Page:Popular Science Monthly Volume 12.djvu/753

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THE SOURCE OF MUSCULAR POWER.

miles in seventy-five consecutive hours; 3. A walk of four hundred and fifty miles in six consecutive days. The proportionate excretion of nitrogen was estimated for these periods of exercise, and was also calculated for eight days of rest. The daily average excretion of nitrogen for the eight days of rest was 60.90 parts for every 100 parts of nitrogen of food. The daily average excretion of nitrogen for the eleven days of walking was 87.34 parts for every 100 parts of nitrogen of food—an increased proportion of 43.44 per cent. These experiments, like those of Prof. Flint, show a very great increase in the proportionate excretion of nitrogen produced by the excessive and prolonged muscular exertion.

Dr. Pavy admits, as the result of his own experiments, the simple fact that muscular exercise increases the proportionate excretion of nitrogen, but he does not accept the view advanced by Prof. Flint, that the muscular system, in exerting force, consumes its own substance, and that this substance is repaired by food. Dr. Pavy made a series of calculations, in connection with his experiments, comparing the force-value of the excess of nitrogen excreted during exercise over the nitrogen excreted during rest with the work actually performed in walking. He attempted to show that the force represented by this excess of nitrogen excreted would not account for the work accomplished. These calculations of Dr. Pavy involve formulas for reducing miles walked to foot-pounds, and estimates of the force exerted in respiratory movements, the action of the heart, etc. Prof. Flint, in his essay, gives an elaborate review of these calculations, and objects to many of the formulæ as necessarily inaccurate. It is impossible, in a short abstract, to give a satisfactory account of Prof. Flint's argument upon these points. The following are the conclusions arrived at by Prof. Flint, as the result of the various experiments which he has discussed:

 
"I. While the various elements of food burned in oxygen out of the body will produce a definite amount of heat which may be calculated as equivalent to a definite number of foot-pounds of force, the application of this law to the changes which food or certain of the constituents of the body undergo in the living organism is uncertain and unsatisfactory, for the following reasons:

"(a.) There is no proof that the elements of food undergo the same changes in the living body as when burned in oxygen, or that definite amounts of heat or force are necessarily manifested by their metamorphoses in such a way that they can be accurately measured.

"(b.) Assuming that the elements of food contain a definite amount of locked-up force, to measure the part of this force which is expended in muscular work, it is indispensable to be able to estimate accurately the force used in circulation, respiration, and the various nutritive processes, and to measure the heat evolved which maintains the standard animal temperature and which compensates the heat lost by evaporation from the general surface. It does not seem that any accurate idea can be formed of the amount of force used in circulation and respiration, and the estimates made by different observers of authority