72 COMPARATIVE ZOOLOGY. 



Vertebrates the food is bolted entire, Mammals masticate 

 it before swallowing. Mastication is more essential in the 

 digestion of vegetable than of animal food ; and hence we 

 find the dental apparatus most efficient in the herbivorous 

 quadrupeds. The food is most perfectly reduced by the 

 ■ Rodents. 



Teeth, as we shall see, are appendages of the skin, not 

 of the skeleton, and, like other superficial organs, are es- 

 pecially liable to be modified in accordance with the hab- 

 its of the creature. They are, therefore, of great zoologi- 

 cal value ; for, sucli is the harmony between them and 

 their uses, the naturalist can predict the food and general 

 structure of an animal from a sight of the teeth alone. 

 For the same reason, they form important guides in the 

 classification of animals; while their durability renders 

 them available to the paleontologist in the detei-mination 

 of the nature and affinities of e.xtinct species, of which 

 they are often the sole remains. Even the structure is 

 so peculiar that a fragment will sometimes suffice. 



4. Deglutition, or How Animals Swallow. — In the 

 lowest forms of life, tlie mouth is but an aperture opening 

 immediately into the body-substance, and tlie food is drawn 

 in by ciliary currents. But in the majority of animals, a 

 muscular tube, called the gullet, or oesophagus, intervenes 

 between the mouth and stomach, the circular fibres of 

 which contract, in a wave-like manner, from above down- 

 ward, propelling the morsel into the stomach." In tlie 

 higher Mollnsks, Arthropods, and Vertebrates, deglutition 

 is generally assisted by the tongue, which presses the food 

 backward, and by a glairy juice, called saliva, which facil- 

 itates its passage through the gullet.'" Vertebrates have 

 a cavity behind the mouth, called the throat, or pharynx, 

 which may be considered as a funnel to the oesophagus." 

 In air-breathers, it has openings leading to the windpipe, 

 nose, and ears. In Man, as in Mammals generally, the 



