142 NATURE AND SOURCES OF ANIMAL FOOD, 
138. We thus conclude our general survey of the Animal 
Kingdom ; which, it is hoped, will be 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 Tamniliar to him. : 
CHAPTER III. 
NATURE AND SOURCES OF ANIMAL FOOD. 
139. Brerore 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. Tt is increased by mental and bodily exercise, and 
