142 NATURE AND SOURCES OP ANIMAL FOOD. 
138. We thus conclude our general survey of the Animal 
Kingdom; which, it is hoped, will he found to answer the 
purpose for which it was designed,—that of giving such an 
amount of preparatory knowledge respecting the principal 
types of animal structure, as may enable even the beginner to 
comprehend what will hereafter be stated of their physiological 
actions. It has not been attempted to observe any proportion 
in the notice of these several types ; the higher forms having 
been slightly passed over, because the details of their vital 
phenomena will constitute the principal subject of the follow¬ 
ing pages; whilst some among the lower have been more 
fully treated, because the ordinary reader cannot be expected 
to have even that outline-acquaintance with their nature and 
actions, which he can scarcely help possessing in the case of 
animals that are familiar to him. 
CHAPTER III. 
NATURE AND SOURCES OF ANIMAL FOOD. 
139. Before we examine the nature of the process by 
which the food of animals is prepared for absorption into 
their bodies, it will be desirable to consider the characters of 
the aliment itself, and the purposes to which it is to be appro¬ 
priated. The term food or aliment may be applied to all 
those substances which, when introduced into the living 
body, serve as materials for its growth, or for the repair of 
the losses which it is continually sustaining (§ 55). When 
animals are deprived of these materials, we see their bodies 
progressively diminishing in bulk, their strength decreases, 
and death at last takes place, after sufferings more or less 
prolonged. In warm-blooded animals, however, a yet more 
urgent demand for food is created by the requirements of the 
heat-producing process ; and many substances are fitted to 
supply this, which cannot serve for the nourishment of the 
tissues. 
140. The demand of the body for food is made known by 
a peculiar sensation, which has its seat in the stomach, namely, 
hunger. It is increased by mental and bodily exercise, and 
