Teeth
The teeth are
specialized structures that bite or tear off the pieces of solid food that
enter the oral cavity and chop and grind this food as it is being mixed with
saliva in preparation for swallowing. The muscles of mastication are
responsible for the movements of the lower teeth in relation to the upper
teeth, and the tongue and cheeks are responsible for positioning food between
the teeth as necessary.
Humans develop two sets of teeth, a
deciduous set (milk teeth), which begin to come in at about the age of 6
months, and a so-called permanent set, which gradually begin to replace the
deciduous set at about the age of 6 years.
The deciduous teeth number 20
in all, 5 on each side of the upper and lower jaws. Starting at the midline of
each jaw and progressing laterally and posteriorly to each side, the deciduous
teeth are named in order: central (medial) incisor, lateral incisor,
canine (cuspid), first molar, and second molar. The four
teeth of the same name are differentiated by designating which jaw and which
side of the jaw, as right or left upper (maxillary) or lower (mandibular)
central incisor. The deciduous teeth are smaller than the permanent teeth that
take their position.
The permanent teeth, once all
have come in, number 32, 8 on each side of the upper and lower jaws. Starting
at the midline of each jaw and progressing laterally and posteriorly to each side,
the permanent teeth are named in order: central (medial) incisor,
lateral incisor, canine (cuspid), first premolar (bicuspid), second
premolar (bicuspid), first molar, second molar, and third molar (wisdom
tooth). The incisors, and to some extent the canines, are adapted for biting
the food, whereas the molars, and to some extent the premolars, are adapted for
grinding and pounding food.
Normally, the upper dental arch is
wider than the lower dental arch, and the upper incisors and canines overlap
the lower incisors and canines. When the jaws are closed (in occlusion), the
teeth of the two jaws come into contact in such a way that their chewing
surfaces fit each other, which means that the teeth of one jaw are not exactly
opposite the corresponding teeth of the other jaw. In spite of this, because
the lower molars, especially the third molars, are longer anteroposteriorly,
the dental arches end at approximately the same place posteriorly. Teeth are
described as having a labial (buccal) surface, a lingual surface, and a contact
surface. Ordinarily, none of the teeth have erupted before birth, but all of
the deciduous teeth usually come in between the sixth month and the end of the
second year. The time of eruption varies considerably as does any timetable of
development. The possible range of the time at which each deciduous tooth may
erupt is indicated in the parentheses below the name of each tooth in the
accompanying illustration.
From the end of the second year until
the sixth year, no visible change in the teeth takes place. At about the sixth
year, the first permanent molar comes in posterior to the second deciduous
molar, and it is important that this be recognized as a permanent tooth and
given the care that a permanent tooth merits. Starting in the seventh year,
gradual replacement of the deciduous teeth by the permanent teeth takes place,
which is usually completed by the twelfth year. The second molar, as a rule,
emerges about this time, and the third molar, if it erupts at all, several
years later. The approximate time at which each permanent tooth may erupt is
specified in the accompanying illustration below the name of the tooth. The
developing permanent teeth are present within the jaw long before they erupt.
Obviously, during the eruption of the teeth, growth changes must occur in the
jaws.
The crown of a tooth is the
portion of the tooth projecting beyond the gum. It differs in shape in
different types of teeth, the difference being related to the functional
adaptation of the tooth. The crown of an incisor is chisel shaped, that of a
canine is large and more conical, and the crowns of the premol rs and molars
are flattened and broad, with tubercles.
The neck of a tooth is the
short, constricted portion that connects the crown and the root.
The root of a tooth is the
portion embedded in the alveolar process of the jaw. It is long, tapering, and
fitted to its socket. The root of an incisor is usually single, the canine has
a single long root, and that of a premolar is usually single, flattened
anteroposteriorly, and grooved, with some tendency to division. Each molar has
two roots, an anterior root and a posterior root, which are apt to be wide,
flattened anteroposteriorly, grooved, and perhaps partially divided. At the tip
(or tips) of each root is a minute opening called the apical foramen, which
allows passage for blood vessels and nerves to the root.
The interior of a tooth contains a
space called the cavity of the tooth (pulp cavity), which is filled in the
natural state by loose connective tissue, capillaries, nerves, and lymphatics,
collectively called the pulp, on the outer surface of which is a layer
of cells called odontoblasts. The cavity extends into each root as the
tapering root canal, which ends at the apical foramen.
Surrounding the cavity is the dentin,
which constitutes the mass of the tooth and is a hard, highly calcified
(only 28% organic matter), homogeneous material. It is traversed by dentinal
tubules (dental canaliculi) extending from the cavity to the outer margin
of the dentin. The dentinal tubules are occupied by processes of the odontoblasts,
which create the dentin.
Forming a cap over the dentin of the
crown is the dense, white, and glistening enamel, the hardest (only
about 3% organic material) and most resistant material in the body. It is made
up of solid, hexagonal prisms (enamel prisms), which are oriented essentially
perpendicular to the related surface of the crown. It is created within the
gums by cells called ameloblasts.
Cementum, modified bone having lamellae, canaliculi, and
lacunae, covers the dentin of the roots. It is very thin at its beginning at
the neck and increases in thickness toward the root’s apex.
The root of the tooth is united to the
wall of the socket by an important layer of vascular fibrous connective tissue,
the alveolar periosteum or periodontal ligament. This layer is
continuous with the lamina propria of the gum at the alveolar process margin
(near the neck of the tooth).
The covering of the internal and
external surfaces of the alveolar processes of the maxilla and mandible, the gums
or gingivae, is made up of stratified squamous epithelium, resting on
a thick, strong lamina propria, which is firmly attached to the
underlying bone. This, being a fusion of mucous membrane and periosteum, could
be called mucoperiosteum. The gum forms a free fold, which surrounds the base
of the crown of the tooth for a short distance like a collar.
The arteries and nerves that supply
the teeth are branches of the superior and inferior alveolar arteries and
nerves. These travel partly to the pulp cavity and partly to the surrounding
periodontal ligament. Branches to the pulp cavity travel by way of the apical
foramen and root canal. The vessels form a rich capillary plexus under the
odontoblast layer.
The enamel of the tooth originates
from the oral ectoderm, which differentiates into ameloblasts, and the rest of
the tooth comes from the mesenchymal tissue of the maxillary and mandibular
arches. The ameloblasts and odontoblasts create a bell-shaped structure as they
lay down enamel and dentin to create the tooth within the mesenchyme. The
mesenchyme remains in the dental pulp, allowing the alveolar vessels and nerves
to reach the developing tissues.
