ORIGIN AND DEVELOPMENT OF THE TEETH. 251 



papillae become pulps, the ivory or dentine and enamel 

 are formed, the enlargement in the maxillary bones, and 

 the ossification of the alveolar processes takes place. 



The rudiments of the second or permanent set of teeth 

 are developed soon after the commencement of the saccular 

 period. The papillae and sacs of the ten milk teeth, in either 

 jaw, are closed about the fourteenth week ; and the upper 

 or superficial part of the primitive groove remains open and 

 receives the name of the secondary dental groove. 



It is in this groove that the permanent teeth begin their 

 development. Behind the operculum of each milk tooth 

 sac a small cavity is seen, called the cavity of reserve. The 

 mucous membrane lines this cavity, and on its floor, as in 

 the primitive groove, a papilla is formed which becomes 

 enclosed in a follicle and finally in a shut sac ; as this is 

 going on, the papilla is found to recede from the surface, 

 and the neck of its sac, which was originally continuous 

 with the common mucous membrane, remains as an oblit- 

 erated cord connected to the internal surface of the gum of 

 each temporary tooth. These cords are called the itinera- 

 dentium. About the fifth month the papilla and follicle of 

 the anterior permanent molar is developed in a portion of 

 the primitive groove, which is behind the posterior tem- 

 porary molar, and is not closed so soon as the anterior 

 portion. 



The dental groove, in closing over the sac of this anterior 

 permanent molar, leaves a space between the sac and the 

 gum, which is the cavity of reserve for the second perma- 

 nent molar, and a similar one for the third molar, or wis- 

 dom tooth. Owing to the sacs of the anterior permanent 

 teeth, and the temporary ones enlarging faster than the 

 maxillary bones can elongate, the cavity for the permanent 

 molars recedes into the root of the coronoid process below, 

 and into the maxillary tuberosity above. But after birth, 

 as the jaw enlarges, the first permanent molar takes its 

 proper place and level in the dental circle, which is now 

 occupied by the cavity of reserve dilating in it, and devel- 

 oping in it the papilla of the second permanent molar. In 



