1246 



THE DIGESTIVE SYSTEM. 



igro 



the dental lamina D.L from 

 the surface epithelium E 

 and the beginning of the 

 adamant germ E.G. 



II. Shows the further growth 

 of the adamant germ and 

 its invagination. 



III. The adamant germ is 

 more invaginated, and its 

 inner layer of cells becomes 

 columnar. A, the dental 

 lamina, grows thinner, but 

 near its posterior or lingual 

 edge there is an enlarge- 

 ment R.G which is the re- 

 serve germ for a permanent 

 tooth. The superficial cells 

 of the ivory papilla P are 

 becoming columnar. 



IV. The inner columnar cells 

 of the adamant germ (called 

 adamant cells) A have 

 formed a cap of adamant EN, 

 inside which the superficial 

 cells of the papilla, the 

 odontoblasts O, have 

 formed a layer of ivory D. 



'-_; ^- D.L 



III 



compartments become more perfect, but are never entirely closed in over the crowns 

 of the teeth. During the eruption of the teeth the upper and anterior part of each of the 

 bony cells is absorbed ; subsequently, however, it is re-formed around each tooth when 

 it has taken its final position. 



9. Eruption. Long before the root is completed, the crown, by some force which is 



not properly understood, but which does 

 not seem to depend on additions to the. 

 root, is pushed through the top of the 

 tooth -sac, and the upper and anterior 

 wall of the roomy alveolus having been 

 absorbed at the same time onwards 

 through the gum until the mouth is 

 reached. Later, 'when the tooth has 

 assumed its final position, the alveolus, 

 as already stated, is re-formed, and 

 closely embraces the completed root. 



10. After the adamant organs of the 

 deciduous teeth have been formed on 

 the inferior aspect of the dental lamina, 

 as described above, the neck of epithelium 

 by which the lamina is still connected 

 with the surface becomes broken up into 

 a cribriform sheet. Its free posterior 

 border, on the other hand, continues to 

 grow inwards in the tissue of the gum 

 towards the cavity of the mouth (Fig. 

 971, III. and IV.), and at a later date 

 there appear on its under surface, near 

 the free edge, and behind the several 

 developing milk teeth, the adamant 

 organs or so-called "reserve germs "- 

 for the corresponding permanent teeth, 

 which are developed in exactly the 

 same manner as the deciduous teeth 

 described above. 



In connexion with the development 

 of the permanent molars, which have no 

 corresponding teeth in the deciduous 

 set, there takes place a prolongation 

 backwards of the posterior extremity 

 of the dental lamina into the tissue of 

 the jaw, behind the last deciduous molar. 

 On the inferior aspect of this prolonga- 

 tion, which has no direct connexion with 

 the surface epithelium, adamant organs 

 are formed for the permanent molars, 

 and their further development goes on 

 in the manner described for the other 

 teeth 



' ^ ^^ ^ Qf ^ 



events in the development of the 



E, teeth OCCUr may be briefly given: 1 tie 



rt 

 he epithelium, the 1 



s ig n of the future teeth, begins about 

 the sixth week of foetal life, and the 

 dental lamina is completed by the end of the seventh week. 



The dental papillae for the eight front teeth appear and become surrounded by their 

 adamant organs about the tenth week, and the papilla for the first permanent molar about 

 the seventeenth week. 



The first traces of calcification, and the formation of the tooth-sacs, take place about 

 the fifth month of foetal life. 



Eruption of Deciduous Teeth. The period at which the eruption of the milk teeth 

 takes place is extremely variable, and no two observers seem to agree upon the question. 



^V 



V. Shows a more advanced 

 stage still. The deposit of 

 ivory is extending down- 

 wards, and enclosing the 

 papilla to form the future 

 pulp, in which a vessel V 

 is seen. 



FIG. 971.-DIAGRAMTO 

 A, Inner layer of adamant germ ; B, Outer layer ; 0, Remains 



DEVELOPMENT 



of intermediate cells; D, Ivory; D.L, Ivory lamina 



Epithelium; E.G, Adamant germ; EN, Adamant ; F, Dental .1-1 n 



furrow ; L.D, Labio-dental furrow ; M, Connective tissue cells; thickening 



eiood vessei asts ' P} Ivor y P a P illa ' R- G, Reserve germ ; v, 



