SECT. 141.] THE TEETH. JOI 



tenth weekj papilla gradually arise in them, ten in each jaw, or 

 twenty in all. These papilla or tooth-germs are soon separated by 

 transverse partitions, so that each comes to lie in a special cavity. 

 In the fourth month, these cavities become narrower and nar- 

 rower, the papillae at the same time assuming the form of the 

 subsequent teeth ; and, at length, are perfectly closed, but in such 

 a manner, that over each cavity or tooth-sac, another small recess 

 is formed as cavity of reserve for the twenty anterior permanent 

 teeth, of which, even in the fifth month of foetal life, the tooth- 

 germs are developed. At first, the new cavities lie over the tooth- 

 sacs of the milk-teeth, but they gradually move to the posterior 

 side of them; and, when the bony alveoli of the milk-teeth appear, 

 are received into small dilatations of them, which, in the incisor 

 and canine teeth, become, at last, completely separate from the 

 others, in the two first molars, on the other hand, open at the 

 bottom of the alveoli of the milk-teeth. The tooth-sacs of all 

 these teeth are drawn out at the apex in form of a solid cord, 

 which extends either to the gum or on the two first molars, to 

 the periosteum at the bottom of the two milk-molars, and which 

 has incorrectly been considered as being a conducting band (guber- 

 naculum) of the teeth in their eruption. 



With respect to the sacs of the three last permanent molars, that 

 of the first arises in the sixteenth or seventeenth week, quite inde- 

 pendently, from the posterior extremity of the primitive dental 

 groove, and closes in such a manner, that a cavity of reserve 

 remains between it and the mucous membrane (my Micr. Anat., 

 fig. 206). In the seventh or eighth month after birth, the cavity 

 of reserve lengthens and extends in an arched form behind the 

 first sac into the alveolar border, produces upon its floor a papilla, 

 upon which it becomes constricted, to form with its lower part 

 the sac of the fourth molar, while the remaining or upper part 

 falls into a line with the other sacs, and is converted into the sac 

 of the wisdom tooth. 



The formation of the milk-teeth commences in the fifth month 

 of foetal life, and in the seventh month they are all in process 

 of ossification. The ossification begins at the apex of the tooth- 

 pulp by the formation of small scales of dentine, which in the 

 molars correspond in number to the prominences of the papilla, 

 or in the crown of the future tooth, but soon coalesce with each 

 other. Immediately after the appearance of a dentinal scale, a 

 thin laver of enamel also arises from the so-called enamel organ 

 at the roof of the tooth-sac (see infra) , which coalesces with the 



