TEETIT. 



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teJ':tii. 



The external layer is formed of Lirge cells of 

 elongated form, provided witli immeroiis 

 processes culled odontoblasts, which are 

 ai-ranged so as to form a kind of colunmar 

 epithelium. They are tinely granular, but 

 have no cell-wall. Three kinds of pro- 

 cesses may be distinguished in these cells — 

 the dentinal, pulp, and lateral processes. 

 The dentinal processes become the before- 

 mentioned dentinal fibres of the ivory. 



Chemically, teeth consist of an organic, 

 cartilaginous basis, agreeing iu composition 

 with that of bone, and of inorganic matter, 

 consisting principally of phosphate of lime 

 with a sni ill quantity of the carbonate. 



JJiVii/ojjment. — At tho beginning of the 

 third mouth, the margins of the jaw form a 

 slight ridge, which consists of a thickening 

 of the embiyunic connective tissue and 

 epithelium of the mucous membrane of the 

 mouth. The rudiments or germs of the first 

 (milk) teeth are met with in the sixth week 

 of fcetal life, ar.d consist of small papillae, 

 one for each tooth, which become visible iu 

 grooves of the mouth, afterwards forming 



Fig. 737. 



Lower jaw of a human nine weeks' foetus, a, tongue, 

 turned back ; 6, right half of the lower lip turned aside ; 

 ft', left half of the lip cut off: c, outer wall of the gum; 

 d, inner wall of the gum; e,f,g. *, papilla; of the teeth; 

 •, fold where the sublingual duct subsequently opens. 



Magnified 9 diameters. 



the alveolar processes. The epithelium 

 forms the enamel, and the other tissue the 

 dentine and cement. Processes from the 

 sides of these dental grooves are then 

 formed, and, approaching each other, en- 

 close the papillte iu distinct follicles, the 

 margins of which gradually grow over the 

 papillse, and uniting, convert them into 



closed sacs or capsules. The pulps then be- 

 conu! moulded into the form of the future 

 teeth, the bases of the pulps dividing into 

 as many portions as the teeth have fangs ; 

 and as tho capsules increase at this stage 

 faster than the pulps, a space is left be- 

 tween them, in which a gelatinous-looking 

 substance is deposited from the wall of the 

 capsule forming the enamel-organ. By 

 some recent authors, however, the existence 

 of the dental grooves is denied. 



Tlie capsule (fig. 738 a) possesses a con- 

 nective coat with vessels and nerves ; and 

 from its base arises the tooth-germ or pulp 

 (fig. 738 h). The pulp consists of an outer 



Fig. 738. 



Capsule of the second incisor tooth of an eight months' 

 human foetus, a, capsule; 6, enamel-pulp ; c, enamel- 

 membrane; d, enamel; f, ivory-cells; h, papilla of 

 tooth or pulp; i, free margin of enamel-organ. 



Magnified 30 diameters. 



non-vascular layer of elongated nucleated 

 cells, with filiform processes, in close appo- 

 sition (fig. 739 rt), covering the surface of 

 the pulp — the ivory-membrane (fig. 738y), 

 not distinctly defined internally, but gradu- 

 ally passing into the vascular parenchyma 

 of the pulp. Tlie inner part of the pulp 

 consists of indistinctly fibrous connective 

 tissue -^ith nuclei, tlie vessels terminating 

 in loops beneath the enamel-membrane 

 (fig. 739 c). 



The enamel-organ (fig. 738 h) covers by 

 its inner concave surface the pulp, its out- 



