ENAMEL 



57 



the outer layer of which are the columnar cells called odontoblasts. The 

 odontoblasts form the dentine, while the remainder of the papilla forms the 

 pulp. The method of the formation of the dentine from the odontoblasts 

 is said to be as follows: The cells form elongated processes at their outer 

 surfaces which are directly converted into the tubules of dentine, figure 66, c, 

 and into the contained fibrils. 



Each papilla early takes the shape of the crown of the tooth to which 

 it corresponds, but as the dentine increases in thickness and papilla dimin- 

 ishes until when the tooth is cut only a small amount remains as the pulp. It 

 is supplied by vessels and nerves which enter at the end of the root. The 

 roots are not completely formed at the time of the eruption of the teeth. 



FIG. 67. Vertical Transverse Section of the Dental Sac, Pulp, etc., of a Kitten, a, 

 Dental papilla or pulp; b, the cap of dentine formed upon the summit; c, its covering of 

 enamel; d, inner layer of epithelium of the enamel organ; e, gelatinous tissue;/, outer 

 epithelial layer of the enamel organ; g, inner layer, and h, outer layer of dental sac. X 14. 

 (Thiersch.) 



The enamel cap is formed by the enamel cells, by the deposit of a keratin- 

 like substance, which subsequently undergoes calcification. Other layers 

 are formed in the same manner meanwhile. 



The temporary or milk-teeth are speedily replaced by the growth of the 

 permanent teeth. 



The development of the temporary teeth commences about the sixth 

 week of intra-uterine life, after the laying down of the bony structure of 

 the jaws. Their permanent successors begin to form about the sixteenth 

 week of intra-uterine life. 



