LESSON 54.] 



THE STRUCTURE OF THE TEETH. 



193 



FIG. 310. 



but wavy ; they also present numerous ramifications and anastomo- 

 ses; towards the crown of a tooth they usually form an elegant 

 plume, which presents a fine object under the mi- 

 croscope. Each tube generally describes two or 

 three large, besides a great number of small cur- 

 vatures, which occasionally have the appearance 

 of zigzags, or spiral windings. 



827. To show the connection of the three 

 tissues of the teeth, and their relation to each 

 other, a figure is given of a longitudinal section 

 of a human canine tooth (Fig. 310). The enamel 

 is marked #, the dentinal tubuli, 5, 6, the pulp 

 cavity, c, the crusta petrosa, or cementum, d, and 

 thin plates of dentine, e. 



828. The cementum is a layer of true bone, 

 which covers the lower part of the teeth, and in the 

 many fanged teeth, not unfrequently cements them 

 all together. The cementum commences above, 

 at the point where the enamel ceases, so that the 

 dentine in a healthy tooth is never left ex- 

 posed to the atmosphere. It begins as a very thin 

 layer, gradually increasing in substance as it de- 

 scends, and attains its greatest dimensions at the 

 lower part of the fang. 



829. As bone, it may readily be imagined that 



it is the least hard of the dental tissues. Like bone, the cementum 

 contains bone cells, with their canaliculi, together with Haversian 

 canals and vessels. The bone cells are conspicuous for the great va- 

 riety which they present in number, form, and size, and for the unu- 

 sual number and length of their canaliculi (little tubes). 



830. The canaliculi often resemble feathers and brushes, and un- 

 less the lacunae (bone cells) are isolated, connect them with each 

 other, and anastomose with the ends of the dentinal canals or tubes. 

 At the point of junction with the enamel, bone cells are invariably 

 absent; they first make their appearance about the middle of the 

 fang, but scattered and solitary. Towards the extremity of the 

 fang their number increases, and they frequently take on a very reg- 

 ular arrangement. 



831. In old teeth it very commonly happens that the pulp 

 becomes obliterated, and the pulp cavity filled with cementum; 

 and in these cases the Haversian canals, lacunae, and their canali- 



13 



Longitudinal section 



of Canine tooth, 



Human. 



