has already been much too long. We shall now consider the vital powers with respect to the two lives of the animal.
II. Difference between the vital properties and those of texture.
In examining the properties of every living organ, we may distinguish them into two kinds. Those of one kind are dependent immediately upon life, begin and finish with it, or rather form its principle and its essence. Those of the other are connected with it only indirectly, and appear rather to depend upon the organization and texture of the parts of the body.
The faculties of perceiving and spontaneously contracting are vital properties: extensibility, and the faculty of contraction upon the cessation of the extending power, are properties of texture; the latter it is true, are possessed of a greater energy when existing in the living fibre, but they remain with the organ when life has ceased; the decomposition of the organs, is the term of their existence. I shall first examine the vital properties.