586 JOHN E. MURLIN 



Gephart and DuBois(a) in the first twenty experiments with the Sage 

 calorimeter upon normal subjects, some of them in the post-absorptive state 

 and others soon after taking foods of various kinds, reported a total heat 

 production of 4577.37 calories by calculation as against 45G9.4 by direct 

 measurement, a discrepancy of only 0.17 per cent. 



Instances might be multiplied further but it is unnecessary. The 

 potential energy of the food in so far as it is oxidized is returned by the 

 body without loss, in kinetic form; and even when measurable work is 

 done the energy can all be accounted for. 



II. The Energy of Muscular Work is Definitely Related 

 to the Potential Energy of the Food 



1. Origin in Non-Nitrogenous Food. When Liebig had completed his 

 classification of the foodstuffs, and had found that all animal tissues con- 

 tained proteins, i. e., are nitrogenous, he suggested that the excretion of 

 nitrogen by the animal might be used as a measure of protein destruction 

 in the animal's body. Carl Voit, who had been a pupil of Liebig, was 

 among the first to put this suggestion to practical use. Among many 

 other important facts, regarding the metabolism of proteins, Voit discov- 

 ered that, contrary to the teaching of Liebig, the protein of the body is 

 not the source of the muscular energy; for, during muscular work, no 

 more nitrogen is eliminated than in muscular rest. Since it had been 

 known from the time of Lavoisier that muscular exercise increased the heat 

 production, it followed, from the observations of Voit, that the non-nitro- 

 genous foodstuffs must be the source of the extra heat production as well 

 as of the energy of muscular contraction. This fact is now thoroughly \ 

 established by almost numberless experiments (Lusk(A) ). An illustration \ 

 may be taken from the work of At water cited above. A subject doing work 

 on the bicycle ergometer produced in twenty-four hours 5,100 calories of 

 heat, of which 43-1 calories came from the protein (N X 6.25 X 4.1). In 

 muscular rest this same individual produced 2,270 Calories, of which 400 

 came from protein. The day's work had increased the total heat pro- 

 duction 2,830 Calories, but the heat from protein had been increased only 

 thirty-four calories. All of the rest, 2,800 Calories (nearly), came from 

 non-nitrogenous food. 



2. Mechanical Efficiency of Muscular Work. Soon after the law of 

 the conservation of energy was enunciated by Mayer, the mechanical effi- 

 ciency of muscular work done by a horse was computed by Joule. He 

 showed that a horse could perform Avork equivalent to twenty-four million 

 foot pounds in one day, during which time the food consisted of 12 pounds 

 of hay and 12 pounds of corn. From original measurements of the heat 

 value of this food Joule inferred that one grain of food consisting of equal 



