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Page:Popular Science Monthly Volume 24.djvu/395

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THE SOURCE OF MUSCULAR ENERGY.

ment, the general fact remains the same, that the quantities of carbonic acid and water eliminated during work are much greater than during rest, in many cases the ratio being as high as two to one. It is also found that the oxygen taken up, though increased during muscular exercise, is not increased in proportion to the carbonic acid eliminated. The result is, that the ratio of the volume of oxygen consumed to the volume of carbonic acid eliminated, which is normally somewhat less than unity, tends to approach unity during muscular work. It should be here remarked that investigations dealing with total respired gases, although doubtless in the main reliable, are not without certain defects. If we could be certain that muscular exercise left all other organic functions unaffected, we could safely attribute the observed changes to the muscular contraction alone. But such is probably not the case. The functions of organs are influenced by the activity of others, and hence the changes noticed in products of elimination or in the consumption of oxygen can not with safety be attributed solely to the muscular work performed, as these substances are consumed or produced by the combined activity of all the living tissues of the organism. Hence the value of the corroborative testimony of the other methods of investigation noticed above.

The influence of muscular exertion on the elimination of nitrogen has also received much attention, inasmuch as the nitrogen eliminated (mainly in the form of urea by the kidneys) may be taken as a measure of the amount of nitrogenous food or tissue decomposed in the organism. The influence, then, of muscular exertion on the excretion of nitrogen is of importance as showing also its influence on the decomposition of albuminoids (foods or tissues). The results of the numerous investigations on this subject have been somewhat at variance. Many have found no material increase in the elimination of nitrogen during muscular exertion; others find a slight increase, but not sufficient to indicate any immediate relation of the nitrogen eliminated to the work performed. Passing over the work of earlier investigators, we will consider briefly the results of some of the later investigators. Voit was one of the first to make careful and exact experiments extending over a considerable period of time, and he determined that the increase in elimination of nitrogen during muscular exertion is very slight; that it bears no constant relation to the work done, and is more influenced by diet than by work. Fick and Wislicenus made an ascent of the Faulhorn in the Alps, with the purpose of determining the possibility or impossibility of albuminoids being the fuel-material for muscular power. They estimated the mechanical work necessary to raise their own bodies through the vertical distance to which they ascended. They then calculated the amount of albuminoids necessary to produce so much force by its combustion. They determined experimentally the amount of nitrogen in their excreta during the period of the ascent, and, having taken no nitrogenous food during that period,