74 



THE CONNECTIVE TISSUES 



[CH. V. 



adamantoblasts which abut on the dental papilla. Each forms a fine 

 deposit of globules staining with osmic acid and resembling keratin 

 in its resistance to mineral acid. At one time it was believed that 

 each adamantoblast was itself calcined and converted into an enamel 

 prism, but this view has been disproved by recent research. The 

 Layer of keratin-like material is outside the bodies of the cells, although 

 a process of each adamantoblast extends into it as a tapering fibre 

 (process of Tomes), and it is usually produced simultaneously with 



the first layer of uncalcified den- 

 tine; when it undergoes calcifica- 

 tion, the first layer of enamel is 

 complete. The adamantoblasts 

 then repeat the process, first 

 causing a deposition of keratin- 

 like material, and this in turn is 

 calcified, and so on. During the 

 formation of layer after layer of 

 enamel, the adamantoblasts retire. 

 By the time the enamel is ap- 

 proaching completion the other 

 layers of the enamel organ have 

 almost disappeared, and they en- 

 tirely disappear when the tooth 

 emerges through the gum. But 

 for some little time there is a 

 somewhat more persistent mem- 

 brane covering the crown ; this is 

 Nasmyth's membrane, or the 

 enamel cuticle ; this is the last 

 formed keratinous layer of enamel 

 which has remained uncalcified. 



As with the dentine, the for- 

 mation of enamel appears first on 

 the apex of each cusp. 



The cement or crusta petrosa is 

 formed from the internal tissue of the tooth sac, the structure and 

 function of which are identical with those of the osteogenetic layer 

 of the periosteum ; or, in other words, ossification in membrane 

 occurs in it. 



The outer layer or portion of the membrane of the tooth sac forms 

 the dental periosteum. 



This periosteum, when the tooth is fully formed, is not only a 

 means of attachment of the tooth to its socket, but also in conjunction 

 with the pulp a source of nourishment to it. Additional laminae of 

 cement are added to the root from time to time during the life of 



FIG. 100. Vertical transverse section of the 

 dental sac, pulp, etc., of a kitten, a, Dental 

 papilla or pulp ; b, the cap of dentine formed 

 upon the summit ; c, its covering of enamel ; 

 d, inner layer of epithelium of the enamel 

 organ ; e, gelatinous tissue ; /, outer epithe- 

 lial layer of the enamel organ ; g, inner layer, 

 and h, outer layer of dental sac. x 14. 

 (Thiersch.) 



