TEETH. 51 



the part which intervenes between the ossifying centre in the 

 shaft and that at each extremity ; while increase in thickness 

 takes place by the formation of layers of osseous tissue beneath 

 the periosteum. The former is an example of ossification in 

 cartilage ; the latter of ossification in membrane. 



Teeth. A tooth is generally described as possessing a crown, 

 neck, and fang or fangs. The crown is the portion which pro- 

 jects 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 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 principally 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 sensitive little mass com- 

 posed of connective tissue, bloodvessels 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 



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. 



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 ap- 

 proaches and covers the lower end or apex of the fang. 



Dentine or ivory in chemical composition closely resembles 



