THE TEETH. 



485 



Fiir. 194. 



pearance. Partitions are now developed, so that each papilla soon lies 



in a special cavity. In the course 



of the fourth month, these cavities 



gradually contract, the papillrc at 



the same time assuming the forms 



of the future teeth, and finally they 



close up completely ; this takes place, 



however, in such a manner that a 



little cavity is developed above each 



" tooth-sac," and thus " reserve 



sacs," in which the pulp begins to 



be developed in the course of the 



fifth month, are formed for the 



twenty anterior permanent teeth. 



Fir. 195. 



These " reserve sacs " at first lie above the milk sacs, but by degrees 

 they retreat backwards, and are received into hollows of the bony alveoli 

 as they are formed round the milk teeth (Fig. 195, g, h). Those of the 

 incisors and canines eventually become completely separated from the 



FIG. 194. Lower jaw of a human foetus, nine weeks old, magnified 9 diameters: a, 

 the tongue thrown back; 6, right half of the lip depressed; b f , left half of the lip cut off; 

 c, outer alveolar wall; d, inner alveolar wall; e, papilla of the first molar; /, papilla of the 

 canine; g, of the second ; /<, of the first incisor; i, folds where the ductus Riviniani subse- 

 quently open, 



FIG. 195. Diagram of the development of a milk-tooth, and of its corresponding perma- 

 nent tooth, after Goodsir : a, dental furrow; b, the same, "with fa papilla; c, the same begin- 

 ning to close, with the rudiment of the reserve cavity; rf, the same, still more closed; c, tooth, 

 sac completed, with a " reserve cavity ;"/, the reserve cavity moving backwards; g, the same 

 become quite posterior, with a pulp ; h, the alveoli of both sacs are forming, the milk-tooth 

 has emerged ; t, the permanent tooth forming, its deeper seated sac has a gubcrnaculum. 



