172 A MANUAL OF ANATOMY 



dentine forms the principal part of the tooth, surrounding the pulp 

 cavity and its prolongations; the enamel covers the exposed part 

 or crown; and the cement covers the root. 



Dentine. — ^This bears a resemblance to bone, but contains rather 

 less animal, and more earthy, matter, the proportion in lOO parts 

 being about 28 of animal matter to 72 of earthy. The dentine 

 has a striated appearance, due to the fact that it is traversed by 

 a number of minute branched channels, called the dentinal tubules, 

 which radiate in a curved manner outwards from the pulp cavity 

 to the deep aspect of the enamel and cement. These tubules 

 contain processes of the odontoblasts which form the membrana 

 eboris, and are known as Tomes' fibres. The part of the dentine 

 adjacent to the enamel and cement is known as the granular layer 

 of Purkinje. It presents a number of irregular spaces, known as 

 the interglobular spaces, which are surrounded by minute globules 

 of calcareous matter. 



Enamel. — ^This caps and protects the dentine of the crown. It 

 is exceedingly hard, which is due to the fact that it contains no 

 animal matter — at least, to any appreciable extent. It consists 

 of solid hexagonal prisms, which are marked by transverse stria- 

 tions. These are received by their deep extremities into depres- 

 sions on the dentine, and are placed vertically on the summit of 

 the crown, but horizontally on its sides. At the period of eruption 

 of a tooth, and for some little time thereafter, the enamel of the 

 crown is covered by a thin membrane, called the enamel cuticle, 

 or Nasmyth's membrane. 



Cement, or Crusta Petrosa. — ^This covers the dentine which forms 

 the root of the tooth. It is true bone, and contains lacunae and 

 canaliculi, but it is destitute of Haversian canals. 



The root of a tooth is maintained in its alveolus by the peri- 

 dental membrane or dental periosteum, which covers the cement, 

 and lines the wall of the alveolus, being continuous with the gum at 

 the neck of the tooth. The articulation is called gomphosis. 



Development of the Teeth. 



The teeth are calcified papillae of the gingival mesoderm, their crowns 

 being covered by calcified ectodermic cells. Three structures are involved 

 in the development of a tooth — namely, (i) the enamel organ, which furnishes 

 the enamel : (2) the dental papilla, which gives rise to the dentine and dental 

 pulp : and (3) the dental sac, from the inner layer of which the cement is 

 formed. The enamel organ is developed from the ectoderm, whilst the dental 

 papilla and the dental sac are of mesodermic origin. 



The development of the temporary teeth commences as ingrowths o.f the 

 epithelium along the margins of the embryonic jaws into the mesodermic 

 tissue of the jaws. Each of these ingrowths, which consists of epithelial 

 cells, soon gives rise to two vertical layers — an outer or labial, and an 

 inner or lingual. The outer layer is called the labio-dcnial lamina, and the 

 inner layer is the true dental lamina. The labio-dental or labio-gingival 

 lamina is concerned with the lip and gum, and it presents superficially a 

 furrow, called the labio-dental or labio-gingival groove, which divides it into 

 two parallel ridges — labial for the lip, and gingival for the gum. The labio- 

 gingival groove itself deepens, and separates the lip from the gum. 



