RESEARCHES, &c.
PART I.
ARTICLE I.
GENERAL DIVISION OF LIFE.
The definition of life is to be sought for in abstract considerations; it will be found, I believe, in this general perception: life is the totality of those functions which resist death.
Such is in fact the mode of existence of living bodies, that every thing which surrounds them tends to their destruction. Inorganic bodies act upon them incessantly; they themselves exercise a continual action, the one upon the other; and would necessarily soon be destroyed, did they not possess a permanent principle of reaction. This principle is life; not understood in its nature, it can be known only by its phenomena: the most general of which is that constant alternation of action on the part of external bodies, and of reaction on the part of the living body, the proportions of which alternation vary according to age.
There is a superabundance of life in the infant, because the reaction is greater than the action. In the adult, an equilibrium is established, and thus this vital turgescence disappears. The reaction of the internal principle is diminished in old age, while the action of external bodies remains the same; thus life languishes and advances insensibly towards that natural term, which must happen when all proportion has ceased.
The measure of life then, in general, is the difference which exists between the effort of external powers, and of internal resistance. The excess of the former announces its weakness; the predominance of the latter is an indication of its strength.
SECTION I.
Division of Life into Animal and Organic.
Such is life considered in the whole; examined more minutely, it offers to our view two remarkable modifications. The one is common to vegetable and animal, the other is peculiar to the last. Let us cast our eyes, for example, on an individual of each of these living kingdoms: we shall see that the one exists only within itself, having no other relation to what surrounds it, than as it respects nutrition, that it springs up, flourishes, and dies, in the spot which received its germe; that the other has, in addition to this internal life which it enjoys in a higher degree, an external life which establishes numerous relations between it and surrounding objects, that its existence is entwined with that of every other being, that it avoids or approaches these according to its fears or wants, and thus appears to appropriate every thing in nature to its exclusive use.
It may be said that the vegetable is the rough sketch, the canvas of the animal; and that to form this last, nothing more is necessary than to display upon this canvas the external organs proper to establish its different relations.
It hence results that the functions of the animal form two very distinct classes. The one is composed of a continual succession of assimilation and excretion; by these it is incessantly converting to its own substance the particles of surrounding bodies, and again ejecting these particles when they have become heterogeneous. It lives within itself only, by this class of functions; by the other, it exists, as it were, out of itself: it is the inhabitant of the world, and not, like the vegetable, of the spot which gave it birth. It feels and perceives what surrounds it, reflects its sensations, moves voluntarily according to their influence, and most generally has the power of communicating by voice, its desires and its fears, its pleasures or its pains.
The assemblage of the functions of the first class, I call organic life, because all organized beings, vegetable or animal, enjoy it in a greater or less degree, and because organic texture is the only condition necessary to its exercise. The united functions of the second class form animal life, thus called, because it is the exclusive attribute of the animal kingdom.
Generation does not enter into the series of phenomena of these two lives, they having relation to the individual, while that regards the species only: nor is it connected, except indirectly, to most of the other functions. It begins its exercise only after the others have been long in action, and is extinct long before they cease to act. In the greater part of animals its periods of activity are separated by long intervals of rest; in man, in whom its remissions are less durable, its relations to the functions are not more numerous. The subtraction of those organs which are its agents, is almost always followed by a general increase of nutrition. The eunuch possesses less vital energy; but the phenomena of life in him, are developed in greater plenitude. Let us, however, leave the consideration of those laws which give us existence, and take up those only which maintain it: we shall recur to the former hereafter.
SECTION II.
Subdivision of each of the lives, Animal and Organic, into two orders of functions.
Each of the two lives, animal and organic, is made up of two orders of functions, which succeed, and are connected to, each other in an inverse ratio.
In animal life the first order is established from the exterior of the body towards the brain, and the second, from this organ towards those of locomotion and voice. The impression of objects is made successively upon the senses, the nerves, and the brain. The first receive, the second transmit, and the last perceives the impression, which being thus received, transmitted, and perceived, constitutes our sensations.
The animal is almost passive in the first order of functions; he becomes active in the second, which results,—from the successive actions of the brain—whence springs volition, the consequence of sensations, from the nerves which transmit this volition, and from the organs of locomotion and voice—the agents of its execution. External bodies act upon the animal by the first order of functions; it reacts upon them by the second.
There exists, in general, a rigorous proportion between these two orders; where the one is plainly marked, the other also is strongly developed. In a series of animals, it will be found, that the one which possesses the greatest share of sensibility, enjoys also the greatest variety of motions. The age of lively sensations, is that also of vivacity of motion; in sleep, when the first order is suspended, the second ceases, or is only irregularly exercised. Those blind of one eye, who only half see what surrounds them, connect their different motions with a tardiness and caution which would soon be lost, if their external communications were increased.
A double action is exercised also in organic life; the one is incessantly composing, while the other decomposes the animal. Such, in fact, (as the ancients and after them some moderns have observed) is his mode of existence, that what he was at one period he ceases to be at another; his organization remains the same, but his elements are continually changing. The nutritive particles alternately absorbed and ejected, pass from the animal to the plant, thence to the brute, and so on again to the animal.
Organic life is well suited to this continual circulation of matter. One order of functions assimilates to the animal, those substances which are to nourish him; another carries off, what has become heterogeneous to, after having for some time made a part of, his organisation.
The first, which is the order of assimilation, results from digestion, circulation, respiration, and nutrition. Every particle foreign to the body, before it can become one of its elements, receives the influence of these four functions.
When it has thus furnished its portion to the formation of our organs, absorption takes it up and conveys it into the circulatory stream, whence it is thrown out by pulmonary or cutaneous exhalation, and the various secretions by which all the fluids are ejected.
Absorption, circulation, exhalation, and secretion form then, the second order of functions of organic life, or the order of dis-assimilation.
Hence it follows that the sanguineous is the mean system, the centre of organic life, as the brain is of animal life, where those particles to be assimilated, and those, which having already undergone assimilation, are destined to be thrown out, confusedly circulate; so that the blood is composed of two parts, the one recrementitious, produced by the aliments and the exhausted materials of nutrition, and the other excrementitious, which is the rubbage, the residuum of all our organs, and which supplies the secretions and external exhalations. These last functions, however, sometimes serve also to throw out certain digestive products, which have had no concurrence in nourishing the parts. This may be seen in the urine and sweat, after copious potations. The skin and kidneys are then excretory organs, not of nutrition but of digestion. This may be observed still more clearly in the production of milk, a fluid manifestly formed out of that portion of the blood, which has not been assimilated by the nutritive process.
There is not the same conformity between the two orders of functions of organic life, as between those of animal life; the enervation of the first does not necessarily bring on a diminution of the second: hence leanness, marasmus, &c. states of the body in which assimilation in part ceases, dis-assimilation is exercised uninterruptedly.
These grand differences between the two lives of the animal, and those not less strongly marked limits which separate the two orders of phenomena of which each is the assemblage, appear to me to offer to the physiologist, the only real division which can be established between the functions.
Artificial methods we shall leave to the other sciences; let us pursue the order of the phenomena, connect the ideas we form of them, and we shall then see, that the greater part of physiological divisions offer only uncertain foundations to him who would raise upon them the edifice of science.
I shall not take notice of those divisions here; the best means of pointing out their insufficiency will be to prove the validity of the system which I have adopted. Let us now go over in detail the grand differences which separate the animal living from without, from the animal existing within itself in an alternate round of assimilation and excretion.
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