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 anv TEETH oF Rarsit, 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 
haye to grind or triturate their food between 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 
