NITROGENOUS FOOD AND WORK 245 



nitrogenous organic matter. In other words, the non-nitrogenous 

 compounds are the main items to be taken into account in 

 making up the value of a cattle food, which value cannot be 

 estimated on a basis of its nitrogen content only. 



II. RELATION OF NITROGENOUS FOOD TO WORK. 



The very special importance that was originally attached to 

 the nitrogenous constituents of food was also seen in the views of 

 Liebig with regard to the source of the work, either external or 

 internal, performed by an animal. He put forward the view 

 that the amount of work done was determined by the amount 

 of nitrogenous material transformed in the body, and therefore 

 that it could be measured by the amount of nitrogen appearing 

 in the urine, since the albuminoids and other nitrogen com- 

 pounds in food which are digested and undergo change in 

 the animal are excreted as urea. Lawes and Gilbert, by their 

 studies of human dietaries, were led to conclude that this 

 view was mistaken, and that the fats and carbohydrates, 

 which are oxidised and leave the body in the respiration 

 products, supply the energy for the work performed in and 

 by the body. Two experiments with pigs, carried out in 1854 

 and 1862 respectively, were adduced as further evidence. The 

 pigs were confined in a frame ; the nitrogen in the food and the 

 nitrogen excreted in urine and faeces respectively were 

 determined. The food was so adjusted that one pig received 

 about twice as much nitrogen as the other (Table LXXXVIL). 



The animals were obviously under equal conditions as 

 regards exercise, both being at rest, yet in each experiment the 

 animal receiving the highly nitrogenous diet excreted rather 

 more than twice as much nitrogen as urea. Thus the amount 

 of nitrogen in the urine, which measures the amount of 

 albuminoids oxidised, could hardly be taken as a measure of 

 the amount of work performed by the respective animals. 



The question was afterwards systematically attacked in 

 various directions by other investigators, and direct proof 

 was obtained that the energy required to carry on work is 



