tially connected to the faculties of sensation and motion as the business of di

gestion for whole classes of animals as insects) possess no circulation, and are nourished, like vegetables, by the mere imbibing of fluids prepared in the intestinal canal.

The blood seems to be merely a vehicle, receiving constantly from the intestines, skin, and lungs, different substances, which it incorporates intimately, and by which its losses, arising from the preservation and growth of parts, are supplied. The nutrition of the body is performed during the course of the blood in the minute extremities of the arteries; here the fluid changes its nature and colour; and it is only by the addition of the various substances just pointed out, that the venous blood again becomes proper for the purposes of nutrition, or, in one word, again becomes arterial.

The venous blood receives the supplies furnished to it by the skin and alimentary canal by a particular set of vessels, called lymphatics; in the same way it receives also the particles detached from various organs, in order to be sent out of the body by the different secretions.

The air entering the lungs seems to produce a sort of combustion in the venous blood, which is necessary for the support of life in all organized bodies. Vegetables, and such animals as have no circulation, respire (for that is the name given to this action of the atmosphere on the nutritive fluid) by their whole surface, or by means of particular vessels which convey air into the interior of the body.Those only, which enjoy true circulation, breathe by means of a particular organ; because, in them, the blood constantly flowing to and from the common source, its vessels have been so arranged, that it is not distributed to the other parts of the body until after passing through the lungs; a circumstance which could not take place, where the nutritive fluid is distributed uniformly through the body without being contained in vessels. Thus respiration is a function of a third order, depending entirely on circulation, and arising as a remote consequence from the faculties which characterize animals.

Generation is the only process in animals, the mode of which does not depend on their peculiar faculties, at least as far as the fecundation of the germs is concerned. Their power of moving and approaching to each other, of desiring and feeling, has allowed them to receive all

the enjoyments of love, while the spermatic fluid is conveyed uncovered in mediately upon the germs; in vegetables, on the contrary, which have no power of propelling this fluid, it is inclosed in small capsules, capable of being transported by the wind, and forming what is called the dust of the stamina. Thus, while the organs of the other functions are more complicated in animals, on account of their peculiar functions, generation is exercised in them, for the very same reason, in a more simple way than in vegetables.

Such are the principal functions that compose the animal economy; they have usually been arranged in three orders. Some of them constitute animals what they are, render them proper to fill the space which nature has marked out for them in the general arrangement of the universe, and would be sufficient for their existence, if that were momentary. These are the faculties of sensation and motion; of which the former determines them in, the choice of such actions as they are capable of, and the latter enables them to execute these actions. Each animal may then be considered as a partial machine, co-ordinate to all the other machines, which, by their assemblage, form this world: the organs of motion are the wheels and levers in a word, all the passive parts; but the active principle, the spring which sets all in motion, resides only in the sensitive faculty, without which the animal would be lost in a constant sleep, and be really reduced to a merely vege tative life. These two functions, then, form the first order, or the animal functions.

But the animal machine, in addition to the powers which those of human construction possess, is endowed with a principle of preservation and repair, consisting in the assemblage of functions which contribute to nutrition, viz. digestion, absorption, circulation, respiration, and secretion; these form the second order, or the vital functions. Lastly, as the duration of each animal is limited according to its species, the generative form a third order of functions, by means of which the individuals that perish are replaced, and the existence of the species preserved.

This threefold division of the objects of physiology is open to many objections, which we have not room to consider in this place; and we therefore add another more complete and natural classification, which will be sufficiently explained in the subjoined tabular view.

To trace out completely all the subjects which this table exhibits would lead into a very wide field of discussion; we shall, after devoting a short space to the consideration of those vital powers which animate living bodies, shortly consider the principal functions.

Of the Vital Powers, Sensibility, and
Contractility.

Struck with the numerous differences that are observable between organized and living, and inorganic matters, philosophers have admitted in the former a peculiar principle of action, a force which maintains the harmony of their functions, and directs them all to one object; the preservation of the individual and of the species. No one at present doubts the existence of a living principle, which subjects the beings endued with it to a different order of laws from those which govern inanimate things, and whose princi

SAge of decrease.

