1218 



THE ORGANS OF DIGESTION 



The Development of the Successional Permanent Teeth the ten anterior ones in either 

 jaw will be first considered. The germs for these teeth are developed from the lingual side 

 of the dental shelf, opposite to the corresponding temporary teeth. The germ for the central 

 incisor appears at about the seventeenth week, and the others follow successively in order of their 

 eruption. The deposition of enamel, dentine, and cementum occurs as in the temporary teeth. 

 The sac of each permanent tooth is also connected with the fibrous tissue of the gum by a 

 slender band of the gubernaculum, which passes to the margin of the jaw behind the corre- 

 sponding milk tooth (see above). 



The Superadded Permanent Teeth three on each side in each jaw develop as primary 

 teeth. The enamel germ for the first molar tooth appears during the sixteenth ireek from tin' 

 end of the dental shelf. The enamel germ for the second molar tooth appears about the fourth 

 month after birth from the neck of the enamel sac of the first molar tooth, while that for the 

 third molar or wisdom tooth does not appear until the third year after birth, from the enamel sac 

 of the second molar tooth. 



Eruption. When the teeth are ready to erupt the bone between them and the gum is absorbed 

 as well as that covering the labial surface of the crown, until one-half of the enamel is exposed. 

 The bone covering the lingual surface is more slowly absorbed, as it protects the permanent 

 o*rm beneath. As a result of this process the toej&tJEaftyaMBd W the removal of the bone 

 and not by a grm^^^^'--'!!^! ' " n * of the 



bed 



Meclcel's cartilage. 



Mandible 



FIG. 933,-Vertical section of the mandible of an early human fetus. (Magnified 25 diameters.) 



in a gutter 5 bone! ^^ ^ betW6en the teeth ' S that the lat ' er soo "'' 



ld e l IS Jn f lMm e enam .t 1 r g an now undergo a series of differentiations. The inner layer 

 anged as columnar epithelium, are the enamel cells, or ameloblasts. The layer is called the 



Denti ? e Enamel 



about tii^ 



The periods for the eruption ot the u ..., 



Lower central incisors . 



Upper incisors 



Lower lateral incisors and first molars 



Canines 



Second molars 



.olete 



935, and 936). The cells of the o 

 ^ distended, and on account 



.onths. 



15 to L'l 



16 to 20 

 20 to 24 



