TEETH 



197 



results in an increase in the number of enamel prisms. The twisting 

 and curvature of the prisms in the developed tooth is a consequence of 

 the torsion of the ameloblast layer during active secretion. While the 

 enamel grows by addition from the outside, the dentine increases in 

 thickness from within. Consequently as the tooth is formed the amelo- 

 blast and odontoblast layers are pushed farther and farther apart. Dur- 

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

 blasts are retained within the dentine to form 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. 



blood vessel 

 in pulp 



dentine 



enamel 



blood vessel in 

 mesenchyme 



odontoblast ' dentinal Time's ameloblast outer epithelium 



layer of enamel organ 



Fig. 150. — Projection drawing of small segment of developing incisor from 130 mm. 

 pig embryo to show formation of enamel and dentine. X3S0. (From Patten's "Em- 

 bryology of the Pig.") 



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- 

 responding to the silver plate covering, the dentine to the bare 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. 

 Membrane bone is formed around the root of the teeth to form the alveoli 

 of the jaw-bone and to hold the teeth firmly in place. 



The mechanics of the eruption of teeth is a problem which needs 

 further elucidation. Among the factors which operate is the elongation 



