120 THE CHEMISTEY OF THE FABM. 



to about 363 foot-tons. During hard work, about one- 

 sixth of the total energy developed in a man's body may 

 appear as external work, the rest will appear as heat. 



It was formerly supposed that muscular force wag 

 produced by the oxidation of the nitrogenous constituents 

 of muscle, and that a diet rich in albuminoids was 

 necessary if hard labour was to be maintained. This 

 idea is now known to be erroneous, it having been shown 

 by repeated experiments that labour does not necessarily 

 increase the production of urea, while it does in every 

 case greatly augment the amount of carbonic acid exhaled. 



The energy required for work may be produced in 

 the body by the oxidation of albuminoids, but it may 

 equally be generated by the oxidation of fat or carbo- 

 hydrates. The animal body has been compared to a 

 steam engine, in which food is burnt in place of coal. 



When labour is demanded from an under-fed animal, 

 the oxidation taking place may be in excess of the food 

 supplied ; in such a case the fat and albuminoids of the 

 animal tissues are oxidised, and the excretion of urea 

 becomes increased. A working animal ill supplied with 

 food will thus suffer seriously in condition. 



When an animal " out of training " is suddenly called 

 upon to perform hard work, it will at first show an in- 

 creased oxidation of albuminoids as the result of labour ; 

 but this will cease as the body becomes fit for work, if 

 sufficient food is supplied. Daring training for in- 

 creased work an albuminous diet will be necessary, as 

 the muscular apparatus has to be built up. 



Maintenance Diets. In the case of an adult animal 

 not increasing in weight, and performing a minimum 



