DEVELOPMENT OF TEETH. 67 



sent. And at the same time small membranous processes are de- 

 veloped from the mouths of the follicles ; these processes are intended 

 to serve the purpose of opercula to the follicles, and they correspond 

 in shape with the form of the crowns of the appertaining teeth. To 

 the follicles of the incisor teeth there are two opercula ; to the canine, 

 three ; and to the molars a number relative to the number of their 

 tubercles, either four or five. During 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 con- 

 tains 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 four- 

 teenth 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 pum 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 extremities, 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 upon 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 corresponding lengthening of the jaws, that 

 the first permanent molars are gradually 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 position 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. 

 The cavity of reserve, which has been previously elongated by the 



