Teeth embryonic development. Early stage of tooth
development. Late stage of tooth development. Permanent teeth laying. Deciduous
teeth replacement by permanent ones. Enamel micro- and ultrastructure. Dentine
and pulp micro- and ultrastructure. Teeth supporting apparatus. Cementum
structure. Pharynx. Tonsilles.
Digestion
begins in the mouth, well before food reaches the stomach. When we see,
smell, taste, or even imagine a tasty meal, our salivary glands, which are located under the tongue
and near the lower jaw, begin producing saliva. This flow of saliva is set in
motion by a brain reflex that's triggered when we sense food or think about
eating. In response to this sensory stimulation, the brain sends impulses
through the nerves that control the salivary glands, telling them to prepare
for a meal.
As the
teeth tear and chop the food, saliva moistens it for easy swallowing. A
digestive enzyme called amylase, which is found in saliva, starts to
break down some of the carbohydrates (starches and sugars) in the food even
before it leaves the mouth.
Swallowing,
which is accomplished by muscle movements in the tongue and mouth, moves the
food into the throat, or pharynx. The pharynx, a passageway for food and air, is about
From
the throat, food travels down a muscular tube in the chest called the esophagus. Waves of muscle contractions called
peristalsis force food down through the esophagus to the stomach.
A person normally isn't aware of the movements of the esophagus, stomach, and
intestine that take place as food passes through the digestive tract.
The digestive system consists of
the digestive tract and its associated glands. Its functions are to obtain from ingested food the metabolites
necessary for the growth and energy needs of the body. Before stored or used as
energy, food is degested and transformed into small molecules that can be
easily absorbed through the lining of the digestive tract. However, a barrier
between the environment and the internal milieu of the body must be maintained.
The first step in the comlex process known as digestion occurs in the mouth,
where food is ground into smaller pieces by mastification and moistened by
saliva, which also initiates the digestion of carbohydrates. Digestion
continues in the stomach and small intestine, the food – transformed into basic
components (aminoacids, monosaccharides, glycerides, etc) – is absorbed. Water
absorption occurs in the large intestine, and as a consequence the undisgested
contents become semisolid.
The digestive process
commences in the oral cavity with the ingestion, fragmentation and moistening of
food but, in addition to its digestive role, the oral cavity is involved in
speech, facial expression, sensory reception and breathing. The major structures of the oral cavity, the
lips, teeth, tongue, oral mucosa and the
associated salivary glands, participate in
all these functions. Mastication is the process by which ingested food 1% made suitable
for swallowing. Chewing not only involves coordinated movements of the mandible and the
cutting and granding action of the teeth but also activity of the lips and
tongue, which continually redirect food between the occlusal surfaces of the
teeth. The watery component of saliva moistens and lubricates the masticatory
process whilst salivary mucus helps to bind the food bolus ready for
swallowing. The entire oral cavity is lined by a protective mucous membrane,
the oral mucosa, which contains many sensory receptors, including the taste
receptors of the tongue.
The epithelium of the oral
mucosa is of the stratified squamous type which tends to be keratinised in
areas subject to considerable friction such as the palate. The oral epithelium
is supported by dense collagenous tissue, the lamina propria. The roof of the
mouth consists of the hard and soft palates, both covered with the same type of
stratified epithelium. In highly mobile areas such as the soft palate and floor
of the mouth, the lamina propria is connected to the underlying muscle by loose
submucosal supporting tissue. In contrast, in areas where the oral mucosa
overlies bone, such as the hard palate and tooth-bearing ridges, the lamina
propria is tightly bound to the periosteum by a relatively thin dense fibrous
submucosa. Throughout the oral mucosa numerous small accessory salivary glands
of both serous and mucous types are distributed in the submucosa.
The palatine uvula is a
small conical process that extends downward from the center of the lower border
of the soft palate. It has a core of muscle and areolar connective tissue
covered by typical oral mucosa.\
The Tooth
A
tooth is a hard structure, set in the upper or lower jaw, that is used for
chewing food. Teeth also give shape to the face and aid in the process of
speaking clearly. The enamel that covers the crown (the part above the gum) in
each tooth can be broken down by acids produced by the mouth for digestive
purposes. This process is called "decay". To prevent decay, good oral
hygiene, consisting of daily brushing and flossing, is necessary. The hardest
substance in the human body is one of the four kinds of tissue which make up
the tooth. It is enamel and covers the crown (area above the gum line) of the
tooth. A bony material called "cementum" covers the root, which fits
into the jaw socket and is joined to it with membranes. "Dentin" is
found under the enamel and the cementum, and this material forms the largest
part of the tooth. At the heart of each tooth is living "pulp," which
contains nerves, connective tissues, blood vessels and lymphatics. When a
person gets a toothache, the pulp is what hurts.
