246 



THE FEEDING EXPERIMENTS 



derived from the oxidation of the food constituents, either 

 albuminoids, fats, or carbohydrates being available for the 

 purpose, though as a rule the two latter are utilised. The 



TABLE LXXXVII. Experiments cut Eothamsted with Pigs in 1854 and 

 1862. Quantities per head per day. 



amount of energy obtainable from each food can be directly 

 measured by the heat it will generate when burnt, and provided 

 the animal receives enough nitrogenous material to repair the 

 normal waste of tissue, the energy required to do work can be 

 wholly derived from the combustion of non-nitrogenous 

 materials. However, when the output of work has to be rapid 

 and at high pressure, it has been found advisable to include a 

 fairly high proportion of easily digestible and concentrated 

 albuminoids in the food ; as Lawes and Gilbert put it in 1852, 

 "a somewhat concentrated supply of nitrogen does, however, 

 in some cases, seem to be required when the system is over- 

 taxed." 



III. THE SOURCE OF FAT IN THE ANIMAL BODY. 



The source of the fat stored up in animals, or given out as 

 milk, was also for a long time a matter of considerable contro- 



