DEVELOPMENT OF THE PERMANENT TEETH. 801 



several parts of a tooth, applies to the permanent as well as to the milk- 

 teeth. 



The origin and progressive development of the sacs of the permanent teeth 

 have still to be considered. There are six more permanent teeth in each jaw 

 than there are milk-teeth, and it is found that the sacs of the ten anterior 

 permanent teeth, which succeed the ten milk-teeth, have a different mode of 

 origin from the six additional or superadded teeth, which are formed further 

 back in the jaw. 



Fig. 559. ENLARGED DIAGRAM OP THE DENTAL Fig. 559. 



ARCH ON THE LEFT SIDE ov THE LOWER JAW OF A 



FCETUS OF ABOUT FOURTEEN WEEKS (slightly 



altered from Goodsir). 



/, the follicles of the five milk-teeth, supposed lo 

 be open, showing the dental papillae within them, 

 and o, the opercula on their borders ; they are 

 numbered from 1 to 5 in the order of their first ap- 

 pearance ; c, to the inside of each is the lunated 

 depression forming the commencement of the germ 

 of the corresponding permanent tooth ; a b, line of 

 the section shown in fig. 551, 5. 



The sacs and pulps of the ten anterior per- 

 manent teeth have their foundations laid 

 before birth, behind those of the milk set. 

 Recurring to the follicular stage of the tem- 

 porary teeth, which is completed about the 



fourteenth week, it will be remembered that behind each milk-follicle there is 

 formed a small lunated recess, similar in form to an impression made by the 

 nail. As already stated, the mucous membrane lining these recesses escapes 

 the general adhesion of the lips and sides of the dental groove, so that when 

 the latter closes they are converted into so many cavities, which are called 

 by Goodsir, " cavities of reserve." They are ten in number in each jaw, 

 and are formed successively from before backwards. They ultimately form 

 the sacs for the permanent incisor, canine, and bicuspid teeth. These cavi- 

 ties soon elongate and recede into the substance of the gum behind the milk 

 follicles, above and behind in the upper jaw, below and behind in the lower. 

 In the meantime, a papilla appears in the bottom of each, (that for the 

 central incisor appearing first, at about the sixth month,) and one or more 

 folds or opercula, as in the case of the temporary teeth, are developed from 

 the sides of the cavity, and, by their subsequent union, divide it into two 

 portions, the lower portion containing the papilla, and now forming the 

 dental sac and pulp of the permanent tooth, and the upper and narrower 

 portion being gradually obliterated in the same manner as the primitive 

 groove was closed over the milk-sacs. When these changes have taken 

 place, the permanent sac adheres to the back of that for the temporary 

 tooth. Both of them continue then to grow rapidly, and after a time it is 

 found that the bony socket not only forms a cell for the reception of the 

 milk-sac, but also a small posterior recess or niche for the permanent sac, 

 with which the recess keeps pace in its growth. Confining our description 

 now, for convenience, to the lower jaw only, it is found that at length the 

 permanent sac so far recedes in the bone as to be lodged in a special osseous 

 cavity at some distance below and behind the milk-tooth, the two being com- 

 pletely separated from each other by a bony partition. In descending into 



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