31(j 



THE TEETH. 



beginning at the posterior cud — and thus by the end of the 15th week 

 the groove is obliterated, and the tooth-germs included in shut sacs ; a 

 condition named by Goodsir the saccular starje (fig. 215, G, 7). He 

 described the closing of the pits as taking place by the growth and 

 mutual union of little lappets or opercula over their orifices (fig. 215, 

 4 and 5 ; fig. 217 o). 



Certain lunated depressions, with enclosed epithelium, which are 

 formed one behind each of the milk-follicles about the fourteenth week, 

 escape the general adhesion of the lips of the groove (figs. 214, Q,fp, 

 217, c, 215). These are the germs of the ten anterior permanent teeth 

 as will be afterwards shown. 



While the above-described changes are going on, the soft embryonic 

 tissue bounding the cavity which contains the tooth-germ, becomes 

 converted into a vascular memdranons sac, and the osseous foundation 

 of the maxilla is laid. The jaw is at first in form of a bony gutter. 



Fig. 216. 



Fig. 2ir. 



Tig. 216. — Enlarged View of the Fpper and Lower Dental Arches ov a Fcetits 

 OP ABOUT Fourteen "Weeks. 



This specimen shows the follicular stage of development of all the milk teeth as 

 described by Goodsir ; in each follicle the papilla is seen projecting ; but this exposure 

 of the papilla and the cavity of the follicle arises from the accidental loss of the 

 eijithelial covering. 



Fig. 217. — Enlarged Diagram of the Dental Arch on the Left Side op the 

 Lower Jaw of a Fcetus op about Fourteen AVeeks (slightly altered from Goodsir). 

 /, the follicles of the five milk teeth, supposed to be open, showing the dental papillse 

 within them, and o, the opercula on their borders ; they are numbered from 1 to 5 in 

 the order of their first appearance ; c, to the inside of each is the lunated depression 

 forming the commencement of the germ of the corresponding permanent tooth. 



in which the teeth-rudiments are lodged ; but this is soon divided by 

 osseous partitions into chambers for the several tooth-sacs, at first with 

 wide openings, which afterwards are narrowed, but so as to allow 

 the contained sacs to cohere with the gum along the border of the jaw. 

 The recesses in question are not the alveoli ; these are formed subsequently 

 around the fangs of the teeth as they rise into their permanent place, and 

 the jaw is deepened by the growth of its alveolar border. 



The dental sacs are well seen in the jaw of an infant a few months old, 

 before the eruption of the teeth. They are represented at this stage in 

 fig. 218. They consist of an outer fibro-vascular coat connected with 

 the periosteum, and an inner highly vascular layer with a little jelly- 



