OFFICES OF THE TEETH. 125 



shall presently describe. The original indentations 

 are obliterated as the beak advances in growth ; but 

 they are permanent in the bill of the duck, where 

 the structure is very similar to that above described 

 in the embryo of the parrot. 



§ 3. Mastication by means of Teeth. 



The teeth, being essential instruments for seizing 

 and holding the food, and effecting that degree of 

 mechanical division necessary to prepare it for the 

 chemical action of the stomach, perform, of course, 

 a very important part in the economy of most ani- 

 mals ; and in none more so than in the Mammalia, 

 the food of which generally requires considerable 

 preparation previously to its digestion. There exist, 

 accordingly, the most intimate relations between the 

 kind of food upon which each animal of this class is 

 intended by nature to subsist, and the form, struc- 

 ture, and position of the teeth ; and similar relations 

 may also be traced in the shape of the jaw, in the 

 mode of its articulation with the head, in the pro- 

 portional size and distribution of the muscles which 

 move the jaw, in the form of the head itself, in the 

 length of the neck, and its position on the trunk, 

 and indeed in the whole conformation of the 

 skeleton. But since the nature of the appropriate 

 food is at once indicated by the structure and 

 arrangement of the teeth, it is evident that these 

 latter organs, in particular, will afford to the 

 naturalist most important characters for establish- 

 ing a systematic classification of animals, and more 

 especially of quadrupeds, where the differences 



