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Page:Popular Science Monthly Volume 24.djvu/400

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the material which by its decomposition produces the force for muscular work is finally decomposed, with evolution mainly of carbonic acid and water. They differ in their views of the nature of the process and the steps by which these ultimate products are obtained.

We have here endeavored to show briefly what has been gained in comparatively recent times by the growth of knowledge in regard to the source of muscular power. Let us attempt a brief summary of the main points brought forward in the preceding discussion: 1. The source of muscular energy is in the chemical decomposition of certain substances, which is accompanied with a release of energy. 2. The muscular contraction produces a greatly increased production of carbonic acid and water, and an increased consumption of oxygen, in the general respiration. To what extent this is due to the mere muscular contraction, to what extent to the influence of muscular exercise on other functions, is difficult to estimate with certainty. 3. The excised muscle, when caused to contract, gives off carbonic acid, and this action is in great part independent of a simultaneous absorption of oxygen. 4. The blood coming from the contracting muscle contains more carbonic acid and less oxygen than that coming from the resting muscle, and less oxygen than that coming to the contracting muscle. 5. The ratio of carbonic acid given off to oxygen taken up is increased by muscular exertion. 6. The nitrogen elimination is but slightly increased during muscular exertion. No considerable amount of nitrogenous muscular tissue is consumed. 7. The immediate fuel-material is mainly non-nitrogenous and carbohydrate in its character, probably in part at least derived from the muscle-glycogen, and perhaps from some other substances stored in some manner in the muscular tissue, possibly also to some extent from sugars conveyed to the tissues by the blood. 8. It is not certain to what extent this glycogen or other non-nitrogenous fuel-material is derived from nitrogenous or albuminoid material during rest or repose of the muscles, but such an origin, for a portion at least of the fuel-material, has some evidence in its favor. 9. The nature of the decomposition of this fuel-material is as yet an unsettled matter. The older theory of direct oxidation has been to a great extent replaced by the more modern theory of fermentative decomposition, i. e., splitting up by combination with water into simpler products with an accompanying release of energy, and this process followed by secondary oxidations exerted by the oxygen of the blood. Satisfactory experimental evidence for deciding with respect to these theories as yet fails us.

In conclusion, it is well, however, to recollect that at best the questions touched upon are but secondary to the more fundamental question upon which no investigation has as yet thrown even the most dim and feeble light, viz., "What is muscular force?" It seems impossible to conceive how a collection of cells with thin, elastic walls, and filled with a fluid or semi-fluid mass, can contract in such a way as to mani-