CRUSTA-PETROSA OR CEMENTUM. 



243 



hexagonal prisms, arranged perpendicularly to the surface 

 of the ivory parallel to each other, and having one of their 

 extremities resting upon the ivory, and the other forming 

 the free surface of the crown, as seen in Fig. 68, A. A thin 

 membrane is described as separating the enamel from the 

 ivory, and is found, by Mr. Nasmyth, to be composed 

 of cells. The enamel, like ivory, consists also of animal 

 and earthy matter, and is chemically composed, according 

 to Berzelius, in 100 parts ;< of 



Phosphate of lime, 85.3 Phosphate of magnesia, 1.5 

 Fluoride of calcium, 3.2 Soda and muriate of soda, 1. 

 Carbonate of lime, 8. Animal matter and water, 1. 

 A FIG. 68. C 



Orusta-petrosa (or cementum) 

 (Fig. 61, A) forms a thin coating 

 of osseous lamina to the root 

 of the tooth, from the neck 

 where the enamel ends, to the 

 apex of the fang. In some animals as the elephant it 

 is continued over the enamel as a thick layer. In struc- 

 ture, it has the calcegerous cells and tubuli, and consists 

 of true bone. Its quantity increases as age advances, and 

 presents, sometimes in old people, an exostosis on the 



FIG. 68, A represents the hexagonal termination of the Enamel fibres. 

 1, 2, 3, show the irregular crevices between the hexagonal fibres. 



FIG. 68, B represents a lateral view of the Enamel fibres. 1 1 Enamel 

 fibres ; 2 2 Transverse stripes. 



FIG. 68, C represents a view of the Enamel on a vertical section. 



