OH. v.] DEVELOPMENT OF TEETH. Si 



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 approaching completion the 

 other layers of the enamel organ have almost disappeared, and 

 they entirely 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 formation 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 con- 

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

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

 the life of the tooth (as is especially well seen in the abnormal con- 

 dition called an exostosis), by the process of ossification taking place 

 in the periosteum. On the other hand, absorption of the root 

 (such as occurs when the milk-teeth are shed) is due to the action 

 of the osteoclasts of the same membrane. 



In this manner the first set of teeth, or the milk-teeth, are 

 formed ; and each tooth, as it grows, presses at length on the wall 

 of the sac enclosing it, and, causing its absorption, is cut, to use a 

 familiar phrase. 



The temporary or milk-teeth are later replaced by the growth 

 of the permanent teeth, which push their way up from beneath 

 them. 



Each temporary tooth is replaced by a tooth of the permanent 

 set which is developed from a small sac which was originally an 

 offshoot from the sac of the temporary tooth which precedes it, 

 and called the cavity of reserve (fig. 98, c, fp). Thus the tem- 

 porary incisors and canines are succeeded by the corresponding 

 permanent ones, the temporary first molar by the first bicuspid ; 

 the temporary second molar developes two offshoots, one for the 

 second bicuspid, the other for the permanent first molar. The 

 permanent second molar is budded off from the first permanent 

 molar, and the wisdom from the permanent second molar. 



K.P. o 



