Page:Popular Science Monthly Volume 41.djvu/106

96 skin and lungs, and in the internal work performed by the metabolic tissues in their constructive processes of nutrition, and the energy used in this internal work finally leaves the body as animal heat, a very large proportion of which is the result of muscular and glandular metabolism.

The work performed in twenty-four hours by the heart alone of a man weighing 150 pounds is estimated at 75,000 kilogramme-metres, an expenditure of energy sufficient to raise his own weight to a height of 3,600 feet, and the work performed by other internal organs, and in vaporizing the water thrown off by the skin and lungs, is quite as significant.

The energy expended in some of the unobtrusive operations of Nature that are likely to escape attention may exceed in amount the more obvious expenditures in mechanical work. We readily recognize the demands for energy by an animal moving a heavy load when working eight or ten hours a day, while we fail to notice that from two to three times as much energy is expended by the same animal in the course of twenty-four hours in vaporizing the water thrown off by the lungs and skin. As this energy is all derived from the food consumed, it must be taken into the account as a significant factor in discussing the physiology of nutrition.

Another important fact should not be overlooked. In the reconstructive processes that are carried on without intermission in the living tissues of the animal machine, a supply of energy, as we have seen, must be constantly provided to replace that which is thrown off from the system in the form of heat, or expended in vaporizing water and in external work; but new materials are not required to replace all the disintegrated constituents of the tissues, as there is a rearrangement, to a certain extent, in the processes of repair of the elements of which they are composed. This is especially the case with muscle, which constitutes so large a proportion of the proteid substance of the body. The work performed by muscle is not at the expense of its nitrogenous substance, and its energy is, to a great extent, if not exclusively, derived from the carbohydrate elements of the food. The demands of the proteid substance of muscle for nitrogen are, therefore, limited, and the available supplies of energy in the various elements of the food determine the efficient activity of the animal machinery.

Energy as a factor in animal physics seems to be entirely overlooked in the application of the popular theory of nutritive ratios. There is a wide difference in the potential energy of feeding rations that have been formulated for the same specific purpose, with practically the same nutritive ratio. On the same page of a popular agricultural paper I find two rations for milk-