Animals, from this brief review, appear to pos sess electrical capacities in a much higher de gree than vegetables, in which the phenomenon is even explicable on ordinary chemical prin ciples, whilst among animals it is unquestion ably one of the effects of vitality.
We have already indicated the existence of two faculties among animals , which become necessary or complemental to them as agents entrusted with their preservation as individuals, and their continuation as kinds ; these are voluntary motion and sensation. But motion in the abstract is a phenomenon of much more extensive occurrence among organized beings than the notion we form of the act as connected with the existence of a muscular system. Mo tion is in fact a quality inherent in organized beings ; they cannot be conceived as existing without change, and change implies motion. In most, or indeed in the whole of the actions which we have glanced at as manifested by them, we have supposed motion. The simplest of all animals, the infusoria, move about in many cases with great briskness; the polypes, composed of an uniform gelatinous mass, also move in various directions; the acalephs, with a similar structure, rise from the bottom and propel themselves through the waters of the ocean by a succession of contractions of their disc, of their tentacula, or of the fringe-like or foliaceous bodies with which several orders of the genus are provided. Many of the en tozoa too, whose bodies consist of a simple gelatinous or mucous tissue, execute motions in various senses.
But it is not only as a whole that a body endowed with life and organization possesses a capacity of motion. Many of its parts, and particularly the globules which enter as essen tial and integral parts of the fluids contained in organized bodies, have inherent powers of motion ; the globules of the blood, for instance, those of the spermatic fluid, and perhaps also the germ included within the ova of the polype, mollusc, &c., have all been observed in motion, and the means by which it is accomplished even demonstrated in many cases. But there is nothing absolutely peculiar in such indivi dual instances, for we must need conceive motion in the first constituent elements of all organisms without exception, long before a muscular, a cellular, a nervous, or any other distinct system has Motion of all kinds, therefore, automatic as well as that which is voluntary, must be held as a quality inherent in organized or living beings. The cause of this phenomenon, as of
so many others manifested in the world of or ganization, has been the subject of much dif ference of opinion and of much dispute among physiologists, and many titles have been ima gined by which the agent or primary cause of the act has been sought to be designated, or the act itself to be explained.
It is quite certain that the capacity to com mence and to continue the phenomena which we designate as vital, or the motions which constitute these phenomena, depends first on a variety of external conditions, such as a certain temperature, intercourse with the air of the atmosphere, supplies of aliment, and the access of light, and it is indubitable that organized beings exhibit phenomena that may be designated excitability, irritability, vital force, &c., which are only other names for these manifestations; but it is also certain that external conditions are of themselves ina dequate to originate manifestations of vitality, and that the phenomena of living organized beings, generally designated excitability, irrita bility, incitability, &c., are consequences of a state of things to explain which they have been conceived as causes, under the title of life, vital principle, soul, &c. The term excitability should be used in physiology, in the very widest sense, to signify a property inherent in organized matter generally, to be determined to manifestations of activity under and in con formity with external influences (Tiedemann). We in fact see organized matter of every de scription—the green matter of Priestley, eon fervw, infusory animals, &c., acquiring organic forms under the dominion of outward influences, and every species of organized being existing within a determinate circle of external agency.