410 PRINCIPLES OF ANIMAL NUTRITION. 
availability of the metabolizable energy of the food will be 100 per 
cent. or we shall have isodynamic replacement. Above that 
amount we shall have an availability depending upon the relation 
of the work of digestion and assimilation to the total metabolizable 
energy. 
Grapuic REPRESENTATION.—The critical amount of food will 
depend chiefly upon two things, viz., the distance below the critical 
thermal environment at which the experiment is made and the 
amount of energy that has to be expended in the digestion and 
assimilation of the food. The greater the former quantity, the 
more of the total metabolism of the animal will be due to the “ chemi- 
cal” regulation and therefore capable of being replaced, while the 
greater the work of digestion the less food must be consumed to 
furnish by its digestive work the heat necessary to a complete 
substitution. 
Y 
On the two codrdinate axes OX and OY let distances along OX 
represent the metabolizable energy of the food consumed and dis- 
tances along OY the effect of this food upon the store of potential 
energy in the body. In the first instance, let us take the case of a 
