Almost endless disputes have arisen among physiologists concerning the essen tial nature of this vital principle. Vitality is one of those attributes which can be more easily discerned and recognized when present in any object, than accu rately defined. Definition indeed would be more likely to confuse than to illustrate it. It is manifested most incontestibly by certain effects, referrible to peculiar powers, which are justly called living or vital, because the actions of the living body are so far depending on these pow ers, that they can, by no means, be ex plained by the physical, mechanical, or chemical qualities of matter. Yet the operation of the latter can be clearly dis cerned in many instances in the animal economy; thus the humours of the eye variously affect the rays of light according to their figure and density : and the me chanical elasticity of the epiglottis, and the chemical affinities exercised in respi ration, are further examples to the same effect. Yet the energy and power of the vital force is most clearly evinced in resisting and overcoming, as we have already stated, the common laws of mat ter. Stahl and his followers, were so struck with the circumstance of living bodies resisting those affinities, which produce putrefaction in dead animal mat ter, that they made life itself to consist in this antiseptic property. The celebrated experiment of Borelli, in which a muscle, deprived of life, was immediately torn by a weight which it could lift easily du ring life, shows how the laws of gravity are overcome. This vital power, in the explanation and illustration of which all physiology is concerned, is so apparent in every living process, that it has been observed by the physiologists of every age, although designated by very various appellations, and defined in very different ways. Calidum innatum, archmus, spiritus vitalis, principium sentiens, &c. are among its numerous appellations. Let it be remembered, that neither these, nor the phrases of vital principle, &c. express any being existing by itself, and Mde pendently of the actions by which it is manifested ; they are only to be consider ed as denoting the assemblage of powers that animate living bodies, and distinguish them from inert matter. Some writers, realizing the offspring of a mere abstrac tion, have talked of the living principle as something distinct from the body, to which they have ascribed powers of see ing and feeling, and even of acting with design.
A more close inspection of any living body will speedily convince us, that this force, which holds together its compo nent parts, in spite of the external pow ers, which tend to separate them, does not confine its influence to this passive result, but that its operation extends even beyond the limits of the living body itself. It seems, at least, that this power does not differ from that by which new parti cles are attracted, and interposed b etwe en the old constituent ingredients of the body; the latter effect seeming to be ex erted as constantly as the power by which the materials of the body are held to gether. For, besides the absorption of alimentary matter from the intestines, and its entrance into the circulating fluid, carrying it to all parts, which processes experience no interruption, but are con tinued from one meal to another, there • is another kind of absorption constantly going on from the surface of the body, and a third which takes place by means of respiration. The two latter alone exist in such living bodies as have not the func tion of digestion ; viz. in plants. Now, since living bodies do not grow indefi nitely, but have certain limits assigned by nature to their size ; they must lose on one side at least, a large part of what they receive on the other ; and, in fact, attentive observation shows us, that per spiration, and several other processes, are constantly destroying parts of their substance.
Hence the idea which we formed at first of the principal phenomenon of life, must be considerably modified. Instead of a constant union of the composing par ticles, we observe them in a state of con tinual circulation from without inwards, and from within outwards. Thus, a living body is a structure into which dead par ticles are successively brought, for the purpose of combining together in various ways, occupying places and exercising offices determined by the nature of the combinations into which they enter, and departing, after a certain period, to he brought under the action of those laws which regulate inanimate matter. We must observe, however, that the propor tion of particles, entering into, or quitting the system, varies according to the age and health of the individual, and that the velocity of the general motion differs ac cording to the different states of each liv ing body.
It appears, too, that life is arrested by causes similar to those which interrupt other known kinds of motion ; and that the induration of fibres, and obstruction of vessels, would render death an inevita ble consequence of life, as rest necessa rily follows all movements which are not performed in vacuo, even if the hour of its approach were not hastened by-a mul titude of extraneous causes.
This general and common motion of all parts constitutes the very essence of life, insomuch that parts separated from a living body immediately die, because they have no power of motion within them. 'selves, and only participate in the general motion produced by the assemblage. Thus, the peculiar mode of existence of any part of a living body arises from the whole ; while, in dead matter, each par ticle has it within itself.
When this nature of life was once clearly recognised by the most constant of its effects, physiologists naturally at tempted to discover its origin, and the mode of its communication to bodies which it animates. They looked at them in their earliest state, approaching as nearly as possible to the instant of their formation ; but they could only discover them completely formed, and already possessing that circulatory motion, of which they were investigating the first cause. However delicate the parts of a foetus, or a vegetable, in the first moments that we can perceive them, they still pos sess life, and have within themselves the germ of all the phenomena which this life will develope in the sequel. These ob servations having been repeated in every class of living bodies, have led to the general conclusion, that there is none which has not formerly constituted part of a body like itself, from which it has been detached; all have participated of the life of another body, bethre the vital motions were carried on independently in themselves ; and it is, indeed, through the means of the vital powers, inherent in the bodies of which they formed part, that they have been so far developed as to become susceptible of an isolated life. For although copulation is necessary in the act of reproduction in several species, it is by no means an essential circum stance, and does not, therefore, change the nature of generation. In reality, then, the peculiar powers of living bodies have their origin in those of the parents; this is the source of the vital impulse, and, consequently, it follows, that life is only produced from life, and that no other ex ists, except what has been transmitted from living bodies to living bodies, in an uninterrupted succession.