THE TEETH. 



229 



FIG. 57. 



Description of a Tooth. Each tooth consists of a crown, 

 neck, and root or fang. The crown is external, naked,'has 

 no investment of periosteum like the bones, hut is covered 

 by a substance called enamel. The neck is the contracted 

 portion of the tooth, surrounded by the gum; while the 

 root, firmly fixed in the alveolar walls, is covered by the 

 periosteum reflected from the alveoli, and has its apex 

 perforated for the entrance of a nerve and artery. 



Description of each class of Teeth, beginning with the 

 permanent division. The Incisors (incido, to cut,) are sit- 

 uated in the anterior and central alveoli of the upper and 

 lower jaw. They are four 

 in number in each jaw 

 two central and two lat- 

 eral. The crown of each 

 is wedge-shaped and 

 sharp, smooth and con- 

 vex in front, concave be-j 

 hind, and covered with a 

 thick coating of enamel. 

 The cutting edge is the, 

 widestpart, and continues 

 narrowing to the extremi- 

 ty of the fang. The root 

 has a conical form is 

 single, and flattened on 

 its side, and has an opening at its apex for the passage of 

 the vessel and nerve to the pulp. Those of the upper jaw 

 are larger and stronger than those of the lower. The 

 central incisors of the upper jaw are larger than the lat- 

 eral; while in the lower, the lateral are larger than the 

 central, though the difference is slight. 



The superior central incisors are the Iroadest of all t*he 

 front teeth, measuring about four lines in their crowns. 

 The inferior central incisors are, on the contrary, the small- 

 est, measuring only about two lines and a half. 



FIG. 57, a a a a Anterior view of the Incisors ; 6 6 6 b Posterior view ; 

 c c c Lateral view. 



