292 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



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. 



283. At about the 14th or 15th week, a little crescentic depression may be 

 observed, immediately behind the inner opercula of each of the " milk" tooth- 

 follicles. This depression gradually becomes deeper, and constitutes what may 

 be 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 pos- 

 terior instead of anterior 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-sac. The implantation of the "permanent" tooth-sacs in the walls 

 of the temporary follicles, gives to the former the appearance of being produced 

 by a gemmiparous process from the latter. This series of changes is represented 

 in Fig. 84, g n. 



284. We now turn 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 begin to be perforated by three canals, which, proceed- 

 ing 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 the enamel-pulp; this closely applies itself to the surface 

 of the papilla, but does not adhere to it. At this period, the tubercles and 

 apices of the papillae or pulps become converted into dentine or tooth-substance, 

 in the manner already stated ( 279) ; 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 

 prismatic epithelium which constitutes the true enamel matrix ( 280). The 

 opercula, which close the mouth of the dental sac, attain a much greater develop- 

 ment 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 has been remarked by Mr. Lintott, that the lines along which 

 the opercula meet, on the crown of the Human molar teeth that is to say, the 

 groove which separates their tubercles is by far the most frequent seat of inci- 

 pient decay ; probably from its tissue having been at the first less perfectly 

 formed than that of the remainder. 



285. 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" ( 282), 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 considerably cavity is left between the 

 sac of the tooth and the surface of the gum. The cavity is a "reserve" of deli- 

 cate mucous membrane, to afford materials for the formation of the second 



