The Source of Muscular Energy. 



67 



The following table by the same investigator shows the great 

 increase in the carbonic acid gas exhaled with increasing labor: 



Carbonic acid exhaled by the horse at rest and at work Smith. 



These tables show the variation in the amount of carbonic acid 

 given off by the lungs of animals during labor, and the rapid 

 increase as the labor increases. No such findings are on record 

 with regard to the excretion of nitrogen in the urea. The con- 

 clusion is irresistible that the carbohydrates and the fat of the 

 food and of the body, and not protein, are the main sources of 

 body energy. 



95. Body heat does not measure the energy of food. On this 

 subject Wolff writes: 1 u The great increase in the combustion 

 of fat during work has led to the assumption that this constitutes 

 the chief source of muscular energy, that the work done is the 

 result of the heat produced, and that in the animal body a con- 

 version of heat into force takes place, just as the steam-engine 

 produces work through the heat of the burning fuel by the inter- 

 vention of steam, or as the hot-air engine executes work by means 

 of the heated air. The non-nitrogenous food stuffs are directly 

 concerned in this heat production, and it has been calculated that 

 20 per cent, of the heat produced by their combustion is con- 

 verted into work, which is a far larger proportion than that yet 

 attained by the most efficient steam-engines, which only convert 

 about 10 per cent, of the heat they receive into work. It is open 

 to question, however, whether the heat produced in the body can 

 be directly converted into mechanical work as in the case of the 



1 Farm Foods, English edition, pp. 82, 83, Cousins, 



