TEETH 



143 



blast and odontoblast layers are pushed farther and farther apart. 

 During the secretion of the dentine, protoplasmic strands from the odonto- 

 blasts are retained within the dentine thus forming the dental canaliculi. 

 The odontoblast cells persist throughout life, and by their continued 

 secretion may in old age entirely obliterate the pulp cavity of the tooth. 

 The crown of the tooth is the first to develop, and for a while the 

 tooth resembles a silver-plated thimble, the thin enamel coating cor- 



ENAMEL PULP 



PERMANEhq:, 

 TOOTH 



PAPILLA' ■»^p?e!i. 



Fig. 131. — A section of the jaw of a nine-months human embryo, showing the anlage 

 of a canine tooth. The enamel organ of the permanent incisor is seen on the lingual 

 side of the milk-tooth. (Redrawn after Corning.) 



responding to the silver plate, the dentine to the underlying metal. 

 As the tooth grows, it increases in length as well as in thickness, adding 

 first a neck and later a root. The opening into the inner pulp cavity 

 becomes more and more restricted as the root elongates until finally only a 

 minute foramen remains to admit blood-vessels and nerves. The nerves 

 grow into the pulp and acquire free terminations among the odontoblast 

 cells. The cement layer is the last to be added. Cement is secreted by 

 bone-cells which penetrate the connective-tissue sac enclosing the tooth. 



