But the animal machine, in addition to the powers which those of human con.' struction possess, is endowed with a prin ciple of preservation and repair, consist ing in the assemblage of functions which contribute to nutrition, viz. digestion, ab sorption, circulation, respiration, and se cretion ; 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 indi viduals 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 sub jects 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 con sider 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, philo sophers 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 sub jects the beings endued with it to a differ ent order of laws from those which go vern inanimate things, and whose princi pal effects are seen, in its removing the bodies which it animates from the agen cy 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 pheno mena 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 se cretory glands ; the capability of receiv ing impressions from external objects, and the power of approaching to, or avoiding them, all demonstrate its exist ence. 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 contrac tility, by which parts, when irritated, con tract, 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 impres sion made on our bodies by external ob jects. 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 de termines the exercise of their contractili ty. This property, subjected to the com mand of the will, is manifested by the sudden shortening of the muscular or gan, which swells, becomes hard, and cau ses 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, qui* independent of the presence of nerves, existing in all or gans, even where no nervous filaments are distributed. Bones, cartilages, liga ments, arteries, and veins, in short, all parts which are not influenced by the will, posssess 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, recognize in the fluids which circulate through them what is proper for their nutrition, and, separating this part, appropriate it to their own substance.
We recognize 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 con scious : a second kind, pervading every part without exception, and presiding over the assimilating functions. 'We ob serve also two kinds of contractility cor responding to the differences of sensibi lity: the one by which the voluntary muscles perform the contractions deter mined by the action of the will ; the other manifested by actions which are equally unknown as the causes which give rise to them.