DEVELOPMENT OF THE TEETH. 



293 



" permanent" Molar, and of the third "permanent" Molar, or " wisdom-tooth." 

 (The process just described is represented in Fig. 85, a c.) It will be conve- 

 nient 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 

 backwards and upwards, into the maxillary tuberosity; and thus it not only 

 draws the surface of the gum in the same direction, but lengthens out the great 

 " cavity of reserve" (Fig. 85, d). During the few months which succeed birth, 



00 



LJLJ 



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

 portion of the Dental Groove. 



however, the jaw is greatly lengthened; and when the infant is eight or nine 

 months old, the anterior "permanent" Molar resumes its former position in the 

 posterior part of the dental arch ; and the great " cavity of reserve" returns to 

 its original size and situation (e). This cavity, however, soon begins to bulge 

 out at its posterior side, and projects itself, as a sac, into the maxillary tuberosity 

 (/) ; a papilla or pulp appears in its fundus; and a process of contraction sepa- 

 rates this portion of it from the remainder. Thus the formation of the second 

 " permanent" Molar from the first, takes place on precisely the same plan with 

 the formation of the " permanent" Bicuspids from the temporary Molars. The 

 new sac at first occupies the maxillary tuberosity (<?); but the lengthening of 

 the jaw gradually allows it to fall downwards and forwards, into the same line, 

 and on a level with the rest (Ti). Before it leaves the tuberosity altogether, the 

 posterior extremity of the remainder of the " cavity of reserve" sends backwards 

 and upwards its last offset the sac and pulp of the " wisdom-tooth" (i) ; this 

 speedily occupies the tuberosity, after the second molar has left it (j) ; and ulti- 

 mately, when the jaw lengthens for the last time, at the age of nineteen or twenty, 

 it takes its place at the posterior extremity of the range of the adult teeth (&). 

 Thus, the " wisdom-teeth" are the second products of the posterior or great 

 " cavities of reserve;" and the final effects of development in the "secondary 

 dental groove." 



286. We have thus sketched the history of the Development of the Teeth, up 

 to the time when they prepare to make their way through the gum. The first 

 stage of this development may be termed the papillary ; and the second the 

 follicular. The latter terminates, when the papillae are completely hidden by 

 the closure of the mouths of the follicles, and of the groove itself. The succeed- 