TEETH. Each tooth may be grossly divided into three segments, the crown, neck
and the root: the crown is that portion which projects into the oral cavity and
is protected by a layer of highly mineralised enamel which coversit entirely.
The bulk of the tooth is made up of dentine, a mineralised tissue which has a
similar chemical composition to bone. The dentine has a central pulp cavity
containing the dental pulp which consists of specialised supporting tissue
containing many sensory nerve fibers.
Neck of the tooth is its
short part between the crown and neck.
The tooth root is embedded
in a bony redge in the faw called the alveolar ridge: the tooth socket is known
as the alveolus and, at the lip or cheek aspect of the alveolus, the bony plate
is generally thinner than at the tongue or palatal aspect. The root of the
tooth is invested by a thin layer of cementum which is connected to the bone of
the socket by a thin fibrous layer called the periodontal ligament or
periodontal membrane. The oral mucosa covering the upper part of the alveolar
ridge is called the gingiva and at the junction of the crown and root
protective cuff around the tooth. The potential space between the gingival cuff
and the enamel of the crown is called the ginglval crevice. All of the tissues
which surround and support the tooth are collectively known as the periodontiym.
The dentine which forms the bulk of the crown and root is composed of a calcified
organic matrix similar to that of bone. The inorganic component (70 %)
constitutes a somewhat larger proportion of the matrix of dentine than that of
bone and exists mainly in the form of hydroxiapatite crystals. From the pulp
cavity, minute parallel tubules, called dentine tubules, radiate to the
periphery of the dentine: in longitudinal sections of teeth, the tubules appear
to follow an S-shaped course.
The crown of the tooth is
covered by enamel, an extremely hard, translucent substance composed of
parallel rods or prisms of highly calcified material cemented together by an
almost equally calcified interprismatic material. The root is invested by a
thin layer of cementum which is generally thicker towards the apex of the root.
The morphological form of
the tooth crown and roots varies considerably in different parts of the mouth;
nevertheless the basic arrangement of the dental tissues is the same in all teeth.
In humans, the primary (deciduous) dentition consists of 20 teeth
comprising two incisors, one canine and two molars in each quadrant. These
begin to be formed at the age of 6 weeks during fetal development and erupt
between the ages of 8 and 30 months after birth. Between the ages of 6 and 12 years, the deciduous teeth are
succeeded by permanent teeth, namely two incisors, one canine and two premolars
in each arch. Distal to these will develop three permanent molars having no
primary precursors: the first permanent molar erupts at age 6, the second at
age 12 and the third
(wisdom tooth) at age 17 to 21 years The points found on the posterior teeth are known
as cusps.
Odontoblasts and dentine.
Dentlne, the dense
calcified tissue which forms the bulk of the tooth, is broadly similar to bone
composition but is more highly mineralised and thus much harder than bone.
The cells responsible for
dentine formation, the odontoblasts, di ferentiateas a single layer of tall
columnar cells on the surface of the dental papilla apposed to the ameloblast
layer of the enamel organ. The odontoblasts initiate tooth formation by
deposition af organic dentine matrix between the odontoblastic and ameloblastic
layers: calclficatlon of this dentine matrix then induces enamel formation by
ameloblasts.
Dentine formation proceeds
by continuing odontobiastic deposition of dentine matrix and its subsequent
calcification: unlike ameloblasts, each odontoblast leaves behind a slender
cytoplasmic extension, the odontoblastic process, within a fine dentlal tubule.
When dentine formation is complete, the dentine is thus pervaded by parallel
odontoblastic processes radiating from the odontoblast layer on the dentinal
surface of the reduced dental papilla which now constitutes the dental pulp.
After tooth formation is complete, a small amount of less organised secondary
dentine continues to be laid down resulting in the progressive obliteration of
the pulp cavity with advancing age. Parallel dentine tubules containing
odontoblastic processes extend through a narrow pale-stained zone of
uncalcified dentine matrix called predentine into the mature dentine.
Underlying the odontoblastic layer, a relatively acellular layer, called the
cell free zone of Weil ,gives way to the highly cellular dental pulp.
