10 PRINCIPLES OF ANIMAL NUTRITION. 
contained in the food. This method may of course be applied 
either to the dry matter or the organic matter of the food as a 
whole or to any single determinable ingredient. 
' Merazotic Propucts.—The digestive tract of an animal, how- 
ever, not only serves as a mechanism for the digestion of food but 
has excretory functions as well, and the rejected matter contains, 
besides the undigested portion of the food, these excreta and the 
metabolic products of intestinal action. In the case of food largely 
or completely digestible, these substances may make up the larger 
portion or even the whole of the feces, while, on the other hand, 
* they constitute but a small proportion of the bulky excreta of 
herbivora. 
It is obvious that these products must be taken account of if it is 
desired to learn the actual digestibility of the food. Unfortunately, 
however, we have at present no trustworthy method for their deter- 
mination. In the past it has been customary to designate the 
difference between food and feces as digestible and, in the case of 
domestic animals at least, to assume that the error involved is not 
serious. 
Apparent Digestibility—Availability—Even with herbivo- 
rous animals, however, the presence of the so-called metabolic 
products in the feces may give rise to serious errors in the deter- 
mination of the real digestibility of some ingredients of the food, 
notably fat and protein. With carnivora, or with the human 
subject, the case is for obvious reasons still worse, and it is 
scarcely possible to determine the digestibility of the food in the 
strict sense of the word. 
The difference between food and feces does represent, however, 
the net gain of matter to the organism resulting from the digestion 
of the food. To express this conception, the use of the word avail- 
able has been proposed by Atwater.* The “available nutrients” of 
a food, according to him, are the actually digestible nutrients minus 
the metabolic products contained in the feces and which may be 
regarded as representing the expenditure of matter, in the form of 
residues of digestive fluids, intestinal mucus, epithelium, ete. 
necessarily incident to the digestion of the food. The term has been 
* Storrs (Conn.) Agr’] Expt. St., Rep., 12, 69. 
