144 THE VITAL FUNCTIONS. 



principally backwards, the posterior portion, being 

 later in forming, is comparatively of a larger size 

 than either the fore or the lateral 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, 

 rather smaller than the others, and are called the 

 tvisdom- teeth ; because they do not usually make 

 their appearance above the gum till the person has 

 attained the age of twenty. In the Negro, how- 

 ever, where the jaw is of greater length, these 

 teeth have sufficient room to come into their places, 

 and are, in general, fully as large as the other 

 molares. 



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 substances ; so that the simple 

 plating of enamel is sufficient to preserve them, 

 even during a long life. But in many herbivorous 

 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 remark- 

 able 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 

 immediately 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 pbice in the IVi/d Boar, and also, though to 



