Dependence upon the circulation differs in different animals. The heart of a salamander may be excised, and yet the animal will live for several hours, or even a day or two after the operation ;* its possession of life being in ferred from the exhibition, not merely of cer tain organic actions, but even of those of rela tion. It is plain, then, that in animals of this tribe, the brain and spinal marrow and other organs do not require so constant an inter course with the blood as in certain other species ; and while we know with tolerable certainty that they do not need it for calorific purposes, it is not improbable that their textures are less frequently repaired than those of the warm blooded classes. Dr. Edwards concludes that life in the above instance is maintained by the organs of innervation, whose function, as we have remarked, continues unimpaired. We should regard the integrity of their action rather as a sign than as a cause of continued vitality ; other signs being perceptible in the persistence of the capillary actions, for which the fluids still remaining in the tissues may be sufficient.
Retention of fluid in the tissues.—Removal of the effete fluid is provided for in the Porifera by ejects; in the Polypifera by expulsion from the central cavity and by transpiration ; in the Acalephve by anal apertures ; and in vascular animals by vessels especially appropriated to the purpose, by transpiration, and by various excretions.- This condition of molecular life is less easily violated than those already spo ken of, because the modes of fulfilling it are more numerous. This is equally true whether we speak of the simple animal forms, or of the tissues of the more complicated ; mortification is less frequently the result of venous than of arterial obstruction. Unquestionably turgescence and inflammation may ensue from the former, and may terminate in gangrene; but it is far more common for the part to be relieved by the excretion of various fluids, constituting hwmorrhage and dropsy, until new channels are found for the returning blood. Hence it ap pears that a redundance of fluid is less dan gerous to organic structures than a deficiency.
Depravation of the fluid of nutrition.—It is obvious that as the structures are elaborated either from the blood in the higher animals, or from the fluids corresponding to it in the in ferior classes, the assimilative processes must be deranged and ultimately brought to a stop, if the liquids are wanting in the proper mate rials. Their quality may be deteriorated in various modes ; by imperfect respiration, by bad or scanty alimentation, and by insufficient or excessive excretion. Each of these causes is traced easily enough in the degenerated tex tures of some animals, but with more difficulty in the simpler classes, because the functions just alluded to are not in the latter concen trated within a space that admits of analysis so well as in the former. The effect of obstructed aeration of the blood however is soon mani fested even in the lowest grades. But we must
observe, that throughout the whole range of animal existence we can more readily ascertain the changes produced in molecular action by diminished respiration, than by the entire sus pension of this function ; because, in the first place, the arrest of the circulation so soon fol lows that of respiration, that the subsequent events are assignable rather to the former than to the latter ; and in the second place, it is impossible to cause one portion of the body to receive unaerated, while the others are sup plied with aerated blood since the function is in some animals too concentrated to allow of an operation calculated to act upon an isolated part, and in other animals too diffuse to enable us to interfere with it effectually in any given space. In the one case we run the risk of cutting off the supply of blood from the whole animal ; in the other we should find it im possible to prevent any one part from receiving from other parts a. compensation for what it loses by the obstruction of its own particular allotment of the respiratory function. Nothing however is more common than to witness the degeneration of structure produced by blood insufficiently arterialized, the imperfection of the process depending either upon disorder in the organs of respiration, or upon a vitiated condition of the atmosphere. From facts of this nature it is legitimate to infer that were it possible for unarterialized blood to circulate, the death of the tissues must sooner or later ensue. Of the destructive tendency of blood depraved by the other causes above enu merated we can likewise judge approxima tively ; in other words, while there can be no question of the deterioration of structures under the operation of those causes, we are not ac quainted with any instances in which we can attribute solely to their agency the entire cessa tion of molecular actions. It almost always happens that other functions have previously failed, and influenced the result in question.
Extinction of irritabilit y .—lrritability might at first seem rather the result of vitality than one of its conditions ; but whether we look at the textural motions in a complex animal, or at their analogues in the entire systems of the simpler formsove shall find irritability to be essential to the continuance of those processes in which living action consists. The alimen tary cavity which contracts upon the nutrient fluid of the zoophyte is no less essential to the existence of the latter, than a similar action of capillary tubes in the tissues of mammalia. In each case the action is requisite, in order to bring the particles within the spheres of the textural affinities. The extinction of irritability is therefore necessarily productive of molecular death. In this instance we are compelled to speak of the privation of a property instead of defining the actual change in the part, because at present it is not ascertained what condition of the part is capable of producing contraction.