MOVEMENTS OF LOWER JAW. 
169 
two large front teeth, the lower jaw has no power of moving 
from side to side, but is rapidly drawn backwards and for¬ 
wards ; and, as the ridges of the molar teeth are arranged in 
the opposite direction, they become very powerful filing in¬ 
struments, by -which the toughest vegetable substances are 
quickly reduced. 
180. In the Human jaw, there is a moderate power of 
motion in all these different directions; and it is furnished 
with all the muscles by which they are effected in the 
different animals that perform them ; but these are not so 
large or strong. The most powerful of the muscles of the 
lower jaw, in all animals, is that by which it is drawn up 
against the upper, so as to close the mouth. This arises from 
the side of the skull in the region of the temple, and is hence 
called the temporal muscle. It covers at its origin a large 
surface of bone; but its fibres approach one another as they 
descend, and pass under a bony arch (which may be felt 
between the cheek and the ear), to attach themselves to 
a process or projection of the 
lower jaw (a, fig. 92), about 
an inch in front of the joint. 
As the distance from the ful¬ 
crum of the point a, at which the 
power is applied, is thus much 
less than that of the front of 
the jaw b, where chiefly the 
resistance is encountered, the 
pow r er of the muscle is applied 
at a mechanical disadvantage; 
and, to overcome a given resist¬ 
ance, the muscle must itself be 
several times more powerful. 
Thus the Tiger and Lion, which 
can lift and carry away the bodies of animals weighing several 
hundred pounds, must possess temporal muscles that shall 
contract with a force of two thousand, or even more. 
181. In Man, as in most of the other Mammalia, there are 
three kinds of teeth, adapted for different purposes. The 
first terminate in a thin cutting edge, and are intended simply 
to divide the food introduced into the mouth ; these are termed 
incisor teeth (fig. 93). Others have more of a conical form,, 
Fig. 92 .—Human Skull. 
