38 



Basic Structure of Vertebrates 



minute foramen at the deep end of the root. The dentinal wall of the 

 pulp-cavity, however, is lined by a thin membrane, the odontoblast 

 layer, consisting essentially of the cells (odontoblasts) which secrete 

 the dentine. Each odontoblast gives off an exceedingly delicate proto- 



Enamel prisms. 



Dentine. 



Enamel. 



Fig. 32. Longitudinal section of the lateral part 

 of the crown of a human molar tooth. (X 240.) 

 (1) Dentinal canaliculi, some extending into the 

 enamel; (2) globules of calcified dentine projecting 

 into the interglobular spaces (3). (Courtesy, 

 Bremer: "A Text-Book of Histology," Philadelphia, 

 The Blakiston Company.) 



plasmic fibril (fiber of Tomes) which occupies the canaliculus of the 

 dentinal tubule which was produced by that cell (Figs. 33, 34). The 

 sensitivity of human dentine to the dentist's drill is probably due to 

 these living fibrils which may act in the manner of nerves. 



In a normal tooth the dentine remains in intimate relation to the 

 living substance which produced it. There is reason to believe that, 

 so long as the odontoblast cells and their dentinal fibrils are alive, the 

 tooth is better able to resist disease. If the blood-vessels and nerves of 

 the pulp are destroyed, the odontoblasts and their fibrils cannot sur- 

 vive. A "dead" tooth is, therefore, more susceptible to decay. Enamel 



ODONTOSIS' 



Fig. 33. Diagrams illustrating the difference in the secretion of dentine (left), 

 and of bone (right). The functional polarization of the odontoblasts and osteoblasts 

 is, however, similar. (After Braus. Courtesy, Neal and Band : " Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



