214 



NORMA L HISTOLOG Y. 



Fig. 192. 



CROWN 



of the papillae likewise elongate and produce the dentin. The 

 cement which constitutes the outer layer of the root of the tooth 

 is bone, and is developed from the foetal connective tissue in that 

 region (Figs. 190 and 191). 



Only a brief description of the structures entering into the forma- 

 tion of the fully developed tooth can be given here. For a more 

 detailed account of them the student is referred to special works on 

 the subject. 



The centre of the tooth is hollow, and the cavity opens oy a 

 small orifice at the tip of the root. This cavity is filled with a 



highly vascular delicate areolar tissue, 

 richly supplied with nerves. Where 

 this pulp is in contact with the tooth 

 its outer layer is made up of modified 

 connective-tissue cells, odontoblasts, 

 which are capable of elaborating den- 

 tin. The body of the tooth is com- 

 posed of dentin. This contains minute 

 canals, analogous to the canaliculi in 

 bone, but much longer. They extend 

 from the pulp-cavity nearly, if not quite, 

 to the outer boundary of the dentin, 

 and, toward their terminations, give off 

 branches. These canals are occupied 

 by long fibrous processes of the odonto- 

 blasts already mentioned. 



The crown of the tooth, down to its 

 neck, is covered with enamel. This 

 is a tissue derived from epithelium, 

 and is composed of long, prismatic ele- 

 ments extending from the surface of 

 the tooth to the dentin. These prisms 

 have a polygonal cross-section and are 

 held together by a hard cement-sub- 

 stance. They are not perfectly recti- 

 linear, but pursue a wavy course, being 

 disposed in lamina? or bundles, in which 

 the prisms have not quite the same direction. 



The root of the tooth, below the point where the enamel ends, is 

 covered with cement, which has the structure of ordinary bone, but 

 is usually devoid of Haversian canals (Fig. 192). 



NECK- 



ROOT—) 



Axial section of a human tooth 

 having but one root: o, enamel; 

 b, dentin; c, cement. 



