192 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. 



These offsets are in the condition of open follicles, communicating with 

 the cavity of the primitive tooth ; but they are gradually closed in, and 

 detached altogether from the capsules of the milk-teeth (Fig. 55, g, h, i). 



322. Soon after the closure of the follicles of the Milk-teeth, the con- 

 version of the cells of the original papilla into Dentine commences, ac- 

 cording to the method already described ( 313). Whilst this is going 

 on, the follicles increase in size, so that a considerable space is left be- 

 tween their inner walls and the surface of the dental papillae ; which 

 space is filled up with a gelatinous granular matter, the Enamel pulp. 

 The portion of this which is converted into enamel, however, is very 

 small ; being only a thin layer, which is left on the inner surface of the 

 capsule after the remainder has disappeared. The interior of the dental 

 sac, at the time when the conversion-process has reached the base of the 

 dentinal pulp, is in the villous and vascular condition of a Mucous mem- 

 brane, which indeed it really is, having been, as we have seen, once 

 continuous with the lining of the mouth ; and the layer of prismatic cells 

 which covers its free surface, and by the calcification of which the enamel 

 is produced, may be regarded as an epithelium. The completion of the 

 Enamel, and the ossification of the capsule so as to form the Cementum, 

 take place at a subsequent period. 



323. We have thus seen that the history of the first development of 

 the Human teeth may be divided into three stages, the papillary, the 

 follicular, and the saccular. The papillary corresponds precisely with 

 the complete mode of dental development in the Shark and other Fish, 

 as already mentioned. The follicular, which commences with the en- 

 closure of the papillae in open follicles, and terminates when the papillae 

 are completely hidden by the closure of the mouths of those follicles, 

 has also its permanent representation in the development of the teeth 

 of many Reptiles and Fishes ; the primitive papillae of which, though 

 enclosed in follicles, are never covered in at the summit, and thus free 

 themselves from their envelopes by simply growing upwards through 

 their open mouths. But in Man, and in all other animals which agree 

 with him in going on to the saccular stage, there must also be an erup- 

 tive stage, which consists in the bursting-forth of the tooth from the 

 enclosing capsule ; the summit of the tooth being carried against the lid 

 of the sac, by the growth of its roots (Fig. 55, A). By the continuance 

 of the same growth, the teeth are caused to penetrate the gum, and are 

 gradually raised above its surface (Fig. 55, i). 



324. All the permanent teeth, which are destined to replace the 

 temporary set, originate, as already stated, in buds or offsets from the 

 capsules of the latter. But behind the last temporary molars, which 

 are replaced by the permanent bicuspids, three permanent molars are 

 to be developed, on each side of either jaw. The first of these is formed 

 on precisely the same plan with the milk-teeth ; but is not completed 

 until a later period. The capsule of the second is formed at a later 

 period from that of the first, by a process of budding exactly analogous 

 to that, by which the other permanent capsules are formed from the 

 corresponding temporary ; and at a still later period, the capsule of the 

 third permanent molar is formed as a bud from that of the second. The 

 evolution of this molar does not usually take place, until the system 



