DEVELOPMENT OF THE TEETH. 



303 



Those epithelial cells (Fig. 127, 3) of the enamel-organ, which lie next the top of 

 the papilla, are cylindrical, and become calcified to form enamel prisms. The 

 layer of cells of the double cap, which is directed towards the tooth-sac (1), 

 becomes flattened, fuses, undergoes a horny transformation, and becomes the 

 cuticula, whilst the cells which lie between both layers undergo an intermediate 

 metamorphosis, so that they come to resemble the branched stellate cells of the 

 mucous tissue (2), and gradually disappear altogether. 



The dentine is formed in the most superficial layer of the projecting connective- 

 tissue of the dental papilla, owing to the calcification of the continuous layer of 

 odontoblasts which occurs there (Figs. 127 and 128, k). During the process, fibres 



Fig. 127. 



a, Dental ridge; b, enamel -organ 

 with 1, outer epithelium; 2, 

 middle stellate layer ; 3, ena- 

 mel-prism cell layer ; c, den- 

 tine-germ with blood-vessels 

 and the long osteoblasts ou 

 the surface ; d, tooth - sac ; 

 e, secondary enamel-germ. 



-Jt 



Fig. 128. 



a, Dental ridge ; b, enamel-organ ; 

 c, dentine-germ ; /, enamel ; g, 

 dentine ; h, interval between 

 enamel-organ and the position of 

 the tooth ; k, layer of odonto- 

 blasts. 



or branches of these cells are left unaffected, and remain as the fibres of Tomes. 

 Exactly the same process occurs as in the formation of bone, the odontoblasts 

 forming around themselves a calcified matrix. The cement is formed from the soft 

 connective-tissue of the dental alveolus. 



Dentition. During the development of the first (temporary or milk) teeth a 

 special enamel-organ (Fig. 127, e) is formed near these, but it does not undergo 

 development until the milk-teeth are shed ; even the papilla is wanting at first. 

 When the permanent tooth begins to develop, it opens into the alveolar wall of the 

 milk-teeth from below. 



The tissue of this dental sac causes erosion, or eating away of the fang and even 

 of the body of the milk-teeth, without its blood-vessels undergoing atrophy. The 

 chief agents in the absorption are the amoeboid cells of the granulation tissue. 

 [Multinuclear giant-cells also erode the fangs of the teeth.] 



Eruption Of the Teeth. The followiug is the order in which the twenty milk- 

 teeth cut the gum i.e., from the seventh month to the second year: Lower central 

 incisors, upper central incisors, upper lateral incisors, lower lateral incisors, first 

 molar, canine, the second molars. 



[The figures indicate in months the period of eruption of each tooth.] 



