188 ANTHROPOLOGY. 
divisions in the upper and two in the lower. These last teeth, however, 
are very variable as to size and other characters, and peor are are not 
protruded, particularly in the upper jaw. 
Every tooth, when divided, presents an external shell or cortex, and an 
internal cavity which extends beneath the crown, partly derredpieniaiae to it 
in form, and contracts as it descends through the root into a fine tube, which 
ends in a minute foramen. In the young and growing tooth this cavity is 
large, and contains a-soft, vascular pulp or bulb; in the course of time it is 
much diminished by surrounding osseous or ivory deposit, and in the adult 
or aged contains little more than a vascular lining membrane. The walls 
of this cavity, though very firm, are perforated by numerous minute pores, 
which lead into the solid texture of the tooth. The substance of a tooth 
consists of three elements: enamel, ivory or dentine, and crusta petrosa or 
cement. The enamel covers the crown; the ivory forms the greater por- 
tion of the body and root; and the crusta, in the form of a thin lamina, 
invests the root only, though, according to some, it is also prolonged over 
the crown and enamel. 
The teeth make their appearance at a very early age of the foetus, 
although requiring a long time for their full development; it is not till the 
sixth or seventh month after birth that the two middle incisors of the lower 
jaw come out. ‘The remaining incisors follow at intervals of from four to 
six weeks; first the two median upper ones; then the two exterior lower, 
and lastly the two exterior upper. The upper and lower anterior molars of 
each side of the jaw next make their appearance, followed by the canines, 
and finally by the posterior molars. At the end of the second year the 
child has twenty teeth. These are called milk teeth, from making their 
appearance during the period of suckling. The incisive and canine milk 
teeth are smaller than the permanent; the posterior molars on the other 
hand are larger. About the seventh year the milk teeth fall out and are 
replaced by the permanent, in nearly the same order as that of their first 
appearance. After all the incisors are changed, the anterior and posterior 
temporary molars are successively shed, and replaced by the permanent 
bicuspids; the canines are not changed before the tenth or eleventh year. 
After the twelfth or thirteenth the second permanent molars appear, and the 
last or dentes sapientize (wisdom teeth) seldom before eighteen or twenty, 
and occasionally at a much later period. 
Pl. 123, fig. 18, the teeth of both jaws from before. Fig. 19, do. from 
the side. Fig. 20, the lower jaw of a child four years old with the milk 
teeth and their sockets. /vg. 21, the same for the upper jaw. S2gs. 22 
and 23, upper and lower jaw opened, with the milk and permanent teeth in 
their sockets or alveoli. fig. 24, dental sac of a milk tooth with its vessels. 
Fig. 25, do. of a permanent tooth. 7g. 26, vertical section of the tooth 
cavity contained by the sac. Fg. 27 a-e, the teeth of the right upper jaw 
of a foetus of about eight months. ig. 28 a-e, teeth of a newborn child. 
Eig. 29 a-e, teeth of a child of four years. /%g. 80, second set of upper 
jaw teeth a a four years’ old child seen from within. Fg. 31, do. from 
below. Fig. 32, teeth of a child at seven years: A, milk teeth; B, per- 
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