Page:Popular Science Monthly Volume 12.djvu/756

736 exercise of muscular power immediately involves the destruction of a certain amount of muscular substance, of which the nitrogen excreted is a measure. Indirectly, nitrogenized food is a source of power, as, by its assimilation by the muscular tissue, it repairs the waste and develops the capacity for work; but food is not directly converted into force in the living body, nor is it a source of muscular power, except that it maintains the muscular system in a proper condition for work. In ordinary daily muscular work, which may be continued indefinitely, except as it is restricted by the conditions of nutrition and the limits of age, the loss of muscular substance produced by work is balanced by the assimilation of alimentary matters. A condition of the existence of the muscular tissue, however, is that it cannot be absolutely stationary, and that disassimilation must go on to a certain extent, even if no work be done. This loss must be repaired by food to maintain life. A similar condition of existence applies to every highly-organized part of the body and marks a broad distinction between a living organism and an artificially constructed machine, which latter can exert no motive power of itself, and can develop no force that is not supplied artificially by the consumption of fuel or otherwise."

Prof. Flint, in an appendix, has added a calculation of the non-nitrogenized food taken by Weston during his five days' walk, in order "to answer the possible objections of those who may contend that, in his discussion, he should have included the heat-producing and force-producing power of non-nitrogenized alimentary substances." This calculation is briefly as follows:

"The actual work represented by walking three hundred and seventeen and one-half miles is estimated at 4,321.33 foot-tons. This leaves 439.56 foot-tons of work which cannot be accounted for in any way, according to the estimates of the observers quoted, leaving a deficiency of a little more than ten per cent.

"These calculations show the fallacy of such estimates, and the impossibility of accounting for work actually performed, even when we include the heat-value and the force-value of non-nitrogenized food."

The estimates of the force used in circulation and respiration, and of the heat produced by the body, were all calculated for a condition of rest, and they are much less than the estimates that should be made for a period of excessive muscular exercise.