rivation of the term, a discourse on natural bodies: but it formerly denoted only an internal reasoning, which terminates in speculation, or abstract contemplation of its object; namely, natural appearances, their causes, &c but the usual acceptation of the word is very different in the present state of science, as we shall see by the following article.

PHYSIOLOGY is the science which treats of the powers that actuate the component parts of living animal bodies, and of the functions which those bodies execute. It presupposes, therefore, a knowledge of the structure of the body, which is the object of anatomy; the latter may be called the science of organization, while physiology is the science of life. The two subjects are so closely connected, that they would be most advantageously considered in connection with each other.

Hence the reader will find many physiological considerations under the articles ANATOMY and COMPARATIVE ANATOMY, which indeed he should peruse as an introduction to the present article..

The term life denotes one of those general and obscure notions produced in our minds by certain series of phenomena, which we have observed to succeed each other in a constant order, and to be connected together by mutual relations. Being ignorant of the bond of union which connects these, although we are convinced of its existence, we have designated the assemblage of phenomena by a name which is often regarded as the sign of a peculiar principle; although it should indicate nothing more than the collection of appearances, which have given rise to its formation. Thus, as our own bodies, and several others, which resemble them more or less strongly in form and structure, appear to resist for a certain time the laws which govern inanimate matter, and even to act on surrounding objects in a manner quite contrary to these laws, we employ the expressions of life and vital power to designate these at least apparent exceptions to general rules. Our only method of fixing the meaning of these words is, to determine exactly in what these exceptions consist. With this object, let us consider the bodies alluded to in their active and passive relations to the rest of nature. Let us contemplate, for instance, the body of a female in the vigour of youth and beauty: those rounded and

voluptuous forms; those graceful and easy motions; those cheeks glowing with the roses of pleasure; those eyes sparkling with the inspirations of genius, or fired by the warmth of love; that physiognomy enlivened by the sallies of wit, or animated by the fire of the passions; all unite to forni a truly enchanting object. A single moment is sufficient to destroy this pleasing illusion: sensation and motion often cease on a sudden, without any, apparent pre-existing cause; the muscles, losing their plumpness, shrink, and expose the angular projections of the bones; the lustre of the eyes is gone, the cheeks and lips grow livid. These are only the prelude to still more unpleasant changes: the flesh turns successively to blue, green, and black; it imbibes the moisture of the atmosphere, and, while one part is evaporated in pestilential emanations, the other melts into a putrid sanies, which also is speedily dissipated. In short, after a few days, nothing remains but a few earthy or saline principles; the other elements having been dispersed in the air or waters, to form new combinations.

This separation is the natural effect of the action of air, moisture, and heat, that is, of all the surrounding external agents, on the dead body; and it arises from the elective attractions which these agents possess for the materials of the body. Yet it was equally surrounded by them during life: their affinities for its component parts were the same: and the latter would have yielded in the same manner, if they had not been held together by a superior power, the influence of which continues to operate until the

moment of death.

This resistance, then, of the laws which act on dead matter, is one of the particu lar ideas entering into the general notion of life, which seems, in a more especial manner, to constitute its essence; for without it life cannot be conceived to exist, and it continues uninterruptedly

until the moment of death.

Almost endless disputes have arisen among physiologists concerning the essential nature of this vital principle. Vitality is one of those attributes, which can be more easily discerned and recognised when present in any object, than accurately defined. Definition indeed would be more likely to confuse than to illustrate it. It is manifested most incontestibly by certain effects, referable 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 explained by the physical, mechanical, or chemical qualities of matter. Yet the operation of the latter can be clearly discerned 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 mechanical elasticity of the epiglottis, and the chemical affinities exercised in respiration, 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 matter. Stahl and his followers were so struck with the circumstance of living bodies resisting those affinities, which produce putrefaction in dead animal matter, 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 during 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, archæus, 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 independently of the actions by which it is manifested; they are only to be considered as denoting the assemblage of powers that animate living bodies, and distinguish them from inert matter. Some writers, realizing the offspring of a mere abstraction, have talked of the living principle as something distinct from the body, to which they have ascribed powers of seeing 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 component parts in spite of the external powers, 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 'particles are attracted, and interposed between the old constituent ingredients of the body; the latter effect seeming to be exerted as constantly as the power by which

the materials of the body are held together. 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 continued 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 function of digestion, viz. in plants. Now, since living bodies do not grow indefinitely, 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 perspiration, 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 particles, we observe them in a state of continual circulation from without inwards, and from within outwards. Thus, a living body is a structure into which dead particles 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 be brought under the action of those laws which regulate inanimate matter. 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 according to the different states of each living body.

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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 inevitable consequence of life, as rest necessarily follows all movements which are not performed in vacuo, even if the hour of its approach were not hastened by a multitude 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 thenselves, 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 particle has it within itself.

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When this nature of life was once clearly recognised by the most constant of its effects, physiologists naturally attempted to discover its origin, and the Imode 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 fœtus, or a vegetable, in the first moments that we can perceive them, they still sess life, and have within themselves the germ of all the phenomena which this life will develope in the sequel. These observations, 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, before 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 exists, except what has been transmitted from living bodies to living bodies, in an uninterrupted succession.

