It was necessary that animals should have within themselves an active princi ple of motion for their nutritive fluid, not only because they were destined to con stant changes of situation and tempera ture ; 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 in stances a power of changing their relative position, consequently the means of carry. ing the nutritive fluid through ditch mul tiplied intricacies must be more powerful than in vegetables, and differently arrang ed. It is contained, in most animals, in innumerable canals, which branch out from two trunks, that communicate to gether 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 pro pelled 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, vi:. from the heart towards all parts by the arte ries, and from all parts to the heart in the veins.
In this rotatory motion consists the cir culation of the blood, which is another secondary function peculiar to animals, chiefly performed and regulated by the heart. This, however, is not so essen tially connected to the faculties of sen sation 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 intesti na canal.
The blood seems to be merely a ve hicle, receiving constantly from the intes tines, skin, and lungs, different substan ces, which it incorporates intimately, and by which its losses, arising from the pre servation and growth of parts, are sup plied. The nutrition of the body is per formed during the course of the blood in the minute extremities of the arteries ; here the fluid changes its nature and co lour ; 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, call ed lymphatics ; in the same way it re ceives 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 ve nous 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 ; i 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 passingthrough the lungs ; a circumstance which could not take place where the nutritive fluid is distributed uniformly through the body without be ing contained in vessels. Thus respira tion is a function of a third order, de pending entirely on circulation, and aris ing as a remote consequence from the fa culties which characterise animals.
Generation is the only process in ani mals, the mode of which does not depend on their peculiar faculties, at least as far as the fecundation of the germs is con cerned. Their power of moving and ap proaching to each other, of desiring and feeling, has allowed them to receive all the enjoyments of love, while the tic fluid is conveyed uncovered immedi ately upon the germs ; in vegetables, on the contrary, which have no power of pro pelling 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 or gans of the other functions are more com plicated in animals, on account of their pe culiar 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 ca pable 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 he 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 func tions.