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posed to attribute the motion to a conversion either of the magnetic force or of the heat into mechanical force. But in respect to the animal functions, this very error has been committed; for in the illustration above cited the magnetic attraction represents the vital affinity or force, the gravitation the repressed chemical affinities of the living tissues, the heat the stimulus, and the fall of the weight the function.

Many arguments may be adduced to show, that while the Correlation Theory affords a consistent and beautiful expression of the relation which exists between the forces of the external world and the developments of the vital force in the growth and nutrition of the body, it is entirely misapplied when it is proposed as an explanation of the connection of the vital force with functional activity.

In the first place, this view entirely ignores the balanced state of the forces in the animal economy, and the accumulation of power arising from the repressed chemical affinity, which it regards merely as operating, after the vital force has discharged the function, in reducing to simpler compounds the devitalised tissue. Surely this is utterly opposed to all we know of the economy of force which prevails throughout nature, and pre-eminently in the living body, in which no power, how subordinate soever, or apparently trivial, is ever wasted.

It is unquestionable, that in this state of equilibrium of the chemical and vital forces there exists an arrangement by which great results might be accomplished. Everything is prepared for the exhibition of a large amount of power by the mere permission of the play of chemical affinity. Would it not be a gratuitous squandering of resources that such a capability for action should be turned to no account?

2ndly. To suppose a conversion of the vital force into

functional action, is to set aside an actual and sufficient cause in favour of one that is entirely hypothetical. The state of chemical tension in the animal body, and the coexistence of chemical change with functional activity, are admitted facts: that this chemical action in the tissues gives rise to the external manifestations of function, is an inference as simple as that the chemical change among the particles of gunpowder is the cause of its explosion. How, then, are we justified in assuming the existence of another process, hard to conceive, and impossible to demonstrate ?

3rdly. The theory in question, while it rejects a cause so natural and obvious, in reality involves the idea of an effect without any adequate cause at all. No intelligible relation of cause and effect can be shown between the stimuli which excite the functions and the conversion of force which they are supposed to cause, or for which they "supply the condition." No proportion is maintained between the amount of the stimulus and the amount of force converted. In what way, for instance, can gentle pressure on the thumbs of the frog, during the season of coitus, produce a conversion. of the vital force of nearly all the muscles of the body into an energetic contractile action?

4thly. Waiving all theoretical objections to the view of the correlation of vital force and functional activity, it may be remarked that the facts do not agree with the principles of that doctrine. The "material substratum" is wanting. In the conversion of the vital force of a muscle into mechanical force, for example, there is no change of the matter in which the force subsists. The conversion supposed is precisely such as would occur if a heated body were suddenly and without adequate cause to lose its heat, and manifest electricity

instead, or shoot into spontaneous motion. The view propounded by Liebig-viz., that the vital force which is converted into mechanical force in muscular contraction is not that of the muscle itself, but may be derived from any other part of the organism, and conveyed to it by the nerves-would be more accordant with the terms of the theory, but we know experimentally that it is not correct.

5thly. The vacillating language used in reference to this part of the subject, by those who have most successfully applied the doctrine of correlation to vital phenomena, betrays the unsoundness of their position.

"Muscular contraction," says Dr. Carpenter, "may be regarded as proceeding from the expenditure or metamorphosis of the cell force, which ceases to exist as a vital power, in giving rise to mechanical agency." But speaking of the external stimuli of muscles, he adds: "These agencies are concerned in occasioning that metamorphosis of living organised tissue into chemical compounds, whereon the development of the muscular force seems to be immediately dependent." 1

Are not two different origins here assigned to muscular contraction? Again, Dr. Carpenter observes (p. 747), "We are, then, to regard the nervous, electrical, and other stimuli under whose influence the muscular force is called forth, less as the immediate sources of that force than as furnishing the conditions under which the vital force, acting through the muscle, is converted into the mechanical force developed in its contraction." But at p. 745, we read: "The nervous force appears convertible into motion through the medium of the muscular apparatus."

With regard to the nervous force, Dr. Carpenter writes as follows: "We find only one kind of tissue 1 Philosophical Transactions, Part ii., p. 746. 1850.

serving for the generation and transmission of nervous. power, this alone affording the material substratum through which the vital force can manifest itself as nervous agency." And again: "Nerve force which has its origin in cell-formation may excite or modify the process of cell-formation in other parts" (p. 743). But, on the following page, he argues, that "all the facts. that have been adduced in support of the identity (of the nervous force and electricity) will be found readily explicable on the idea of their correlation or mutual convertibility."

Can the nerve force be both a manifestation of vital force and a result of the conversion of electricity? Can it have its origin at once in cell formation and in a galvanic current ? And yet, further, are there not the same reasons for holding that the electrical stimulus only furnishes the conditions under which the vital force is converted into the nervous force, as exist in respect to muscular contraction?

Even Liebig's perspicuity fails him upon this subject In his observations "On the Phenomena of Motion in Living Bodies," he writes thus: "All experience proves that there is in the organism only one source of mechanical power; and this source is the conversion of living parts into lifeless amorphous compounds."

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But at p. 220,"As an immediate effect of the manifestation of mechanical force, we see that a part of the muscular structure loses its vital properties, its character of life."

Is not the same change thus made both cause and effect?

The last writer on this topic, Dr. Reynolds, in an able article "On the Objects and Scientific Position of 1 Op. cit., p. 242.

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Physiology," is not more definite in his language. Compare the following passages:-"The partial disintegration of the tissues (of the muscular and nervous systems) is one condition or source of their action" (p. 112). "We have therefore to regard these animal properties (sensibility and muscular contraction) as functions of the vital force inherent in the cell, and as constituting two of its special endowments" (p. 118).

In the passages above cited-and many more of the same character might be adduced-two contradictory ideas appear to have been struggling in the writer's mind, and alternately giving the colour to his language. One is, that motion, or nervous action, as the case may be, is a direct expression of the vital force; and the other, that it is the result of the chemical disintegration of the muscular or nervous tissues. Owing to this cause, the words used virtually assert that the retrograde metamorphosis of the tissues, or their conversion into lifeless compounds, is a result or manifestation of the vital force, which is in its very terms a contradiction.

To these considerations it may be added, that to affirm the function to be the result of the accompanying disintegration, is to adopt the negative side of the argument. It enables us to reject altogether sensibility and contractility, as separate properties of the nervous and muscular tissues, apart from their known tendency to chemical change. And no principle in science is better grounded than that nothing may be assumed to exist without a proved necessity.

The substance of what has been advanced may be briefly stated thus. Dynamically considered, the changes which take place in the inorganic world are divisible. into two classes-those which directly result from the

1 British and Foreign Medico-Chirurgical Review, No. 31.

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