142 NATURE AND SOURCES OP 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 familiar to him. 



CHAPTEE 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 



