Page 132 - My FlipBook

116 HUNTER ON THE TEETH.
About the fifth month both the processes themselves and the
The following description of the formation of the different parts
of the dental follicle has been generally adopted on the authority
of Goodsir. About the sixth week of foetal life a groove, the primitive
dental groove, is formed along the edge of the jaw, in the mucous mem-
brane of the gum ; from its floor ten papillae rise in succession in each
jaw. The papillae appear in the following order:—the first milk molar
(7th week) ; the canine (8th week) ; the two incisors (about the 9th
week) ; the second milk molar (10th week). The papillae in the upper
jaw appear a little before those in the lower. With the appearance of
the last papilla the papillarij stage terminates. The next stage is the
follicular. This consists in the thickening and deepening of the margins
of the dental groove, and the separation of the papillae by the formation
of membranous partitions which pass across from one margin of the
groove to the other. This stage is completed about the fourteenth week.
In the early part of this stage the papillae grow rapidly and begin to
assume peculiarities of form according with those of the crowns of the
future teeth. The follicular stage is converted into the saccular about
the end of the fifteenth week. The change consists in the formation of
small laminae or opercula of membrane which project from the sides of
the follicle, and at last, by meeting and cohering, convert the follicle
into a closed sac. The lips of the dental groove also become thickened
and unite, so that the groove is at length entirely obliterated. The num-
ber of opercida in each follicle is said to vary with the character of the
future tooth, there being four or five in the case of the molar ; two,
one external and one internal, in that of the incisor; and three,
two internal and one external for the canine. About the fourteenth
week a lunated depression is formed behind each milk follicle ; these
escape the general adhesion of the margins of the groove. In them the
pulps of the ten anterior permanent teeth are subsequently developed
;
they are called by Mr. Goodsir " cavities of reserve." The formation of
the pulp and the closure of the cavity by 02?ercula is similar to that
already described in the case of the follicles of the milk teeth. The pulp
of the first permanent molar arises as a papilla about the sixteenth week
in a portion of the primitive dental groove which remains unclosed
behind the sac of the second milk molar. The papilla seated at the bot-
tom of this "posterior cavity of reserve'" becomes shut off and enclosed
in a sac by the formation and cohesion of opercula, whilst the upper por-
tion of the cavity lengthens backwards, and forms a second cavity of
reserve in which the papilla for the second molar tooth appears about
the seventh month after birth. Ultimately the same change again occurs,
and gives rise to the formation of the papilla and sac of the wisdom
tooth, which takes place about the sixth year.
Considerable doubt, however, has been thrown by various observers
on the account given by Arnold and Goodsir. Valentine and Rasch-