659
CHAPTER SEVENTEEN
Digestive System
salts and is the hardest substance in the body. If abrasive
action or injury damages enamel, it is not replaced. Enamel
may wear away with age.
The bulk of a tooth beneath the enamel is composed of
a living cellular tissue called
dentin,
a substance much like
bone, but somewhat harder. Dentin, in turn, surrounds the
tooth’s central cavity (pulp cavity), which contains a com-
bination of blood vessels, nerves, and connective tissue,
collectively called pulp. Blood vessels and nerves reach this
cavity through tubular
root canals,
extending into the root.
Tooth loss is most often associated with diseases of the gums
(gingivitis) and the dental pulp (endodontitis).
of the food particles, enabling digestive enzymes to interact
more effectively with nutrient molecules.
Different teeth are adapted to handle food in different
ways. The
incisors
are chisel-shaped, and their sharp edges
bite off large pieces of food. The
canines
are cone-shaped, and
they grasp and tear food. The
premolars
and
molars
have fl at-
tened surfaces and are specialized for grinding food particles.
Each tooth consists of two main portions—the
crown,
which projects beyond the gum, and the
root,
anchored
to the alveolar process of the jaw. The region where these
portions meet is called the
neck
of the tooth. Glossy, white
enamel
covers the crown. Enamel mainly consists of calcium
Incisors
Canine (cuspid)
Canine (cuspid)
Premolars
(bicuspids)
Premolars
(bicuspids)
Molars
Incisors
(a)
Molars
Central
incisors
(b)
Lateral
incisors
Canines
First
premolars
Second
premolars
FIGURE 17.8
This partially dissected child’s skull reveals primary
and developing secondary teeth in the maxilla and mandible.
FIGURE 17.9
Permanent teeth. (
a
) The secondary teeth of the
upper and lower jaws. (
b
) Anterior view of the secondary teeth.
TABLE
17.2
|
Primary and Secondary Teeth
Primary Teeth
(Deciduous)
Secondary Teeth
(Permanent)
Type
Number
Type
Number
Incisor
Incisor
Centra
l
4
Centra
l
4
Latera
l
4
Latera
l
4
Canine (cuspid)
4
Canine (cuspid)
4
Premolar (bicuspid)
First
4
Second
4
Molar
Molar
First
4
First
4
Second
4
Second
4
± ± ±Third
4
Total
20
Total
32
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