184 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



ridge, which has been also increasing. The follicles at this time grow faster than the pa- 

 pilla', so that the latter recede into the former. The primitive dental groove then contains 

 ten papillae, inclosed in as many follicles; and thus all necessary provision is made for the 

 production of the first set of teeth. (This series of changes is represented in Fig. 88, a g.) 

 The groove is now situated, however, on a higher level than at first ; and it has undergone 

 such a change by the closure of its edges, as to entitle it to the distinctive appellation of 

 secondary dental groove. It is in this secondary groove that those structures originate, which 

 are destined for the development of the Second or Permanent set of Teeth, of those at 

 least which replace the Milk Teeth. This is accomplished in the following manner. 



b. At about the fourteenth or fifteenth week, a little crescentic depression may be observed, 

 immediately behind the inner Opercula of each of the Milk-tooth follicles. This depres- 

 sion gradually becomes deeper, and constitutes what maybe termed a cavity of 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 gradually 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 in- 

 ferior to the milk sacs; and at last they imbed themselves in the submucous cellular tissue, 

 which lias all along constituted the external layer of the milk-sac. The implantation of the 

 Permanent tooth-sacs in the walls of the Temporary follicles, gives toth former the appear- 

 ance of being produced by a gemmiparous process from the latter. This series of changes 

 is represented in Fig. 88, g n. 



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

 close, become gradually moulded into their peculiarly Human shape. The Molar pulps be- 

 gin 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 deve- 

 loped into the fangs of the future teeth. Whilst this is going on, the sacs grow more rapidly 

 than the papillse, so that there is an intervening space, which is filled with a gelatinous gra- 

 nular substance the enamel blastema ; this closely applies itself to the surface of the pa- 

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

 sac, ramifies minutely in its'pruper membrane, but does not send the smallest twig into the 

 granular substance. At this period, the tubercles and apices of the papillee or pulps become 

 converted into real dentine or tooth-substance, in the manner already stated ( 213) ; 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 villousand vas- 

 cular condition of a true Mucous membrane, having upon it a very thin Inyerof the granular 

 substance, or enamel-pulp, which may be considered as a sort of Epithelium ; and it is by 

 the deposition of calcareous matter in the long prismatic cells of this, that the enamel is 



Fig. 89. 



00 



J 



Diagrams illustrative of the formation of the three Permanent Molar teeth, from the non-adherent 



portion of the Dental Groove. 



formed. The opercula, which close the mouth of the dental sac, attain a much greater de- 

 velopment in the Molar teeth of Herbivorous animals; where they dip down into the midst 

 of the dentinal pulp, and give origin to insulated spots both of enamel and cementum. It 

 lias been remarked by Mr. Lintott, that the lines along which the opercula meet, on the 



