TEETH. 319 



and when they exist in any number, or extend nearly through 

 its entire thickness, they produce a pearly appearance of the 

 enamel, and render it brittle. 



Thin longitudinal sections of the enamel in connexion 

 with the dentine generally exhibit numerous linear fractures, 

 which extend through its entire thickness, and which most 

 probably arise from their mode of preparation. Sections of 

 enamel also usually present numerous wavy lines, and which 

 are occasioned by the instrument employed in making the 

 cuttings. 



Structure of the Pulp. 



The centre of the dentine of all teeth is hollowed out into 

 a cavity ; this is occupied with a soft and reddish mass easily 

 separable from the walls of the cavity, the pulp. 



The pulp is made up of numerous blood-vessels, the walls 

 of which are constituted of delicate and nucleated cells, of 

 nerves, of ganglionic cells, and larger granular cells placed 

 principally on the surface of the pulp, external to the other 

 structures which enter into its formation. 



These external granular cells are supposed to play an im- 

 portant part in the development of the dentine, and to 

 which more particular reference will be made hereafter. 



The pain experienced in toothache arises from inflamma- 

 tion of the nerves of the pulp, and which is frequently left 

 exposed to the contact of the air in consequence of the 

 removal of the dentine, by which in sound teeth it is en- 

 closed, as the result of caries. 



DEVELOPMENT OF THE TEETH. 



The subject of the development of the teeth may be con- 

 sidered under two heads : under the first the general develop- 

 ment of the teeth will be briefly noticed, and under the 



