860
UNIT 4
Maintenance of the Body
23
Similarly, the permanent dentition [two incisors, one canine,
two premolars (PM), and three molars] is
Tooth Structure
Each tooth has two major regions: the crown and the root
(Figure 23.11)
. Te enamel-covered
crown
is the exposed part
of the tooth above the
gingiva
(jin
9
jĭ-vah), or
gum
, which sur-
rounds the tooth like a tight collar.
Enamel
, a brittle ceramic-
like material thick as a dime, directly bears the force of chewing.
Te hardest substance in the body, it is heavily mineralized with
calcium salts, and its densely packed hydroxyapatite (mineral)
crystals are oriented in force-resisting columns perpendicular
to the tooth’s surface. Te cells that produce enamel degenerate
when the tooth erupts; consequently, decayed or cracked areas
of enamel will not heal and must be artificially filled.
Te
root
is the portion of the tooth embedded in the jaw-
bone. Canine teeth, incisors, and premolars have one root,
although the first upper premolars commonly have two. Te
first two upper molars have three roots, while the correspond-
pieces of food. Te conical or fanglike
canines
(cuspids or eye-
teeth) tear and pierce. Te
premolars
(bicuspids) and
molars
have broad crowns with rounded cusps (tips) best suited for
grinding or crushing. Te molars, with four or five cusps, are
the best grinders. During chewing, the upper and lower molars
repeatedly lock together, an action that generates tremendous
crushing forces.
Te
dental formula
is a shorthand way of indicating the
numbers and relative positions of the different types of teeth.
Tis formula is written as a ratio, uppers over lowers, for
one-
half
of the mouth. Since the other side is a mirror image, we
obtain total dentition by multiplying the dental formula by 2.
Te primary dentition consists of two incisors (I), one canine
(C), and two molars (M) on each side of each jaw, and its dental
formula is written as
Incisors
Central (6–8 mo)
Incisors
Central (7 yr)
Canine (eyetooth)
(16–20 mo)
Canine (eyetooth)
(11 yr)
Premolars
(bicuspids)
First premolar
(11 yr)
Molars
First molar
(10–15 mo)
Molars
First molar (6–7 yr)
Lateral (8–10 mo)
Lateral (8 yr)
Second molar
(about 2 yr)
Second molar
(12–13 yr)
Third molar
(wisdom tooth)
(17–25 yr)
Permanent
teeth
Deciduous
(milk) teeth
Second premolar
(12–13 yr)
Figure 23.10
Human dentition.
Teeth of the lower jaw: the
deciduous and permanent sets. Approximate age at which tooth
erupts is shown in parentheses. The shapes of individual teeth are
shown on the right.
2I, 1C, 2M (upper jaw)
3
2
(20 teeth)
2I, 1C, 2M (lower jaw)
2I, 1C, 2PM, 3M
3
2
(32 teeth)
2I, 1C, 2PM, 3M
Crown
Neck
Root
Enamel
Dentin
Dentinal tubules
Pulp cavity
(contains
blood vessels
and nerves)
Gingiva
(gum)
Gingival
sulcus
Cement
Root canal
Periodontal
ligament
Apical
foramen
Bone
Figure 23.11
Longitudinal section of a canine tooth within
its bony socket (alveolus).
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