THE TEETH. 



359 



in a double curved or wavy direction from both the 

 enamel and dentine to the pulp. In their course they 

 divide and subdivide dichotomously, giving to the sur- 

 face of the dentine, when cut, a striated appearance. 

 They are filled with a delicate rod of protoplasm, which 

 is continuous with the processes of the odon to blasts on 

 the surface of the pulp. No nerve-filaments have been 

 found in dentine, but this protoplasm is probably the 

 medium through which painful sensations are trans- 

 mitted to the nerves of the pulp. The chemical compo- 

 sition of dentine is about twenty-eight per cent, organic 

 matter and seventy-two inorganic or earthy matter, con- 

 sisting of phos- 

 phate and carbo- 

 nate of lime, and 

 traces of fluoride 

 of lime, phosphate 

 of magnesium, and 

 other salts. Sec- 

 ondary dentine is 

 a formation of 

 dentine within the walls of the pulp-cavity, due to an 

 external or internal irritant which has stimulated certain 

 odontoblasts. 



The enamel is the external covering of the crown, 

 and is the hardest and most brittle structure of the body. 

 It is composed almost entirely of inorganic matter, con- 

 taining only from about three to five per cent, of inor- 

 ganic material. In arrangement, it is made up of 

 hexagonal prisms, radiating from the centres of develop- 

 ment, one for each cusp, and thus in the fissures we 

 often have the enamel imperfectly protecting the dentine 

 by the formation of V-shaped spaces, which allow ingress 

 to various external causes of decay. It is thickest on 



FIG. 157. ENAMEL PRISMS. 



A, in longitudinal view ; B, in cross-section. 



