DEVELOPMENT OF THE TEETH. ote 
6. Turning now to the jaw itself: The connective tissue of the gum surrounding the 
tooth germ (as the developing tooth with its enamel organ and dentine papilla are called) 
early becomes condensed and vascular (Fig. 659, V.), and later on forms a membranous bag 
—the tooth sac or follicle—which completely shuts off the developing tooth from the 
surrounding structures. On the floor of this sac the tooth gerin sits, the base of its 
dentine papilla being continuous with the tissue of the floor of the sac, and the young 
tooth being enclosed by the sac, as a kernel is enclosed by its shell. ; 
7. Reverting to the tooth: When the crown is completed the deposit of dentine, but 
not of enamel, is continued downwards to form the root. This latter is composed chiefly 
of dentine continuous above with that of the crown, and like it formed by the odontoblasts 
of the dentine papilla. As the dentine is deposited, and the root is being built up, the 
connective tissue of the tooth sac comes to surround the root more closely, and deposits on 
its surface, after the manner of a periosteum, a layer of bone, the cementum or crusta 
petrosa. The cementum having been formed, the connective tissue of the sac then per- 
sists as the alveolar periosteum. The development of the root takes place very slowly, 
and its lower end is not completed as a rule for some time after the eruption of 
the tooth has taken place. 
8. During the development of the teeth the ossification of the jaw has being going on, 
and as it grows up on each side, the young teeth, enclosed in their tooth sacs, come to lie 
in an open bony groove, which is subsequeutly divided by septa into compartments—the 
alveoli—for the individual tooth sacs. The bone continuing to grow after birth, these 
compartments become more perfect, but are never entirely closed in over the crowns of 
the teeth. During the eruption of the teeth the upper and anterior part of these bony 
cells is absorbed ; subsequently, however, it is reformed around each tooth when it has 
taken its final position. 
9. Eruption.—Long before the root is completed, the crown, by some force which is 
not properly understood, but which does not seem to depend on additions to the root, is 
pushed through the top of the tooth sac, and—the upper and anterior wall of the roomy 
alveolus having been absorbed at the same time—onwards through the gum until the 
mouth is reached. Later on, when the tooth has assumed its final position, the alveolus, 
as already stated, is reformed, and closely embraces the completed root. 
10. After the enamel organs of the milk teeth have been formed on the inferior aspect 
of the dental lamina, as described above, the neck of epithelium by which the lamina is 
still connected with the surface becomes broken up into a cribriform sheet. Its free 
posterior border, on the other hand, continues to grow backwards in the tissue of the 
gum towards the cavity of the mouth (Fig. 659, III. and IV.), and at a later date there 
appear on its under surface, near the free edge, and behind the several developing milk 
teeth, the enamel organs—or so-called ‘“ reserve germs ”—for the corresponding permanent 
teeth, which are developed from these in exactly the same manner as the milk teeth 
described above. 
In connexion with the development of the permanent molars, which have no corre- 
sponding teeth in the milk set, there takes place a prolongation backwards of the posterior 
extremity of the dental lamina into the tissue of the Jaw, behind the last milk molar. On 
the inferior aspect of this prolongation, which has no direct connexion with the surface 
epithelium, enamel organs are formed for the permanent molars, and their further 
development goes on in the manner described for the other teeth. 
The dates at which some of the chief events in the development of the teeth occur 
may be briefly given :—The thickening of the epithelium, the first sign of the future 
teeth, begins about the sixth week of foetal life, and the dental lamina is completed by the 
end of the seventh week. 
The dentine papilla for the eight front teeth appear and become surrounded by their 
enamel organs about the tenth week, and the papilla for the first permanent molar about 
the seventeenth week. 
The first traces of calcification, and the formation of the tooth sacs, takes place about 
the fifth month of foetal life. 
Eruption of Deciduous Teeth.—The period at which the eruption of the milk teeth 
takes place is extremely variable, and no two observers seem to agree upon the ques- 
tion. The following, according to Tomes, may be taken as representing the average. 
The lower central incisors appear first, usually between the sixth and ninth months ; 
then follows a rest of a few months. Next come the four upper incisors, followed by a 
rest of a few months. Then the lower lateral incisors and the four first molars erupt, 
succeeded by a rest of afew months. Next appear the canines, and finally the four second 
molars, which are all cut by the end of the second year. 
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