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The degree to which the phenomena of Life are dependent upon Physical agencies, has been a subject of inquiry and speculation among scientific investigators of almost every school. That many of the actions taking place in the living body are conformable to the laws of mechanics, has been hastily assumed as justifying the conclusion thatall its actions are mechanical; and hence arose theiatro-mathematical doctrines, which obtained considerable currency among the physicians and physiologists of the seventeenth century. In like manner, the fact that many of the changes of composition which take place within living bodies are analogous to those occurring externally to them, was assumed by another party as the foundation of the hypothesis thatall the phenomena of life are of the nature of Chemical actions; and of that hypothesis theiatro-chemial doctrines which superseded the system of Galen, and which held their ground under various modifications for several centuries, were the natural expressions. The insufficiency of either of these hypotheses, or of both of them combined, to explain the phenomena of life, gave origin to a third, which was undoubtedly more correct in its fundamental conception than either of its predecessors had been; the position assumed being, that the phenomena of each livingbody proceed from avital agency, oranima , peculiar to each organized structure, and having nothing in common with chemical or mechanical principles. The sect of the Vitalists, however, did not steer clear of the exclusiveness which had been the great fault of the chemists and physicists; but, in looking at every action of the living body as the immediate result of vital agency, claimed for that agency much that is clearly attributable to the operation of chemical and physical forces. Among modern Physiologists there is a distinct recognition of the fact, that many of the phenomena of living bodies may be placed in the same category with those of inanimate matter, and that such are not otherwise affected by vital agency than as this prepares or modifies the conditions under which they occur. But there is also a distinct recognition of the fact, that living bodies present a large class of phenomena which are altogether peculiar to them, and which can only be attributed to agencies of which the inorganic world is altogether independent; and hence has arisen the notion ofvital agency as the foundation of Physiological science, just as the notion ofaffinity is the foundation of Chemistry, and that ofmutual attraction of General Physics. And putting aside all hypothetical considerations with regard to the abstract nature of that agency, Physiologists have been aiming to determine the laws of its operation; following the same mode of inquiry for this purpose, as that which has been found successful in other departments of scientific investigation. In doing this, it has been necessary for them toisolate , as much as possible, those phenomena which may be regarded as Chemical or Physical, from those which must be distinguished as Vital; in order that, by the collocation and comparison of the latter, their mutual relations may be discovered. Still, after making every possible allowance for the operation of chemical and physical agencies, in thedirect production of the changes of composition, mechanical movements, &amp;c. which connect living beings (so to speak) with the universe around them, it is impossible for the discriminating inquirer not to see, that the influence of these agencies isindirectly exerted, to a yet greater extent, in the production or modification of purely vital phenomena. Thus, to take a very simple case, it cannot be for a moment doubted that heat and light exert an influence upon the vegetable germ, which is essential to its growth and development into the perfect plant, and to the performance of all the actions of the latter, whether these have reference to the extension of its own fabric, to the formation of organic compounds from the materials supplied by the inorganic world, or to the production of the germs of new individuals which are in like manner to go through the same series of phases. Hence light and heat have been designated as “vital stimuli;” the current idea being, that their agency upon the vegetable germ excites or awakens the forces which were dormant in it; and that, by enabling it thus to assimilate the new materials supplied by the inorganic world, and to give to these the structure of organized bodies, they contribute to develope the latent powers of these materials, which in their turn exhibit vital properties as they are made to form part of organized structures. Such, at least, is the doctrine of those who have most clearly expressed themselves upon the relation of the “vital stimuli” to the "vital properties” of organized bodies; and the author has not been able to find in physiological writings, any indication of a more intimate relationship between the physical forces and vital phenomena, than that just stated,—save on the part of those who have vaguely identified Heat or Electricity with the “vital principle,” with about the same amount of philosophical discrimination as that which was exercised by the iatro-chemists and iatro-mathematicians of the sixteenth and seventeenth centuries.

XXXVI. On the mutual relations of the vital and physical forces