514 DIGESTION. 



A 



The number of the teeth is sixteen in each jaw. These are divided 

 into classes, according to their shape and use. There are, in each jaw, 

 four incisores; two cuspidati or canine teeth; four bicuspidati ; and six 

 molares or grinders. Each tooth has three parts: the crown, neck, 

 and fang or root; the first being the part above the gum ; the second 

 that embraced by the gum; and the third, the part contained in the 

 alveolus or socket. The crown varies in the different classes. In the 

 incisors, it is wedge-shaped; in the canine, conical; and in the molar, 

 cubical. In all, it is of extreme hardness, but in time wears away 

 by the constant friction to which it is exposed. The incisor and 

 canine teeth have only one root; the molares of the lower jaw, two; 

 and the upper, three. In all cases, they are of a conical shape, the 

 base of the cone corresponding to the corona, and the apex to the bot- 

 tom of the alveolus. The alveolar margin of the jaws is covered by a 

 thick, fibrous, resisting substance, called gum. It surrounds accurately 

 the inferior part of the crown of the tooth, adheres to it strongly, and 

 thus adds to the solidity of the junction of the teeth with the jaws. It 

 is capable of sustaining considerable pressure without inconvenience. 

 But we shall have to return to the subject of the teeth hereafter. 



The articulation of the lower jaw is of such a nature as to admit of 

 depression and elevation; of horizontal motion forwards, backwards, 

 and laterally; and of a semi-rotation upon one of its condyles. The 

 muscles that move it may be thrown into two classes: elevators and 

 depressors. These, by a combination of their contraction, can produce 

 every intermediate movement between elevation and depression. The 

 raisers or levator muscles of the jaw extend from the cranium and upper 

 jaw to the lower, They are four in number on each side, the temporal, 



and masseter, which 



Fig. 2ti are entirely concerned 



in the function; the 

 external pterygoid, 

 which, whilst it raises 

 the jaw, carries it at 

 the same time forward, 

 and to one side; and 

 the internal pterygoid, 

 which, according as it 

 unites its action with 

 the temporal or with 

 Skull of the Polar Bear. the external pterygoid, 



is an elevator of the 



jaw or a lateral motor. The depressors may be divided into imme- 

 diate and mediate, according as they are, or are not, attached to the 

 lower jaw itself. There are only three of the former class: 1, the 

 digastricus, the anterior fasciculus of which, or that which passes from 

 the os hyoides to the lower jaw, depresses the latter; 2, the genio- 

 Jiyoideus; and 3, the mylo-hyoideus, all of which concur in the forma- 

 tion of the floor of the mouth. The indirect or mediate depressors 

 are all those, that are situate between the trunk and the lower jaw, 

 without being directly attached to the latter; as the thyro-hyoideus, 



