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 oi 

 the jaw b, where chiefly the 

 resistance is encountered, the 

 power 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. 



