NUTRITION. 45 
The objects of nutrition are growth, repair, and propa- 
gation. The first object of life is to grow, for no animal 
is born finished. Some animals, like plants, grow as long 
as they live ;* but the majority soon attain a fixed size. In 
all animals, however, without exception, food is wanted for 
another purpose than growth, namely, to repair the waste 
which is constantly going on. For every exercise of the 
muscles and nerves involves the death and decay of those 
tissues, as shown by the excretions. The amount of mat- 
ter expelled from the body, and the amount of nourish- 
ment needed to make good the loss, increase with the ac- 
tivity of the animal. The supply must equal the demand, 
in order to maintain the life of the individual; and as an 
organism can make nothing, it must seek it from without. 
Not only are thegmuscles and nerves wasted by use, but 
every organ in the body; so that the whole structure needs 
constant renewal. An animal begins to die the moment it 
begins to live. The function of nutrition, therefore, is con- 
structive, while motion and sensation are destructive. 
Another source of demand for food is the production of 
germs, to propagate the race, and the nourishment of such 
offspring in the egg and infantile state. This reproduc- 
tion and development of parts which can maintain an in- 
dependent existence is a vegetative phenomenon (for plants 
have it), and is a part of the general process of Nutrition. 
But it will be more convenient to consider it hereafter 
(chapters xix., xx.). Still, another necessity for aliment 
among the higher animals is the maintenance of bodily 
heat. This will be treated under the head of Respira- 
tion. 
For the present, we will study Nutrition, as manifested 
in maintaining the life of an adult individual. 
In all animals, this process essentially consists in the 
introduction of food, its conversion into tissue, and the 
removal of worn-out material. 
