272 



STRUCTURE AND DEVELOPMENT OF THE TEETH. 



STRUCTURE AND DEVELOPMENT OF THE TEETH. 



The tooth is to be regarded as a modified papilla of the mucous membrane 

 of the jaw, of exceptional size and peculiar structure. In its simplest form it 

 appears as a horny tooth, as, for instance, in the lamprey and the duck-bill, in 

 which the connective-tissue framework of the papilla is covered externally with 

 layers of hard, horny epithelium, comparable with the formation of hair and of 

 bristles. In the formation of human teeth a thick layer covering the papillary 

 cone is transformed into the firm layer of calcified dentine. The epithelium of the 

 papilla produces the enamel, while an accessory deposit takes place around the 

 base of the cone in the form of a thin covering of bone (cement) . 



The dentine, ivory, or tooth-bone, which surrounds the cavity of the tooth 

 (Fig. 99) and the root-canal, is firm, elastic and brittle. It appears, when 

 subjected to special treatment, to be composed of fibrils, which unite to form 

 lamellae, and these in turn make up the dentine and are traversed perpendicularly 



by the dentinal tubules. These numerous, long, 

 corkscrew-like, spiral dentinal tubules begin 

 with free openings from 1.3 // to 2.2 u in diam- 

 eter in the interior of the tooth, and traverse 

 the dentine to its outermost layer. The tubtiles 

 are bounded by an extremely resistent, thin 

 cuticle-like layer, the dentinal sheath (Fig. 

 100), which is most unyielding to chemical 

 agents. Within the cavities of the dentinal 

 tubules and completely filling them lie soft 

 fibers, the dentinal fibrils, which are to be con- 

 sidered as enormously elongated processes from 

 the superficial pulp-cells, the odontoblasts. 



The dentinal tubules, and also their con- 

 tents, the dentinal fibrils, anastomose throughout 

 their entire course by means of processes. Most 

 of them terminate near the enamel, or they 

 penetrate by means of delicate processes into the 

 cement substance between the enamel prisms. 

 Only a few bend over, forming an arch and 

 joining one another (Fig. 102, A, c}, while 

 others pass over into the interglobular spaces 

 (Fig. 101). The latter are small, uncalcified 

 areas of the ground-substance, or dilated tubules 

 located in greater number particularly near the 

 periphery of the dentine, and bound by spherical 

 surfaces. With the naked eye peculiar lines 

 can be seen in the dentine, particularly that of 

 the elephant's tooth, running parallel with the 

 contour of the tooth (Schreger's lines) , which 

 depend upon the fact that, at these points, all 

 of the dentinal tubules pursue a similar course 

 as respects their main curves. A special canal- 

 system, rising from the root, lies between the 

 dentine on one side and the enamel and cement 

 on the other, and communicates with the other cavities of the tooth. 



The enamel (vitreous substance), the hardest substance in the body, as 

 hard as apatite or quartz, covers the free projecting crown of the tooth. It 

 consists of perpendicular, hexagonal prisms (Fig. 102, B, C), arranged side by side 

 like palisades, and united by cement-substance. These prisms are from 3 fi to 5 p 

 wide, varying in thickness throughout their course, at the same time arching 

 in different directions, and they exhibit, after the action of acids, a coarse, trans- 

 verse striation, which, however, is absent in entirely fresh prisms. As regards 

 their nature, the enamel-prisms are elongated and calcified cylindrical epithelium 

 of the dentinal papilla. Retzius described, in enamel, the presence of dark, 

 brownish bands, running parallel to the outer border of the enamel^ and due to 

 the deposition of air in the enamel (Fig. 99). Fully formed enamel is in marked 

 degree negatively doubly refracting and uniaxial, while developing enamel is posi- 

 tively doubly refractive. 



The cuticula, the membranous capsule of the enamel, covers the free surface 



FIG. 99. Longitudinal Section through 

 an Incisor Tooth: s, enamel; d, 

 dentine; cd, tooth-cavity; c, cement. 



