418 GENERATION. 



the dentine which forms the root of the tooth. As these 

 processes go on, the tooth projects more and more, the upper 

 part of the wall of the follicle gives way, and the tooth 

 finally appears at the surface. 



The permanent teeth are developed beneath the follicles 

 of the temporary, or milk-teeth. The first appearance is a 

 prolongation or diverticulum from the enamel-organ of the 

 temporary tooth, which dips more deeply into the mucous 

 structure. This becomes the enamel-organ of the permanent 

 tooth ; and the successive stages of development of the dental 

 follicles and the dental pulp progress in the same way as in 

 the temporary teeth. As the permanent teeth increase in 

 size, they gradually encroach upon the roots of the temporary 

 teeth. The roots of the latter are absorbed, the permanent 

 teeth advance more and more toward the surface, and the 

 crown of each temporary tooth is finally pushed out. The 

 number of the temporary teeth is twenty, while there are 

 thirty-two permanent teeth. Thus there are three perma- 

 nent teeth on either side of both jaws, which are developed 

 de novo, and are not preceded by temporary structures. 1 



The first dental follicles usually appear in regular succes- 

 sion. The follicles for the internal incisors of the lower jaw 

 appear first, this occurring at about the ninth week. All of 

 the follicles for the temporary teeth are completely formed 

 at about the eleventh or the twelfth week. 



The temporary teeth appear successively, the correspond- 

 ing teeth appearing a little earlier in the lower jaw. The 

 usual order, subject to certain exceptional variations, is as 

 follows : 



The four central iftcisors appear from six to eight months after birth. 

 The four lateral incisors appear from seven to twelve months after birth. 

 The four anterior molars appear from twelve to eighteen months after birth. 

 The four canines appear from sixteen to twenty-four months after birth. 

 The four posterior molars appear from twenty-four to thirty-six months after 

 birth. 



1 For an account of the structure of the teeth, see vol. ii., Alimentation, p. 

 139, et seq. 



