72 HANDBOOK OF PHYSIOLOGY. 



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, 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 that each tooth 

 antagonizes 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, therefoi'3, 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 an- 

 tagonists. 



It is worthy of note that from the age of four years to the shedding 

 of the first milk-tooth the child has no fewer than forty-eight teeth, 

 twenty milk-teeth and twenty-eight calcified germs of permanent teeth 

 (all in fact except the four wisdom teeth, which show no signs of devel- 

 opment until the third year). 



Structure of a Tooth. 



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

 or roots. 



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 be- 

 low this. 



On making longitudinal and transverse sections through its centre 

 (fig. 73, A, B), a tooth is found to be principally composed of a hard 

 material, dentine or ivory, which is hollowed out into a central cavity 

 which resembles in general shape the outline of the tooth, and is called 

 the pulp cavity from its containing the very vascular and sensitive pulp. 



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

 nerves, and large numbers of cells of varying shapes, e.y., fusiform, stel- 

 late, and on the surface in close connection with the dentine a specialized 

 layer of cells called odontoblasts, which are elongated columnar-looking 

 cells with a large nucleus at the tapering ends or those farthest from 



