
70 STORRS AGRICULTURAL EXPERIMENT STATION. 
material or fuel, and hence are not available in the sense in 
which the word is here employed. The amounts of available 
nutrients are found by subtracting the ingredients of the feces 
from the corresponding ingredients of the food. ‘The term 
digestible has often been used in this sense, but the distinction 
here made is evidently an important one. How the available 
nutrients of the food are actually utilized in any given case: 
how much benefit the body gains from a given amount of 
nutrients in any given diet, is another matter. 
AVAILABILITY-OF ENERGY. 
We have spoken of the metabolic products as residues from 
materials used to digest the food and thus make it available 
for general uses. ‘Their energy is not metabolized. In both 
material and energy they represent part of the cost of making 
the food available. But the process of digestion involves cer- 
tain mechanical operations, especially the chewing of the food 
and the peristaltic movement which accompanies the passage 
of the food through the alimentary canal. For these a certain 
amount of mechanical energy is required. More or less energy 
is still further used in the secretion of the digestive juices, and 
finaily there are the processes of cleavage and synthesis involv- 
ing more or less transformation of energy. The energy actu- 
ally used in all these operations, which belong mainly to the 
general process of digestion, comes from the part of the food 
which we have designated as available. Since it is used for 
digestion it is not available for the other work of the body. 
The available energy, using the word available in the broadet 
sense, includes the energy required for the work of digestion. 
This energy of digestive work has not yet been exactly meas- 
ured. Reasonably close approximations have been made, nota- 
bly by Zuntz and Hagemann, with domestic animals. With 
feeding stuffs containing large amounts of cellulose and other 
undigestible materials, the energy required for digestive work 
was found to be very large. In the digestion of straw and the 
poorer qualities of hay this cost of handling the coarse material 
which resists the action of the digestive juices is so large as to 
require a not inconsiderable share of the total available energy. 
With the concentrated feeding stuffs the amount of energy 
required for digestive work is much smaller. | 
Iyxperiments for determining the amount of energy required 
for the digestion of food by man have not been made. ‘The 

