168 TEETH OF RODENTS. MOTION OF JAWS. 



with pulp. Such have no proper root ; for additional matter 

 is being continually formed at their base, and thus the whole 

 tooth is pushed upwards. This is the case with the Elephant's 

 tusks ; and also with the large teeth that occupy the front of 

 the jaw in Rabbits, Squirrels, Rats, and other gnawing ani- 

 mals (fig. 91). The 

 upper edges of these 

 teeth are being con- 

 stantly worn away by 

 use : and they are 

 kept up to their 

 proper level by the 

 growth of the tooth 



Fig. 91. JAW AND TEETH OF RABBIT. from below. But it 



sometimes happens that one of these teeth is broken off ; and 

 the one opposite to it in the other jaw is then thrown into dis- 

 use. It continues, however, to grow up from below; but, 

 not being worn down at the top, its length increases greatly, 

 so that it may become a source of great inconvenience to the 

 animal 



178. The teeth are but passive instruments in the act of 

 mastication. They are put in movement by the jaws in which 

 they are fixed ; and these are made to act against each other 

 by various muscles. The upper jaw is usually fixed to the 

 head; and has not, therefore, any power of moving inde- 

 pendently of it. But the lower jaw is connected with the 

 skull by a regular joint on either side ; and is so moved by 

 the muscles attached to it, as to cut, crush, or grind the food, 

 according to the nature of the teeth. 



179. There is considerable variety, in different animals, as 

 to the extent of motion which the lower jaw possesses. In 

 the purely Carnivorous quadrupeds, it has merely a hinge-like 

 action, that of opening and shutting ; and by the sharpness of 

 the edges of the molar teeth, it is thus rendered a powerful 

 cutting instrument. But in the Herbivorous animals, which 

 have to grind or triturate their food 1 ., 'tween the roughened 

 surfaces of their molars, such a limited motion would be of 

 no avail ; and we accordingly notice, if we watch an ox or a 

 horse whilst masticating its food, that the lower jaw has con- 

 siderable power of motion from side to side. On the other 

 hand, in the Rodents, or gnawing animals furnished with 



