118 THE VITAL FUNCTIONS. 



the absorption of the fangs of the temporary teeth, and, push- 

 ing them out, occupy tlieir places.^ 



As the jaw bone, during its growth, extends principally 

 backwards, the posterior portion, being later in forming, is 

 comparatively of a larger size than either the fore or the la- 

 teral parts; and it admits, therefore, of teeth of the full size, 

 which, consequently, are permanent. The molar teeth, 

 which are last formed, are, for want of space, ratlier smaller 

 than the others, and are called the wisdom-teeth, because 

 they do not usually make their appearance above the gum 

 till the person has attained the age of twenty. In the negro, 

 however, where the jaw is of greater length, these teeth have 

 sufficient room to come into their places, and are, in gene- 

 ral, fully as large as the other molars. 



The teeth of carnivorous animals are, from the nature of 

 their food, less liable to be worn, than those of animals 

 living on grain, or on the harder kinds of vegetable sub- 

 stances; so that the simple plating of enamel is sufficient to 

 preserve them, even during a long life. But in many herbi- 

 vorous quadrupeds we find that, in proportion as the front 

 teeth are worn away in mastication, other teeth are formed, 

 and advance from the back of the jaw to replace them. This 

 happens, in a most remarkable manner, in the elephant, and 

 is the cause of the curved form which the roots assume; for, 

 in proportion as the front teeth are worn away, those imme- 

 diately behind them are pushed forwards by the growth Of 

 a new tooth at the back of the jaw; and this process goes on 

 continually, giving rise to a succession of teeth, each of 

 which is larger than that which has preceded it, during the 

 whole period that the animal lives. A similar succession of 

 teeth takes place in the wild boar, and, also, though to a less 

 extent, in the Sus Q^thiopicusA This mode of dentition 



• It Is stated by Rousseau that the shedding- of the first molar tooth both 

 of the Guinea-pig, and the Capihara, and its replacement by the permanent 

 tooth, take place a few days before birth. Anatomic Compai-ce du systeme 

 dentaire, p. 164-. 



t Home, Phil. Ti-ans. for 1799, p. 237; and 1801, p. 319. 



