X.] 



MOUSE RATIONS 



201 



we have explained before, with the coarse fodders much 

 of the energy is spent in the work of digestion, so that 

 only a portion is available for work, though all the heat 

 value is available for maintenance. In the ordinary 

 horse ration, we may deduct about lo per cent from the 

 digestible food constituents reckoned as starch in order 

 to obtain figures comparable to those given in the 

 bottom line of the table. In a later chapter more exact 

 figures will be given for the starch equivalent 



Table XX.— Energy Requirements of Working Horses. 



So far, the food has only been considered as a source 

 of energy, and this is much its most important function, 

 so that the best measure of the value of a food is the 

 amount of available energy it possesses. Food, how- 

 ever, has also to effect the renewal of the tissues of the 

 body, and this function must be considered indepen- 

 dently of the supply of energy. 



An animal cannot be maintained without a certain 

 minimal supply of protein containing nitrogen, and 

 though the protein can supply energy and when in 

 excess can even give rise to fat, its prime purpose is to 

 repair the nitrogenous waste of the tissues, and for this 

 no other nutrient will suffice. Proteins give off heat 

 when burnt in the body and so can take the place 

 of fats or carbohydrates, but as these two latter con- 



