NET AVAILABLE ENERGY—MAINTENANCE. 411 
fasting animal and suppose the thermal environment to be at the 
critical point. The distance OA may then represent the loss of 
potential energy (tissue) from the body caused by the internal work. 
If now we supply the animal with food 80 per cent. of whose met- 
abolizable energy is available, with any given amount of energy 
thus supplied, as OB= AC, 80 per cent. of that energy, represented 
by CD, will serve to maintain the store of potential energy in the 
body, while 20 per cent., or DB’, will be absorbed by the work of 
digestion, étc., and converted into heat. Accordingly if we assume 
that the work of digestion is proportional to the amount of food 
eaten, the line AD will indicate the availability of the particular 
food and may be represented algebraically by the equation 
y=az, 
in which a=tan DAC =the percentage availability. 
We may also represent the heat production on the same axes. 
With no food it will be OF equal to OA. With an amount of food 
equal to OB it will be equal to OH+DB’=BF, and the line EF, 
expressed algebraically by . 
y=(1—a)z, 
will represent the law of heat production. 
Let us next suppose that, the animal being again deprived of food, 
the external demand for heat is increased, by a fall of temperature, 
e.g., and that to meet this demand the metabolism is increased by 
an amount AG, and the heat production consequently by the equal 
amount EH. If we now give the same food as before, its real availa- 
bility will be unchanged and will be represented by the line GJ, 
parallel to AD. Up to the critical amount of food, however, the 
heat resulting from the digestive work will, as we believe, be sub- 
stituted progressively for that represented by HH and resulting 
from the metabolism AG. The apparent availability, therefore, 
will be represented by the line GK, making an angle of 45° with the 
axes, and the heat production by the line HL, parallel to OX. 
When the food consumed reaches an amount OM at which the line 
GK intersects AD, the limit of this substitution is reached, since 
the amount of digestive work, KN, equals the amount of additional 
metabolism AG caused by the fall in temperature. In other words, 
