FORMATION OF THE TISSUES TEETH. 493 



reserve,- adapted to furnish delicate mucous membrane for the future formation of the 

 sacs and pulps of the ten anterior Permanent teeth. These cavities of reserve are gradu- 

 ally separated from the secondary dental groove, by the adhesion of their edges ; and they 

 thus become minute compressed sacs, situated between the surface of the gum and the 

 milk-sacs. They gradually recede, however, from the surface of the gum, so as to be 

 posterior instead of inferior to the milk sacs; and at last they imbed themselves in the 

 submucous cellular tissue, which has all along constituted the external layer of the milk- 

 sacs. The implantation of the Permanent tooth-sacs in the walls of the Temporary folli- 

 cles, gives to the former the appearance of being produced by a gemmiparous process 

 from the latter. This series of changes is represented in Fig. 130 , n. 



c. We now return to the Milk-teeth, the papillae of which, from the time that their fol- 

 licles close, become gradually moulded into their peculiarly Human shape. The Molar 

 pulps begin to be perforated by three canals, which, proceeding from the surface towards 

 the centre, gradually divide their primary bases into three secondary bases; and these 

 become developed into the fangs of the future teeth. Whilst this is going on, the sacs 

 grow more rapidly than the papillae, so that there is an intervening space, which is filled 

 with a gelatinous granular substance; this closely applies itself to the surface of the 

 papilla, but does not adhere to it. The branch of the dental artery which proceeds to 

 each sac, ramifies minutely in its proper membrane, but does not send the smallest twig 

 into the granular substance. At this period, the tubercles and apices of the papillae or 

 pulps become converted into real dentine or tooth-substance, in the manner hereafter 

 stated ( 633); and the granular matter is absorbed as fast as this appears; so that, 

 when the process of conversion has reached the base of the pulp, the interior of the 

 dental sac is left in the villous and vascular condition of a true Mucous membrane, 

 having upon it a very thin layer of the granular substance, which may be considered as 

 a sort of Epithelium; and it is by the deposition of calcareous matter in the long pris- 

 matic cells of this, that the enamel is formed. 



d. Whilst these changes are going on, other important preparations are being made 

 for the Permanent set. The general adhesion of the edges of the Primitive Dental 

 Groove, ( a) does not invade the portion which is situated behind the Posterior Milk 

 follicle; this retains its original appearance for a fortnight or three weeks longer, and 

 affords a nidus for the development of the papilla and follicle of the Anterior Permanent 

 Molar tooth, which is developed in all respects on the same plan with the milk teeth. 

 After its follicle has closed, the edges of the dental groove meet over its mouth; but as 

 the walls- of the groove do not adhere, a considerable cavity is left between the sac of the 

 tooth and the surface of the gum. The cavity is a reserve of delicate mucous membrane, 

 to afford materials for the formation of the Second Permanent Molar, and of the Third 

 Permanent Molar, or Wisdom-tooth. The process just described is represented in Fig. 

 131, a c. It will be convenient here to continue the account of the development of 

 these teeth, although it takes place at a much later period. Towards the end of 'foetal 

 life, the increase of the bulk of the Milk-tooth sacs takes place so much more rapidly 

 than the growth of the jaw, that the sac of the Anterior Permanent Molar is forced 



00 



HXXJ 



Digrams illustrative of the formation of three permanent Molar Teeth, from the non-adherent portion of 

 the dental groove. (After Goodsir.) 



42 



