290 ANIMAL PHYSIOLOGY. 
DIGESTION OF FOOD. 
The processes of digestion may be considered as having 
for their end the preparation of food for entrance into the 
blood. 
This is in part attained when the insoluble parts have been 
rendered soluble. At this stage it becomes necessary to inquire 
as to what constitutes food or a food. 
Inasmuch as animals, unlike plants, derive none of their 
food from the atmosphere, it is manifest that what they take in 
by the mouth must contain every chemical element, in some 
form, that enters into the composition of the body. 
But actual experience demonstrates that the food of animals 
must, if we except certain salts, be in organized form—i. e., it 
must approximate to the condition of the tissues of the body in 
a large degree. Plants, in fact, are necessary to animals in 
working up the elements of the earth and air into form suit- 
able for them. 
Foodstuffs are divisible into : 
I. Organic. 
1. Nitrogenous. 
(a.) Albumins. 
(b.) Albuminoids (as gelatine). 
2. Non-nitrogenous. : 
(a.) Carbohydrates (sugars, starches). 
(0.) Fats. 
II. Inorganic. 
1. Water. 
2. Salts. 
Animals may derive the whole of their food from the 
bodies of other animals (carnivora); from vegetable matter 
exclusively (herbivora); or from a mixture of the animal and 
vegetable, as in the case of the pig, bear, and man himself 
(omnivora). 
It has been found by feeding experiments, carried out mostly 
on dogs, that animals die when they lack any one of the con- 
stituents of food, though they live longer on the nitrogenous 
than any other kind. In some instances, as when fed on gela- 
tine and water, or sugar and water, the animals died almost as 
soon as if they had been wholly deprived of food. But it has 
also been observed that some animals will all but starve rather 
than eat certain kinds of food, though chemically sufficient. 
