TEETH 



IOQ 



the root of the tooth forms after the crown is essentially complete. The 

 innermost layers are formed last. In addition to the contour lines, den- 

 tine seen in reflected light shows the radial Schreger's lines, which follow 

 the course of the dentinal canals but are said to be due to the fibrillar 

 structure of the matrix between them. 



Dentine when fully developed is not so hard as enamel and contains 

 a much larger amount of organic matter (approximately 25%). When 

 the inorganic substances are removed from enamel, the remaining tissue 

 scarcely holds together, but dentine and bone, when so treated, leave a 

 gelatinous matrix which preserves the form of the original object. The 

 dentinal canaliculi pass radially through the dentine, often following a 

 somewhat S-shaped course as shown in Fig. 94, B. In addition to these 

 primary curves, they may show spiral twists and secondary curves. As 

 they cross the dentine, they divide dichotomously a few times and give 

 off many slender lateral branches, some of which anastomose with those 

 from adjacent canaliculi (Fig. 99). They finally become very slender 



Enamel prisms. 



Dentine. Enamel. 



FIG. 99- FROM A LONGITUDINAL SECTION OF THE LATERAL 

 PART OF THE CROWN OF A HUMAN MOLAR TOOTH. 

 X 240. 



i, Dentinal canaliculi, some extending into the enamel; 2, 

 globules of calcified dentine projecting into the inter- 

 globular spaces, 3. 



Cement. 



FIG. 100. FROM A LONGITUDINAL SEC- 

 TION OF THE ROOT OF A HUMAN 



MOLAR TOOTH. X 240. 

 I, Dentinal canaliculi interrupted by a 

 stratum with many small interglobular 

 spaces, 2. 3, bone lacunas and canaliculi. 



and generally end blindly, but some terminal loops have been described. 

 Each canal is surrounded by a resistant uncalcified layer known as Neu- 

 mann's sheath. This sheath may be isolated with acids, and thus it is 

 comparable with the "corpuscles" of bone and the capsules of cartilage. 

 It is difficult to determine whether the processes from the odontoblasts 

 extend the whole length of the canaliculi, but they are believed to do so. 

 Tomes observed that the peripheral portion of the dentine is more sen- 

 sitive than the deeper part, and considered that the fine ramifications of 

 the odontoblasts respond like nerve fibers to stimulation. Nerves 

 have been traced to the odontoblast layer at the base of the dentine, but 

 it is doubtful whether they extend into the dentinal canals as some have 

 reported. 



