78 



CONNECTIVE TISSUES TEETH. 



cisors ; and at the tenth week that for the posterior milk molar tooth. The 

 dental ridges now coalesce (/, Fig. 41), owing to the meeting and fusion of 

 two (incisors), three (canine), or four (molar) processes (opercula) from their 

 margins, which takes place at the fourteenth week. The tooth-germs are 

 now inclosed in saccuii, and on section the characters seen in Fig. 40 are 

 presented. 



Between the papilla or dentine germ and the connective tissue forming 

 the wall of the sacculus is seen a spongy tissue (3) composed of cells, which 

 forms the enamel organ. The innermost layer of cells, which are in imme- 

 diate contact with the papilla, are columnar and regularly hexagonal in 

 section. Their extremities are destitute of a cell-wall, though elsewhere it 

 is well defined. Near the base of the papilla the cells become more cubical, 

 and are reflected at its base upon the inner surface of the wall of the sacculus. 

 The polygonal cells intervening between these two layers become stellate and 

 anastomose with each other, the interspaces being filled with a gelatinous 

 substance, which appears to form a kind a pabulum for the growth of the 

 columnar enamel cells. At all events, it undergoes atrophy, so that the 



Thin section of the inner portion of the Dentine, and of the surface of the Pulp, of an adult Incisor 

 Tooth : a, portion in which calcification is complete, showing separate ylntjular masses at the line of 

 junction with the uncalcified substance, b; at care seen oval masses of germinal matter (cells), with 

 formed material on their outer surface ; d, terminal portions of uerve-fihres. 



outer and inner epithelial layers come into close apposition ; and in teeth 

 examined just at the period of eruption a membrane can be detached from 

 the enamel, composed of one or more layers of very flat epithelial cells, 

 which Waldeyer entertains no doubt represents the outer epithelium, with 

 more or less of the stratum intermedium. The enamel itself proceeds from 

 the direct calcification of the long columnar cells, the deposition of the cal- 

 careous salts taking place with tolerable uniformity in their outer ends. 

 When the calcification has proceeded to a certain extent, the calcified part 

 may be raised in the form of a membrane (Raschkow's membrana pivt'orma- 

 tiva) from the subjacent softer tissue; and thus, as Huxley first demon- 

 strated, the enamel is a petrified epithelium. The first germ of the dentine 

 appears as a dark semilunar area at the base of the enamel germ. As de- 

 velopment proceeds the papilla is gradually formed by the differentiation of 

 the mucous tissue, and is composed of cells lying in delicate connective tissue. 

 The cells or odontoblasts lying near the surface become elongated, and 

 arranged vertically to the surface, the more deeply situated extremes of 

 each communicating with other odontoblasts lying nearer the axis of the 

 papilla. Calcification first invades the outer extremities of these cells. A 



