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-cavity, 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. 81 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. 33 Vertebrates have 
a cavity behind the mouth, called the throat, or z>harynx, 
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 
