96 TEE POPULAR SCIENCE MONTELY. 



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 kilo- 

 gramme-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 vapor- 

 izing 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 re- 

 constructive 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 per- 

 formed by muscle is not at the expense of its nitrogenous sub- 

 stance, and its energy is, to a great extent, if not exclusively, de- 

 rived 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 ele- 

 ments 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 feed- 

 ing rations that have been formulated for the same specific pur- 

 pose, with practically the same nutritive ratio. On the same 

 page of a popular agricultural paper I find two rations for milk- 



