Irritability is merely an expression of the fact that the substance of which it is predicated, undergoes contractions inexplicable on common physical principles. We detect nothing in the substance, the existence of which enables us to pronounce with certainty that it may be the subject of the actions alluded to. Some have maintained that irritability ought to be ad mitted as an ultimate fact, of which we know as much as of gravity. But we apprehend that there is this great difference in our knowledge of the two properties, viz. that although igno rant of the cause of the attraction of gravitation, we are certain that the phenomena are co extensive with the essential properties of mat ter; but we are utterly unacquainted with that collection of properties to which irritability necessarily belongs. The muscle which has ceased to quiver under the galvanic wire is, for all that we can tell to the contrary, the same in composition as that which is still ca pable of exhibiting the phenomenon. More over the action is observed in a great variety of tissues, both in individual animals, and in the whole series ; tissues which appear to have little in common saving a fibrous arrangement of their particles. But as the action in question is stopped by causes which in no way affect the fibre as such, it is plain that this is not the only requisite. Moreover there are unequi vocal exhibitions of contractility in animals, in which it is 'difficult to imagine that there can be any shortening of fibres; we allude to the Infusoria, Rotifera, Medusw, &c. Tiedemann makes a separate species of this contractility, under the designation of " contractilit6 des animaux gelatineux." * There is reason to
suspect that ganglionic tissue is importantly concerned in the action, partly because it is almost universally distributed through irritable substances, and partly because contraction is prevented by causes which operate upon this tissue. As long however as there are animals wbich manifest contractions, but in which no such tissue can be detected, it is impossible to consider the latter an essential element in the action generally; though it may be quite es sential in the animals in which it is found ; just as a heart, though by no means necessary to the function of circulation in the abstract, is indispensable in the animal of whose system it forms a part.
Irritability may be destroyed' by substances, either applied directly to the part or acting upon the general system. Thus, the fibres of the heart may be paralysed by a solution of opium injected into its cavities, or by essential oil of tobacco given by the mouth. Light ning annihilates the property all over the body. The motions of Infusoria may be arrested by a shock of galvanism,t by solutions of opium and camphor, and by the vapour of sulphur. Arsenical preparations have a similar effect. The contractions of capillary vessels in the higher animals may be arrested by a certain description of injunes of the brain and spinal marrow.t But it is needless to multiply ex amples.
Such then are the causes of molecular death. For a history of its phenomena, -when partial, NYC refer to the article MOR:1IFICATION. Its characters when universal, that is, when the consequence of systemic death, will be con sidered when we come to speak of the signs of the reality of death.