TEETH 107 



quently the prisms, when isolated, appear beaded, with transverse bands 

 at the places of constriction. 



When seen in cross section the prisms have highly refractive outlines, 

 from 3-6 fi in diameter. They were formerly described as polygonal 

 and primarily hexagonal (Fig. 94, E) but Smreker finds that they are 

 crescentic, as shown in Fig. 94, F (Arch. f. mikr. Anat., 1905, vol. 66, 

 pp. 312-331). The convex side of the crescent, along which the interpris- 

 matic cement is most abundant, is always toward the dentine. The 

 hollow of the crescent receives an adjacent prism which appears to have 

 been pressed into it. Isolated prisms of this sort are therefore hollowed 

 out on one side, and it is possible that they connect with one another by 

 flanges or bridges (von Ebner, Arch. f. mikr. Anat., 1905. vol. 67, pp. 18-81). 



DENTAL PAPILLA, DENTINE, AND PULP. 



The dental papilla has already been described as a mass of dense 

 mesenchyma, enclosed and probably moulded by the enamel organ. At 

 the end of the fourth month, shortly before the formation of enamel has 

 begun, the outermost cells of the papilla become 

 elongated and arranged in an epithelioid layer. 

 Since they produce the dentine, which is the 

 principal part of the tooth, these cells are known 

 as odontoblasts . At first they rest against the 

 inner enamel cells. Later a thin layer of pre- 

 dentine extends like a membrane between the 

 ameloblasts and odontoblasts; it is seen as a 

 white line in Fig. 92. As the layer of predentine 

 widens and becomes calcified, the odontoblasts 

 remain on its inner surface, which is toward the 

 pulp. Five of them are shown in Fig. 95, 

 together with their branching processes, one of 



FIG. 95- FIVE ODONTOBLASTS. 



which proceeds from the cuticular border of each FROM WHICH TOMES'S FIBERS 



EXTEND UPWARD INTO THE 



cell and occupies a canal in the dentine. These DENTINE, FROM A TOOTH OF A 



NEWBORN CAT. (Prenant.) 



dental or dentinal canals (canaliculi dentales) are 



readily observed in adult teeth. Their existence, and the fact that they 

 open into the pulp cavity, were recorded by Leeuwenhoek in 1687. 

 "The presence of fibrils of soft tissue within the dentinal tubes" was 

 established by Tomes in 1856 (Phil. Trans., pp. 515-522). He found 

 that if a section of a fresh tooth is placed in dilute hydrochloric acid and 

 then torn across the tubes, fibrils will be seen projecting from the broken 

 edges; and that if the pulp is pulled away from the dentine, fibrils can be 

 drawn out from the tubes. By the latter method the cells shown in Fig. 

 96 were obtained. The fibers within the dentinal canaliculi are called 

 dentinal, dental or Tomes' s fibers. 



