It were a grave mistake to suppose this agency either chemical or mechanical in its na ture; when of such potency as to act either chemically or mechanically it is destructive instead of productive of vital phenomena. These phenomena, therefore, and external influences are rather in opposition to one another than identical. External influences ex cite organized beings to manifest their inherent capacities; they do not bestow these capacities ; all organized beings, indeed, and each parti cular tissue of every individual among them, are excited in different modes by the various influences from without ; the stimulus is iden tical, the effects are infinitely different.
But organized beings, and especially ani mals, are not dependent on external influences alone for the manifestation of their peculiar properties; they have themselves the additional power of engendering stimuli proper to arouse into activity the various organs and systems of which they are composed. The fluids circu lating through every part of their bodies may be regarded in the light of the most generally distributed stimuli of this description. The nrevous system is another and important source of excitation, the influence of which is felt in every part of the organism of all animals above the very lowest. The various instincts, appe tites, propensities, sentiments, and intellectual faculties, also, which all emanate from the nervous system, are inherent causes of a vast variety of manifestations of activity among the more. perfect animals. There are yet other stimuli of a mechanical, or chemical, or pecu culiar nature, which excite unusual or ano malous manifestations ; in this category may be placed contagions of different kinds, the causes of epidemic diseases, medicines, &c.
With regard to the essence or cause of this property of the organic globule to commence, and of the perfectly developed organism to manifest the various phenomena whose sum constitutes their vitality, and endows them with their various cognizable properties, all we can say is that it appears to inhere immedi ately in the particular state of matter which composes them. What this state is in itself we cannot tell ; but we are familiar with the phenomena which ensue from, and which in deed reveal to us its existence. It is evidently as diversified as species, and as the systems or organs possessed by the individuals severally composing these : there is a power—the nisus formations, the vis plastica, in the matter sus ceptible of formation,—the organic globule, the germ,—which presides over and regulates its acts; and there are powers inherent in the parts or organisms to which the plastic force gives rise, in accordance with which they manifest the special acts that distinguish them. It would be improper, however, to regard this power or these powers as forces apart from and other than the globules, germs or organisms themselves ; in the present state of our know ledge we cannot separate exciting causes from manifestations of activity; all we can venture to say is that germs exist, that organisms exist with inherent capacities of action in harmony with the peculiar states of their constituent elements, thus : the germ of the infusory ani mal exists with its inherent capacity to en gender an infusory animal, the germ of the polype with its inherent power to produce a polype; in the same way the various tissues, vessels, glands, &c. of vegetables and animals
exist with their special capacities of excitation, which are manifested in the particular functions they severally perform. Excitability is there fore a multiform property and a consequence, not a single peculiar power inherent in orga nized beings, the fundamental cause of their actions and identical with or itself the living principle. Dependent on the integrity and continuance of the functions of nutrition, how can it be the cause of these? Only manifested in kind, with the occurrence of specific organs, how can it be the cause of their several ma nifestations ? We are altogether in the dark with regard to the mode in which the motions and other actions of organized beings are performed, how or by what law the globule that in the infusion of organic matter is to become an infusory animal moves, as well as of the manner in which the contractility of a muscle is excited by the stimuli fitted to call this quality into action. The contractility of the infusoria, po lypi, medusze, and other similar tribes appears to be peculiar. The motions exhibited by the confervze, tremellm, and simplest vegetables are also peculiar to them; they differ from those manifested by the simplest animals in being entirely under the influence of external influences, and showing nothing like spon taneity. The tissues of all animals, even the most complicated, show traces of a vital ten sion or contractility, different from simple elasticity and not depending on muscularity; the cellular membrane, Skin, fibrous tissues generally, excretory ducts, and vessels of all descriptions tend to contract upon the parts and fluids they surround and include. This tonicity or peculiar contractility disappears in great part with the cessation of life : wound made in a dead body never gapes as it does in a living one. Something of the same kind exists in vegetables ; the sap as cends with greatly increased velocity in the young shoots under the influence of stimuli of different kinds, and its flow is checked by nar cotics and altogether arrested by poisons ; it is probable, therefore, that it takes place in con sequence of a vital tonicity or contractility in the sides of the sap-vessels•which contain it.