Old Age.
Decrepitude.

pal effects are seen, in its removing the bodies which it animates from the agency of chemical affinities, to which the multiplicity of their elements would otherwise have rendered them prone; and in its maintaining their temperature at an uniform standard. All the phenomena observed in the living animal body might be cited in proof of this principle. The effects produced on the food by the digestive organs; its absorption by the lacteals; the circulation of the nutritive juices in the blood-vessels; the changes which they undergo in the lungs and secretory glands; the capability of receiving impressions from external objects, and the power of approaching to, or avoiding them, all demonstrate its existence. But we prove it more directly by means of the two properties, with which the organs of these functions are endued. These are, sensibility, or the faculty of feeling; the aptness to receive, from the contact of foreign bodies, more or less vi

vid impressions, which change the order of their motions, accelerate or retard, suspend or terminate them; and contractility, by which parts, when irritated, contract, act, or execute motions.

By means of the senses, and of the nerves which are continued from them to the brain, we perceive or feel the impressions made on our bodies by external objects. The brain, which is the true seat of this relative sensibility, (or, as it might well be called, perceptibility,) being excited by these impressions, influences the moving powers of the muscles, and determines the exercise of their contractility. This property, subjected to the conmand of the will, is manifested by the sudden shortening of the muscular organ, which swells, becomes hard, and causes those parts of the skeleton to which it is attached to move. The nerves and the brain are the essential organs of these two properties: division of the former destroys sensation, and the voluntary motion of those parts to which the nerves are distributed. But there is another kind of sensibility, quite independent of the presence of nerves, existing in all organs, even where no nervous filaments are distributed. Bones, cartilages, ligaments, arteries, and veins, in short, all parts which are not influenced by the will, possess no nerves. Yet, though in their natural state they transmit to the brain no perceptible impression, though they may be injured without giving the animal any pain, and though the will has no influence over them, yet they enjoy a sensibility and contractility, by virtue of which they perceive impressions and contract in their own manner, recognise in the fluids which circulate through them what is proper for the nutrition, and, separating this part, appropriate it to their own substance.

We recognise then in the parts of our body two modes of sensation, as well as two species of motion: a sensibility, by means of which certain parts transmit to the brain impressions which they feel, and of which we therefore become conscious: a second kind, pervading every part without exception, and presiding over the assimilating functions. We observe also two kinds of contractility corresponding to the differences of sensibility: the one by which the voluntary muscles perform the contractions determined by the action of the will; the other manifested by actions which are equally unknown as the causes which give rise to them.

When we have once clearly distin-
VOL. X.

guished these two grand modifications of sensibility and contractility, we shall find out, without difficulty, the source of those eternal disputes, raised by Haller and his followers, concerning the irritable and sensible nature of parts. Bones, tendons, cartilages, &c. to which this great physiologist denied these two properties, enjoy only that lateral sensibility and obscure contractility which are common to all living beings, and without which we cannot conceive the existence of life. In the healthy state they are completely destitut the power of transmitting perceptible impressions to the brain, or of being influenced by that organ to any manifest motion. It has also been disputed whether sensibility and contractility depended on the existence of nerves; whether these were its necessary instruments, and whether their injuries were followed by a loss of those vital powers in the parts which have nerves. We may answer in the affirmative, as far as regards perceptive sensibility and voluntary motion, which is entirely subordinate to it; but in the negative, with respect to the sensibility and contractility which are indispensable in the processes of assimilation.

Sensibility may then be either perceptive or latent. The former is attended with a consciousness of the impressions or perceptibility, and requires a peculiar apparatus. The latter, unaccompanied by consciousness, is common to every thing that lives; it has no particular organs, but is universally expanded in all living parts, whether of vegetables or animals. Contractility may be either voluntary and sensible (vis nervosa), which is subordinate to perceptibility; involuntary and insensible, which corresponds to latent sensibility; or involuntary and sensible (vis insita), as in the action of the heart, stomach, &c.

