60 



SPECIAL HISTOLOGY. 



each papilla soon lies in a special cavity. In the course of the 

 fourth month, these cavities gradually contract, the papillae at 



Fig. 194. 



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 de- 

 veloped above each "tooth 

 sac," and thus "reserve 

 sacs," in which pulps begin 

 to be developed in the course 

 of the fifth month, are 

 formed for the twenty an- 

 terior permanent teeth. 



Fig. 195. 



Fig. 194. Lower jaw of a human foetus, nine weeks old, x 9 : a, tongue thrown 

 back; b, right half of the lip depressed; b', left half of the lip cut off; c, outer 

 alveolar wall; d, inner alveolar wall; e, papilla of the first molar ; f, papilla of the 

 canine ; g, of the second ; h, of the first incisor ; i, folds where the ductus Riviniani 

 subsequently open. 



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

 permanent tooth, after Goodsir : a, dental furrow ; b, the same, with its papilla ; c, 

 the same beginning to close, with the rudiment of the reserve cavity; d, the same 

 still more closed ; e, 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 ; i, the permanent tooth 

 forming, its deeper seated sac has a gubemaculum. 



