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, such 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 determination 

 of the nature and affinities of extinct 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, the mouth is but an aperture opening 

 immediately into the body-substance, and the 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. 31 In the 

 higher Mollusks, 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. 32 Vertebrates have 

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

 which may be considered as a funnel to the oesophagus. 33 

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

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



