7 2 THE CONNECTIVE TISSUES. [CH. v. 



the other teeth ; cases of whole families in which their absence is 

 a characteristic feature are occasionally met with. 



When the teeth are fully erupted it will be observed that the 

 upper incisors and canines project obliquely over the lower front 

 teeth and the external cusps of the upper bicuspids and molars lie 

 outside those of the corresponding teeth in the lower jaw. This 

 arrangement allows to some extent of a scissor-like action in 

 dividing and biting food in the case of incisors ; and a grinding 

 motion in that of the bicuspids and molars when the side to side 

 movements of the lower jaw bring the external cusps of the lower 

 teeth into direct articulation with those of the upper, and then 

 cause them to glide down the inclined surfaces of the external 

 and up the internal cusps of these same upper teeth during the 

 act of mastication. 



The work of the canine teeth in man is similar to that of his 

 incisors. Besides being a firmly implanted tooth and one of 

 stronger substance than the others, the canine tooth is important 

 in preserving the shape of the angle of the mouth, and by its 

 shape, whether pointed or blunt, long or short, it becomes a 

 character tooth of the dentition as a whole in both males and 

 females. 



Another feature in the fully developed and properly articulated 

 set of teeth is that no two teeth oppose each other only, but each 

 tooth is in opposition with two, except the upper Wisdom, usually 

 a small tooth. This is the result of the greater width of the 

 upper incisors, which so arranges the "bite " of the other teeth 

 that the lower canine closes in front of the upper one. 



Should a tooth be lost, therefore, it does not follow that its 

 former opponent remaining in the mouth is rendered useless and 

 thereby liable to be removed from the jaw by a gradual process 

 of extrusion commonly seen in teeth that have no work to perform 

 by reason of absence of antagonists. 



Structure of a Tooth. 



A tooth is generally described as possessing a crown, neck, and 

 root. 



The crown is the portion which projects beyond the level of the 

 gum. The neck is that constricted portion just below the crown 

 which is embraced by the free edges of the gum, and the root 

 includes all below this. 



Ou making longitudinal and transverse sections through its 

 centre (figs. 93, 94), a tooth is found to be composed of a hard 



