PORMATIOX OF THE TEETH. 313 



bone, run abundantly through the cement. In chemical composition it 

 resembles bone, and contains 30 per cent, of animal matter. The 

 cement is, according to some, extremely sensitive at the neck of the 

 tooth, if it be exposed by retraction of the gum. This is probably due 

 to the nerves of its periosteal covering. By its connection with the 

 surrounding membranous structures it contributes to fix the tooth in 

 the socket. It is the seat of the bony growths or exostoses sometimes 

 found upon the teeth. 



FORMATION OF THE TEETH. 



A tooth is formed on the same fundamental type of development as a 

 hair. In the latter case a process grows down from the Malpighian 

 layer of the epidermis into the subjacent cutaneous corium, in which a 

 depression is simultaneously produced for its reception. A papilla, soon 

 becoming vascular, rises up from the bottom of the depression into the 

 cellular mass, and the primitive tissue forming the wall of the recess is 

 converted into the coats of a follicle. In the formation of a tooth there 

 is in like manner a downgrowth from the Malpighian layer of the oral 

 epithelium (which corresponds with the epidermis and is derived from 

 the same embryonic layer). The cellular process is received into a recess 

 of the subjacent mucous membrane. In this also a vascular papilla 

 grows up from the bottom, and the simple wall of the cavity is differ- 

 entiated into a vascular sac or follicle. 



The first recognized steps in the development of the teeth take place 

 as early as the seventh week of intra-uterine life. At this time the 

 oral epithelium becomes thickened along the border of the jaws, and its 

 Malpighian layer grows down into a corresponding groove, which is 

 formed to receive it in the soft embryonic tissue of which the jaw then 

 consists. The groove, although filled and covered in by the epithelium, 

 is still faintly indicated by a shallow superficial furrow. This down- 

 growth of epithelium, which is named the " enamel-germ," forms 

 the foundation of the special structures or organs which generate the 

 enamel in the several teeth, and for the sake of distinction may be 

 termed the coinmon enaniel-gerni. The groove, as well as the 

 changes subsequently occurring in it, was observed by F. Arnold and 

 by Goodsir, who named it the " primitive dental groove," but neither 

 of these observers appears to have noticed the contained epithelium 

 (or at least to have recognized its importance), probably in conse- 

 quence of its soft and friable substance having been accidentally wiped 

 away. The common enamel-germ, simultaneously with the groove, next 

 increases in depth, and, at the same time, its deeper portion inclines 

 outwards, forming an angle with its more superficial part. It also swells 

 out below, so that a transverse vertical section of it is club or flask- 

 shaped. An increased development then takes place at particular 

 points, corresponding in situation with the ten milk-teeth ; and the 

 common enamel-germ thus becomes parted in its deeper portion, or 

 extended by further growth, into as many distinct aggregations of cells, 

 or special enaiuel-gernis, — one for each tooth — of a club or flask-shape, 

 connected by a narrowed neck with what remains of the common 

 epithelial ingrowth (fig. 214, A, /). These tooth-germs, as they may 

 now be called, are lodged each in its own recess, which at this time 

 is merely a pit in the soft embryonic tissue, without the membranous 