Since we cannot go back to the first origin of living bodies, our only resource in investigating the true nature of the powers which animate them, consists in examining their structure, and tracing the union of their elements. Our knowledge of these points is too imperfect for us to draw all the necessary inferences. The minute branches of vessels and nerves, and the intimate structure of the organs in general, elude our imperfect means of research our analysis of fluids is also very incomplete, and there are probably several of which we have no means at all of subjecting to examination. Yet, though

our knowledge of organization be not sufficient to enable us to explain all the facts presented to our observation by living bodies, we may, by means of it, recognise them, even in an inactive state, and trace their remains after death. No inanimate matter has that fibrous and cellular texture, nor that multiplicity of volatile elements which form the characters of living bodies, whether alive or dead. Thus, while inorganic solids are only composed of many-sided particles, attracting each other by their surfaces, and receding only for the purpose of se parating; while they are resolved into a very limited number of elementary substances, and are formed merely by the combination of these elements, and the aggregation of these particles; while they grow only by the juxtaposition of new particles, which are deposited exteriorly to those already existing, and are destroyed only by the mechanical separation of their parts, or the decomposing agency of chemical means; organized bodies, made up of fibres and laminæ, whose intervals are filled by fluids, are resolved almost entirely into volatile elements, grow on bodies similar to themselves, and separate from these only when they are sufficiently developed to act by their own powers; constantly assimilate foreign matters to themselves, and, interposing these between their own particles, grow by the operation of an internal power, and perish at last by this interior principle; indeed, by the very effect of their

life.

An origin by generation, a growth by nutrition, and a termination by death, are the general characters common to all organized bodies; and if several of such such as immediately depend on them, bodies possess these functions only, and and have only the organs required for their performance, there are many others exercising particular functions, which require appropriate organs, and also modify the general functions and their organs.

Of all the less general powers, which presuppose organization, but which do not seem to be necessary results of structure, those of sensation and voluntary motion are the most remarkable, and exert the greatest influence over the other functions. We are conscious of the exist ence of these powers in ourselves, and we attribute them, by an analogical mode of reasoning, to many other beings, which we therefore name animated beings or animals. They seem to be necessarily connected together; for the idea of voluntary motion contains in itself that of

sensation; since volition cannot be conceived without desire, and without a feeling of pleasure or pain. The goodness which we observe in all the works of nature will not allow us to believe that she has formed beings with the power of sensation, that is, with a susceptibility of pleasure and pain, without enabling them at the same time to approach to the one, and fly from the other, as least to a certain degree. And if, among the too real misfortunes which afflict our species, one of the most affecting is the sight of a man of sensibility deprived by superior force of the power of resisting oppression: the poetic fictions, most apt to excite our pity, are those which represent sensible beings inclosed in immoveable bodies; and the tears of Clorinda, flowing with her blood from the trunk of a cypress, ought to arrest the blows of the most savage man.

Independently of the chain which unites these two powers, and of the double apparatus of organs which they require, they produce also several modifications in the faculties. common to all organized bodies; and these modifications, joined to the two peculiar powers, constitute more particularly the essential nature of animals. Thus, in respect to nutrition, vegetables being attached to the earth absorb nutrive fluids directly by their roots; these, almost infinitely subdivided, penetrate the smallest intervals of the soil, and, if we may use the expression, travel to a distance in quest of nourishment for the plant to which they belong; their action is quiet and constant, being liable to interruption only when drought deprives them of the necessary juices. Animals, on the contrary, fixed to no spot, but frequently changing their abode, required the power of transporting with them the provision of fluids necessary for their nutrition; they have therefore an interior cavity to receive their food; and on its inner surface there are the openings of absorbing vessels, which, to use the energetic language of Boerhaave, are real internal roots. The size of this cavity, and of its orifices, allowed in several animals, the introduction of solid substances. These required instruments for their division, and liquors for their solution; in a word, nutrition was no longer performed by the immediate absorption of matters in the state in which the earth or atmosphere furnished them; it was necessarily preceded by various preparatory operations, which, taken altogether, constitute digestion.

Thus digestion is a function of a secondary class, peculiar to animals. Its existence, as well as that of the alimentary cavity in which it takes place, is rendered necessary by the power which animals have of voluntary motion; but it is not the only consequence of that power.

Vegetables, having few faculties, are simple in their organization; being composed almost entirely of parallel or slightly diverging fibres. Moreover, their fixed position admitted of the general motion of their nutritive fluid being kept up by simple external agents; thus it ascends by means of suction in their spongy or capillary texture, and also through the influence of evaporation from the surface; it is rapid in a direct ratio to this evaporation, and may even become retrograde when that process ceases, or when it is changed into absorption by the moisture of the atmosphere.

It was necessary that animals should have within themselves an active principle of motion for their nutritive fluid, not only because they were destined to constant changes of situation and temperature; but also from their more numerous and highly developed faculties requiring a much greater complication of organs. Hence the component parts became very intricate in their composition, and often very distant, and possessed in many instances a power of changing their relative position, consequently the means of carrying the nutritive fluid through such multiplied intricacies must be more powerful than in vegetables, and differently arranged. It is contained, in most animals, in innumerable canals, which branch out from two trunks, that communicate together in such a way, that the fluid urged into the branches of one is received by the roots of the other, and carried back to a common centre, from which it is propelled afresh.

At the point of communication between the two great trunks is placed the heart, whose contractions impel the nutritive fluid into all the branches of the arterial trunk; for the orifices of the heart pos sess valves, disposed in such a way that the circulating juices can only move in the directions now described, viz. from the heart towards all parts by the arteries, and from all parts to the heart in the veins.

In this rotatory motion consists the circulation of the blood, which is another secondary function peculiar to animals, chiefly performed and regulated by the heart. This, however, is not so essen