Mouth
The mouth, also known as the buccal cavity (Latin bucca= cheek) or oral cavity (Latin cavum oris), is the opening through which humans take in food and issue vocal sounds called voice. It is the cavity found at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing the tongue and teeth.
The lips are two fleshy folds that
surround the oral orifice. They are covered on the outside by skin and are
lined on the inside by mucous membrane. The substance of the lips is made up by
the orbicularis oris muscle and the muscles that radiate from the lips into the
face. Also included are the labial blood vessels and nerves, connective tissue,
and many small salivary glands. The philtrum
is the shallow vertical groove seen in the midline on the outer surface
of the upper lip. Median folds of mucous membrane-the labial frenulae, connect the inner surface of the lips to the
gums.
The Mouth Cavity
The mouth extends from the lips to the
pharynx. The entrance into the pharynx, the oropharyngeal isthmus is formed on each side by the palatoglossal
fold. The mouth is divided into the vestibule and the mouth cavity proper.
Vestibule
The vestibule lies between the lips and
the cheeks externally and the gums and the teeth internally. This slit like
space communicates with the exterior through the oral fissure between the lips.
When the jaws are closed, it communicates with the mouth proper behind the
third molar tooth on each side. The vestibule is limited above and below by the
reflection of the mucous membrane from the lips and cheeks to the gums.
The lateral wall of the vestibule is
formed by the cheek, which is made up by the buccinator muscle and is lined with
mucous membrane. The tone of the buccinator muscle and that of the muscles of
the lips keeps the walls of the vestibule in contact with one another. The duct of the parotid salivary gland opens
on a small papilla into the vestibule opposite the upper second molar tooth.
Mouth Proper
The mouth proper has a roof and a floor.
The roof of the mouth is formed by the hard palate in front and the soft palate
behind. The floor is formed largely by the anterior two thirds of the tongue
and by the reflection of the mucous membrane from the sides of the tongue to
the gum of the mandible. A fold of mucous membrane called the frenulum of the tongue connects the
undersurface of the tongue in the midline to the floor of the mouth.
Lateral to the frenulum the mucous membrane
forms a fringed fold, the plica
fimbriata. The submandibular duct of the submandibular gland opens onto
the floor of the mouth on the summit of a small papilla on either side of the
frenulum of the tongue .The sublingual gland projects up into the mouth,
producing a low fold of mucous membrane, the sublingual fold. Numerous ducts of the gland open on the summit of
the fold.
Mucous Membrane of the Mouth
In the vestibule, the mucous membrane is
tethered to the buccinator muscle by elastic fibers in the submucosa that
prevent redundant folds of mucous membrane from being bitten between the teeth
when the jaws are closed. The mucous membrane of the gingiva, or gum, is
strongly attached to the alveolar periosteum.
Sensory Innervation of the Mouth
Roof: The
greater palatine and nasopalatine nerves from the maxillary division of the
trigeminal nerve.
Floor: The
lingual nerve (common sensation), a branch of the mandibular division of the
trigeminal nerve. The taste fibers travel in the chorda tympani nerve, a branch
of the facial nerve.
Cheek: The
buccal nerve, a branch of the mandibular division of the trigeminal nerve and the
buccinator muscle by the buccal branch of the facial nerve
The
Teeth
Deciduous Teeth-
There is 20 deciduous teeth in each jaw-
·
4 incisors
·
2 canines
·
4 molars
They begin to erupt about 6 months after
birth and have all erupted by the end of 2 years. The teeth of the lower jaw
usually appear before those of the upper jaw.
·
4 incisors
·
2 canines
·
4 premolars
·
6 molars
They begin to erupt at 6 years of age.
The last tooth to erupt is the third molar, which may happen between the ages
of 17 and 30. The teeth of the lower jaw appear before those of the upper jaw.
Parts
Enamel is the hardest and most
highly mineralized substance of the
body. It has its origin from oral ectoderm.
It is one of the four major tissues which make up the tooth, along with dentin, cementum,
and dental pulp. It is normally visible and must
be supported by underlying dentin. 96% of enamel consists of mineral, with
water and organic material comprising the rest.
The normal
color of enamel varies from light yellow to grayish white. At the edges of
teeth where there is no dentin underlying the enamel, the color sometimes has a
slightly blue tone. Since enamel is semi translucent, the color of dentin and
any restorative dental material underneath the enamel strongly affects the
appearance of a tooth.
Enamel's
primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. The large amount of minerals
in enamel accounts not only for its strength but also for its
brittleness. Dentin, which is less mineralized and less brittle,
compensates for enamel and is necessary as a support.
Unlike dentin
and bone, enamel does not contain collagen. Proteins of note in the development of enamel are-
These proteins
help in the development of enamel by serving as framework support, among other
functions. In rare circumstances enamel can fail to form, leaving the
underlying dentine exposed on the surface.
Dentin
Dentin is the
substance between enamel or cementum and the pulp chamber. It is secreted by
the odontoblasts of the dental pulp. The process
is known as dentinogenesis. The porous, yellow-hued material is
made up of the following-
|
·
Inorganic
materials |
70% |
|
·
Organic
materials |
20% |
|
·
Water by weight |
10%. |
Because it is
softer than enamel, it decays more rapidly but dentin still acts as a
protective layer and supports the crown of the tooth. Dentin is a
mineralized connective tissue with an organic matrix of
collagenous proteins. Dentin has microscopic channels, called dentinal tubules,
which radiate outward through the dentin from the pulp cavity to the exterior
cementum or enamel border. The diameter of these tubules range from 2.5 μ
near the pulp, to 1.2 μ in the mid portion, and 900 nm near the
dentino-enamel junction. Length is variable. These tubules do not intersect
with each other.
Types of dentin
There are
three types of dentin-
·
Primary
·
Secondary
·
Tertiary
Primary
dentine is original structure. Secondary dentin is a layer of dentin produced
after root formation and continues to form with age. Tertiary dentin is created in response to
stimulus, such as cavities and tooth wear.
Cementum
Cementum is a
specialized bone like substance covering the root of a tooth. It is composed
approximately of the following-
|
·
Inorganic
material mainly hydroxyapatite |
45% |
|
·
Organic
material mainly collagen |
33% |
|
·
Water |
22% |
Cementum is
excreted by cementoblasts within the root of the tooth
and is thickest at the root apex. Its coloration is yellowish and it is softer
than dentin and enamel. The principal role of cementum is to serve as a medium
by which the periodontal
ligaments can
attach to the tooth for stability.
At the cement
to enamel junction, the cementum is acellular due to its lack of cellular
components, and this acellular type covers at least ⅔ of the root. The
more permeable form of cementum, cellular cementum, covers about ⅓ of the root
apex.
Dental
pulp
The dental
pulp is the central part of the tooth filled with soft connective
tissue. This tissue contains blood vessels and nerves that enter the tooth
from a hole at the apex of the root. Along the border between the dentin
and the pulp are odontoblasts, which initiate the formation of
dentin. Other cells in the pulp include fibroblasts,
preodontoblasts, macrophages and T lymphocytes. The pulp is commonly called
the nerve of the tooth.
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