COMPOSITION OF THE TEETH. 101 



When examined with the microscope, the bone or ivory 

 appears to consist of minute branching fibres, which begin at the 

 pulp and run toward the periphery of the tooth, and aje im- 

 pacted in some homogenous bony tissue lodged between them. 

 These fibres have been shown by Mr. Nasmyth to be nothing 

 more than little opaque bodies, the nuclei of the bone or ivory 

 cells arranged in a linear series. 



The enamel or vitreous substance, (see Fig. 24,) so named 

 from its resemblance to vitrified minerals, has been with 

 greater propriety called by Blake, the cortex striatum, from the 

 lines which it presents upon its sides. It forms a covering 

 nearly a line in thickness upon the crown of the teeth, and is 

 thinned down at its termination upon the neck. It is composed 

 of minute hexagonal crystalline fibres, consisting like those of 

 the ivory of minute cells filled with calcareous substance piled 

 one upon another, perpendicularly to the bony part, and so 

 closely compressed together, as to leave no obvious interval 

 between them. All the wear of the teeth takes place, therefore, 

 at the end of these fibres and not upon their sides ; and the 

 enamel is rendered by this arrangement much less liable to frac- 

 ture. 



No vessels have been traced to this substance, nor has it 

 ever been seen like the bony portion, colored by madder in 

 young animals fed on this substance during the development 

 of the teeth. But Mascagni, infatuated with his discoveries 

 in the absorbent system, absurdly regarded this substance as 

 entirely formed of absorbent vessels.* It is exceedingly hard 

 and strikes fire, on collision with steel. While covering the 

 bone, it presents a milky white appearance ; removed from it, 

 it is semi-transparent and opaline. 



The enamel is thickest on those parts of the teeth most 

 exposed to friction, as on the horizontal surfaces of the 

 grinders, the edges of the incisors, and the points of the cuspi- 

 dati. The position of the enamel and its arrangement into 

 fibres is well seen in Fig. 24. 



* Vide Prodrome. 

 9* 





