THE INCISORS. 



609 



the external dental cavity depends, 

 in a great measure, upon the thick- 

 ness of the cementous deposit. It 

 is much more rare to find it due 

 to an exaggerated length of the 

 enamel diverticulum, as most 

 authors have written. 



The cement, which gives bulk 

 and strength to the teeth, does not 

 offer, by a great deal, the same 

 resistance to wear which the other 

 substances possess. It is spread 

 over the faces of the tooth in the 

 form of a very thin layer, and 

 gradually disappears by reason of 

 the friction of the aliment, the 

 lips, and the tongue. It persists, 

 on the contrary, at the bottom of 

 the external dental cavity, while 

 the free portion of the latter exists 

 upon the dental table, in the form 

 of a blackish spot or " mark," more 

 and more confined and surrounded 

 by a round band of enamel. 



The cement is very similar 

 to bone, as its microscopic exami- 

 nation demonstrates. It is formed 

 by the alveolar periosteum, prin- 

 cipally at the limit between the 

 root and the crown of the tooth, 

 and is not the result, as Si- 

 monds has advanced, of the trans- 

 formation of the dentine into bone. 

 In very old teeth (Fig. 284) it ex- 

 ists in great abundance around 

 the roots, and the following ap- 

 pears to us to be the mechanism 

 of this new cementous formation. 

 As the maxillary bones, little 

 by little, push the greater part 

 of the tooth from its alveolus 

 as age advances, the root which 

 then constitutes the wearing sur- 

 face is no longer attached with 

 sufficient solidity to resist the en- 

 ergetic actions of the jaws, par- 

 ticularly of the inferior, which 

 rubs against the superior ; it there- 

 fore becomes, little by little, dis- 



FiG. 283.— Longitudinal and median section of a 

 permanent inferior pincer (enlarged). 



FJ, anterior face; FP, posterior face ; C, cement; 

 39 £, enamel; /, ivory. 



