CH. V.] THE TEETH 67 



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 articula- 

 tion 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. 



On making longitudinal and transverse sections through its centre 

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

 dentine or ivory, which is moulded around a central cavity which 

 resembles in general shape the outline of the tooth ; the cavity is 

 called the pulp cavity from its containing the very vascular and 

 sensitive pulp. 



The tooth-pulp is composed of loose connective tissue, blood-vessels, 



