TEETH. 5 r 



embraced by the free edges of the gum, and the fang 

 includes all below this. 



On making a longitudinal section through the centre 

 of a tooth (figs. 20 and 21), 



it is found to be princi- Fig. 20.* 



pally composed of a hard 

 matter, dentine or ivory ; 

 while in the centre this 

 dentine is hollowed out 

 into a cavity resembling in 

 general shape the outline 

 of the tooth, and called the 



pulp -cavity, from its containing a very vascular and sensi- 

 tive little mass composed of connective tissue, blood-vessels 

 and nerves, which is called the tooth-pulp. The pulp is 

 continuous below, through an opening at the end of the 

 fang, with the mucous membrane of the gum. Capping 

 that part of the dentine which projects beyond the level 

 of the gum, is a layer of very hard calcareous matter, the 

 enamel, while sheathing the portion of dentine which is 

 beneath the level of the gum, is a layer of true bone, called 

 the cement or crusta petrosa. At the neck of the tooth the 

 cement is exceedingly thin, but it gradually becomes thicker 

 as it approaches and covers the lower end or apex of the 

 fang. 



Dentine or ivory in chemical composition closely re- 

 sembles bone. It contains, however, rather less animal 

 matter; the proportion in 100 parts being about 28 of animal 

 matter to 72 of earthy. The former, like the animal matter 

 of bone, may be resolved into gelatin by boiling. The 



* Fig. 20. Sections of an Incisor and Molar Tooth. The longitudinal 

 sections show the whole of the pulp-cavity in the incisor and molar 

 teeth, its extension upwards within the crown, and its prolongation 

 downwards into the fangs, with the small aperture at the point of each : 

 these and the cross section show the relation of the dentine and enamel. 



E 2 