The former species of sensibility being that which is observed in the func

tions which connect the animal with external objects, is called, by Bichat, animal sensibility; and the corresponding contractility is distinguished by the same term. The other kind of these two vital powers, which are exerted in the internal processes of nutrition, &c. common to animals and plants, that is, to all organized bodies, is named the organic.

Organic sensibility is merely the faculty of receiving an impression; animal sensibility is the same faculty, with the additional power of conveying it to a common centre. In the former case the effect terminates in the organ. The latter

D

belongs only to animals, whose perfection is in a direct ratio to the quantity of this sensibility. There is some reason for supposing that these two are not different powers, but that they differ only in quantity. For inflammation, which is an increased action of parts, raises organic into animal sensibility in diseases of bones, &c.

Different stimuli, applied to the same organ, determine the developement of one or other of these powers: thus, no sensation is transmitted to the brain from the passage of blood in the arteries, but when an extraneous fluid is injected, the animal's cries shew that he feels it. The coats of the stomach experience in the healthy state no perceptible impression from the food, but very distinct and even painful sensations are transmitted to the brain, when a few grains of poisonous matter are mingled with the aliment. The animal sensibility excited on mucous membranes by foreign bodies (as bougies in the urethra, &c.) is quickly lost, and subsides into organic.

Each organ seems to have, independently of accidental variations, an original quantity or dose of sensibility, to which it returns after any deviation. In this consists the peculiar life of each organ, and from this arise the relations which it has to extraneous substances. Hence excretory ducts, opening on mucous membranes, refuse admission to the substances passing along those canals. Hence the lacteals absorb the chyle only. These particular relations may also take place with matters foreign to the body, as well as with animal fluids, as we see in the case of medicines acting on particular organs, as cantharides, mercury, &c.

Contraction is the cominon, but not the universal, mode of animal motions. For the iris, corpus cavernosum, &c. dilate when they move. Organic contractility is always and immediately connected with organic sensibility, for there is no intermediate function between these; the organ itself is the point in which the sensation ends, and from which the principle of contraction begins. The animal sensibility and contractility are not so closely united; we may feel without moving: here the nerves and brain perform their functions between the action of the two powers.

Sensible organic, or, in other words, involuntary and sensible, contractility corresponds very nearly to irritability; while the insensible seems more like tenacity. To consider irritability as the exclusive

endowment of muscles, is taking a very contracted view of the subject. These organs have indeed the largest portion, but every part possessing life reacts, although less manifestly, on the application of certain stimuli. No rule is more fallacious, than that of estimating the mus cularity of a part of the action of artificial irritants. The organic and animal contractilities cannot be converted into each other as the corresponding sensibilities can.

The parts of the living body possess also some powers which result merely from their organization, and have been denoted by physiologists under the epithet of vis mortua. Thus they admit of extension beyond the natural state from extraneous impulse, and of contraction when that impulse ceases to operate. This extensibility and contractility are independent of life, and are terminated only by death. The stretching of mus cles by moving a limb, the extension of the skin over a tumour, its retraction when divided, &c. are examples of these powers. They have been confounded by some physiologists with the insensible organic contractility.

A muscle exhibits all the powers now enumerated. It contracts, in obedience to the will, by its animal contractility; from the application of stimuli, by its organic sensible contractility. Its nutrition and growth show the existence of organic insensible contractility; and its retraction on a section exemplifies the contractility of organization. The internal organs of the body have only the three last powers, and the white organs (cartilage, tendon, ligament, &c.) only the two last. While, therefore, the two first properties exist only in certain parts, the latter are found in all. Hence the organic insensible contractility may be selected as the general character of all living parts; and the contractility of organization as the common attribute of all living or dead parts that are organically constructed.

As for porosity, divisibility, elasticity, and the other properties which living bodies have in common with inanimate matter, they hardly deserve mention here, because they are never exerted in their whole extent, or in their genuine purity, if we may use that phrase. Their results are always affected by the influence of the vital powers, which constantly modify those effects which seem to flow most directly from physical, mechanical, or chemical causes.