DEVELOPMENT OF THE TEETH. 185 



crown of the Human molar teeth, that is to say. the groove which separates their tubercles, 

 is by far the most frequent seat of incipient decay ; probably from its tissue having been 

 at the first less perfectly formed than that of the remainder. 



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) 

 docs 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 de- 

 velopment of the papilla and follicle of the Anterior Permanent Molar tooth, which is de- 

 veloped 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. 89, 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 An- 

 terior 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. 99, rf). During the few months which succeed birth, how- 

 ever, 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 con- 

 traction separates it from the remainder of the cavity of reserve. 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 (*r) ; 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 (/a). 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. In the Elephant, in which there is a con- 

 tinual new production of molar teeth at the back of the jaw, it is probable that from each 

 sac a cavity of reserve is formed, which produces the succeeding tooth ; and thus the only 

 essential difference between its dentition and that of Man, consists in the degree of continu- 

 ance of this gemmiparous process; which ceases in Man, after being twice performed, but 

 is repeated in the Elephant until nearly the close of its life. 



e. 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 

 papilla? are completely hidden by the closure of the mouths of the follicles, and of the groove 

 itself. The succeeding stage, which has long been known as the saccular, is the one during 

 which the whole formation of the Tooth-substance, and of the Enamel, takes place. It is 

 during this period, also, that the ossification of the jaw is being effected ; and that the bony 

 sockets are formed for the teeth, by the consolidation of the anterior and posterior ridges 

 bounding the alveolar groove (in which the dental groove was originally imbedded), and of 

 the interfollicular septa, which are produced by the meeting of transverse projections from 

 these ridges. The history of development in the Lower Jaw is very nearly the same; the 

 chief difference being in the origin and situation of the primitive dental groove. 



/. We have now only to consider the fourth or eruptive stage, that in which the Teeth 

 make their way through the gum. This process chiefly results from the lengthening of the 

 fang, by the addition of new bony matter ; and the crown of the tooth is thus made to press 

 against the closed mouth of the sac (Fig. 98, m). This at last gives way, so that the sac as- 

 sumes its previous condition of an open follicle. When the edge of the tooth has once made 

 its way through the gum, it advances more rapidly than can well be accounted for by the 

 usual rate of lengthening of its fang ; and this appears to be due to the separation of the 

 bottom of the sac from the fundus of the alveolus ; so that the whole tooth-apparatus is car- 

 ried nearer to the surface, leaving a space at the bottom of the alveolar cavity, in which the 

 further lengthening of the root can take place (n). The open portion of the sac remains as 

 the narrow portion of the gum, which forms a vascular border and groove round the neck 

 of the perfected tooth (o). The deeper portion of the sac adheres to the fang of the tooth, 



16* 



