Page:Popular Science Monthly Volume 15.djvu/833

 energy equivalent to that which coiled it. One of the forms which this energy takes on is that of muscular motion, which we thus trace back to the potential energy of food, and through this to that great source of all energy to our earth, the sun.

We are not, however, satisfied with knowing in this general way that it is the food we eat which serves as a vehicle to convey to us our needful supply of sun-force. We were already acquainted with the necessity for food, but we wish to know which of the ingredients of our food performs this function, or, if all do it, which one performs it to the best advantage.

Until a comparatively recent date it was assumed unhesitatingly that the albuminoids—that is, bodies like albumen (white of egg), fibrine (muscular fiber), caseine (the basis of cheese), etc., which contain the element nitrogen as a characteristic ingredient, and which we shall designate collectively as proteine—were the proximate source of muscular power. It was taught that work was performed by means of an increased oxidation of the fibrine, of which the muscles are largely composed, and that the proteine of the food served to repair the wear thus caused. This view is still found in many especially of the smaller text-books of physiology, and seems to be the one generally current. Even so eminent a physiologist as Professor Austin Flint, Jr., has recently devoted a small book ("The Source of Muscular Power," D. Appleton & Co., 1878) to its defense; but nevertheless it was never founded upon experimental evidence, and has now been rendered untenable in its original form.

Karl Voit, of Munich, was the first to make exact experiments on this subject, and in 1860 he published the results of his researches, which showed conclusively that, contrary to the then generally accepted theory, muscular exertion did not increase in the least the amount of proteine decomposed in the body, although it was accompanied by a large increase in the amount of non-nitrogenous matters oxidized. This fact was immediately accepted by many physiologists as a proof that the commonly received view of the source of muscular power was incorrect, and that that power was in reality derived from the non-nitrogenous components of food—its fat, starch, sugar, etc. According to them, the muscles are, like a steam-engine, simply an apparatus for the transmission of energy furnished by some other substance, while the fat, etc., is the fuel of the living machine.

Voit and his followers, on the contrary, still hold that proteine is the proximate source of muscular power, though their views have naturally been materially modified by the experimental results just mentioned.

Voit compares the constant decomposition of proteine which goes on in the body to the constant flow of water in a stream. A mill situated by the stream may use the whole power of the water, a half, a quarter, or any desired fraction, without in the least altering the