

FORMATION OF DENTAL CAPSULE. 191 



the layer is very thin where it covers the Enamel, and being soft, it is 

 soon worn away by use. In the teeth of many Herbivorous Mammals, 

 it dips down with the Enamel to form the vertical plates of the interior 

 of the tooth ; and in the teeth of the Edentata as well as of many Rep- 

 tiles and Fishes, it forms a thick continuous envelope over the whole of 

 the surface, until worn away at the crown. 



320. The development of these additional structures is provided for 

 by the enclosure of the primitive papilla, from which the Dentine is 

 formed, within a Capsule, which, at one period, completely covers it in : 

 between the inner surface of the capsule, and the outer surface of the 

 dentinal papilla, a sort of epithelium is developed, by the calcification 

 of which, the Enamel is formed ; and the Cementum is generated by 

 the conversion of the capsule itself into a bony substance. The pro- 

 cesses by which this capsular investment is produced, and the tooth 

 completed and evolved, will now be briefly described, as they occur in 

 the Human foetus. 



321. The dental papillae begin to make their appearance, at about 

 the seventh week of embryonic life, upon the mucous membrane cover- 

 ing the bottom of a deep narrow groove (Fig. 55, a), that runs along 

 the edge of the jaw (Fig. 55, 5) ; and during the tenth week, processes 

 from the sides of this "primitive dental groove," particularly the ex- 

 ternal one, begin to approach one another, so as to divide it, by their 

 meeting, into a series of open follicles, at the bottom of which the pa- 

 pillae may still be seen. At the thirteenth week all the follicles being 

 completed, the papillae, which were at first round blunt masses of cells, 

 began to assume forms more characteristic of the teeth which are to be 

 developed from them ; and by their rapid growth, they protrude from 



Fig. 55. 



Successive stages of the development of the deciduous or temporary teeth, and of the origin of the 

 sacs of the permanent set. 



the mouths of the follicles (Fig. 55, c). At the same time, the edges 

 of the follicles are lengthened into little valve-like processes, or oper- 

 cula, which are destined to meet and form covers to the follicles (Fig. 

 55, d). There are two of these opercula in the Incisive follicles, three 

 for the Canines, and four or five for the Molars. And by the fourteenth 

 week, the two lips of the dental groove meet over the mouths of the 

 follicles, so as completely to enclose each papilla in a distinct capsule 

 (Fig. 55, e). At this period, before the calcification of the primitive 

 pulps commences, a provision is made for the production of the secon 

 or permanent molars; whose capsules originate in buds or offsets from 

 the upper part of the capsules of the temporary or milk-teeth (Fig. 55,/). 



