106 SCIENTIFIC FEEDING OF ANIMALS 



(b) The nitrogen- free nutrients (fats and carbo- 

 hydrates) as a source of muscular energy. 



Whenever comparisons were made with animals, 

 of whatever species, it was always found that the 

 performance of work caused a considerable increase 

 in the excretion of carbon dioxide, even where no 

 more protein was split up than when the animal 

 was at rest. This shows at once that the nitrogen- 

 free substances must take part in the production 

 of muscular energy, for the energy which an animal 

 requires for the performance of work can only come 

 from the decomposition of substances in the body. 

 If it is found that during work an animal uses 

 only small amounts of protein but more nitrogen- 

 free substances than when at rest, it is safe to con- 

 clude that the energy comes, at least partially, from 

 the extra nitrogen-free substances. Numerous in- 

 vestigations lead to the same conclusion, and a few 

 examples of the results obtained with horses may 

 be quoted here. 



I. In the experiment just quoted, where the 

 animal received a ration (hay and beans) rich in 

 proteins, more and more of these were split up, 

 until at the end 280 gr. more per day were decom- 

 posed than during light work. When the same 

 animal was given a ration (hay and maize) poor 

 in protein but richer to about three kilos in fats 

 and carbohydrates, the excretion of nitrogen and 

 the body weight remained unaltered whether the 



