96 DEVELOPMENT OF TEETH. 



ing the fourteenth and fifteenth weeks the opercula have completely closed 

 the follicles, so as to convert them into dental sacs, and at the same time 

 the papillae have become pulps. 



The deep portion of the primitive dental groove, viz. that which contains 

 the dental sacs of the deciduous teeth, being thus closed in, the remaining 

 portion, that which is nearer the surface of the gum, is still left open, and 

 to this Mr. Goodsir has given the title of secondary dental groove ; as it 

 serves for the development of all the permanent teeth, with the exception 

 of the anterior molars. During the fourteenth and fifteenth weeks small 

 lunated inflections of the mucous membrane are formed, immediately to 

 the inner side of the closing opercula of the deciduous dental follicles, 

 commencing behind the incisors and proceeding onwards through the rest; 

 these are the rudiments of the follicles or cavities of reserve of the four 

 permanent incisors, two permanent canines, and the four bicuspides. As 

 the secondary dental groove gradually closes, these follicular inflections 

 ,of the mucous membrane are converted into closed cavities of reserve, 

 which recede from the surface of the gum and lie immediately to the 

 inner side and in close contact with the dental sacs of the deciduous teeth, 

 being enclosed in their submucous cellular tissue. At about the fifth 

 month the anterior of these cavities of reserve dilate at their distal extre- 

 mities, and a fold or papilla projects into their fundus, constituting the 

 rudiment of the germ of the permanent tooth ; at the same time two small 

 opercular folds are produced at their proximal or small extremities, and 

 convert them into true dental sacs. 



During the fifth month the posterior part of the primitive dental groove 

 behind the sac of the last deciduous tooth has remained open, and in it 

 has developed the papilla and follicle of the first permanent molar. Upon 

 the closure of this follicle by its opercula, the secondary dental groove 

 upon the summit of its crown forms a large cavity of reserve, lying in 

 contact with the dental sac upon the one side and with the gum on the 

 superficial side. At this period the deciduous teeth, and the sacs of the 

 ten anterior permanent teeth, increase so much in size, without a corre- 

 sponding lengthening of the jaws, that the first permanent molars are gra- 

 dually pressed backwards and upwards into the maxillary tuberosity in 

 the upper jaw, and into the base of the coronoid process of the lower jaw ; 

 a positjon which they occupy at the eighth and ninth months of foetal life. 

 In the infant of seven or eight months the jaws have grown in length, and 

 the first permanent molar returns to its proper position in the dental range. 

 Thf cavity of reserve, which has been previously elongated by the upward 

 movement of the first permanent molar, now dilates into the cavity which 

 that tooth has just quitted ; a papilla is developed from its fundus, the 

 cavity becomes constricted, and the dental sac of the second molar tooth 

 is formed, still leaving a portion of the great cavity of reserve in connexion 

 with the superficial side of the sac. As the jaws continue to grow in 

 length, the second permanent dental sac descends from its elevated posi- 

 tion and advances forwards into the dental range, following the same 

 curve with the first permanent molar. The remainder of the cavity of 

 reserve, already lengthened backwards by the previous position of the 

 ^econd molar, again dilates for the last time, developes a papilla and sac 

 in the same manner with the preceding, and forms the third permanent 

 nolar or wisdom tooth, which at the age of nineteen or twenty, upon the 