Enamel hardest
material of the body, it is composed almost entirely of the mineral
hydroxyapatite (Ca10(PO4)6(OH)2), which is arranged in highly packed hexagonal
enamel rods or prisms about
a
The small interstices
between adjacent rods are occupied by hydroxyapatite crystals. A small amount
of organic matrix (protein and polysaccharide) represents the remnants of the
matrix synthesized and excreted by the enamel-producing cell, the ameloblasts,
and prior to mineralization of the enamel. Enamel covers the dentine only in
the region of the exposed crown; in the root the dentine is covered by
cementum.
Dental pulp. The dental pulp consits of a delicate supporting connective tissue
resembling primitive mesenchyme. lt contains numerous stellate fibroblasts,
reticuline fibres, flne poorly organised collagen fibres and much ground
substance. The pulp contains a rich network of thin-walled capillaries supplied by
arterioles which enter the pulp canal from the periodontal membrane, usually
via one foramen at each root apex. The pulp is also richly innervated by a
plexus of myelinated nerve fibres from which fine non-myellnated branches
extend into the odontoblastic layer. Despite the acute sensitivity of dentine,
nerve fibres are rarely demonstrable in dentine and the mechanism of sensory
reception is unknown: it is suggested that the odontoblastic processes may act
as sensory receptors.
CEMENTUM. The dentine, comprising the root, is covered by a thin layer of cementum
which is elaborated by cells called cementocytes lying on the surface of the
cementum. The cementum is an amorphous calcified tissue into which the fibers
of the periodontal membrane are anchored. Fragments of alveolar bone have
remained attached to the roots after extraction of these specimens.
Cementum consists of a
dense, calcified organic material similar to the matrix of bone and is
generally acellular. Towards the root apex the cementum layer becomes
progressively thicker and irregular and cementocytes are often entrapped in
lacunae within the cementum (cellular cementum). Cementocytes resemble
osteocytes and remain viable throughout the life, being nourished through
canaliculi which link the lacunae. They can become activated to produce new
cementum when required.
In addition to
cementocytes, which are scattered throughout the cellular cementum, there is a
layer of cells called cementoblasts, which are similar to the actively
synthetic osteoblasts of bone. Cementoblasts lie against the surface of the
periodontal ligament and probably produce most new cementum by appositional
deposition.
THE PERIODONTAL MEMBRANE
forms a thin fibrous
attachment between the tooth root and the alveolar bone; it consists of dense
collagenous tissue. The collagen fibres, known as Sharpeys fibres, run
obliquely downwards from their attachment in the alveolar bone to their
anchorage in the cemehtum at a more apical position on the root surface.
The periodontal membrane
thus acts as a sling for the tooth within its socket, permitting slight
movements which cushion the impact of masticatory forces. The points of
attachment of the collagen fibres in both cementum and bone are in a contrast
state of reorganisation to accomodate changing functional stresses upon the
teeth.
Osteoclastic resorption is
often seen at one aspect of a tooth socket and complementary osteoblastic
deposition of the tooth through the bone: this is the mechanism which permits
tooth movement during orthodontic treatment. The periodontal membrane is richly
supplied by blood vessels and nerves from the surrounding alveolar bone, the
apical region and the gingiva. Small clumps of epithelial cells are often found
scattered throughout the periodontal membrane: these cells are remnants of
Hertwigs sheath and are known as epithelial rests of Malassez.
During tissue preparation
the enamel has been completely dissolved from the surface of the crown, but the
extent of the outer surface of the enamel, can be visualised by shreds of
regaining organic debris which had been adherent to the tooth surface.
TOOTH DEVELOPMENT. The
tissues of the teeth are derived from two embryological sources. The
enamel is of epithelial (ectodermal) origin while the dentine, cementum, pulp
and periodontal ligament are of mesenchymal (mesodermal) origin.
The first evidence of tooth
development in human occurs at 6 weeks of fetal life with the proliferation of a
horseshoe-shape epithelial ridge from the basal layer of the primitive oral
epithelium into the underlying mesoderm.
In the position of the
future jaws: this is known as the dental lamina. In each quadrant of the mouth,
the lamina then develops four globular swellings which will become the enamel
organs of the future deciduous central and lateral incisors, canines and first
molar teeth.
Subsequently, the dental
lamina proliferates backwards in each rich successively giving rise to the
enamel organs of the future second deciduous molar and the three permanent
molars.
The permanent successors of
the deciduous teeth will later develop from enamel organs which bud off from
the inner aspect of the enamel organs of their deciduous predecessors.
The primitive mesenchyme
immediately subjacent to the developing enamel organ proliferates to form a
cellular mass whilst, at the same time, the enamel organ becomes progressively
cap-shaped, enveloping the mesenchymal mass which becomes known as the dental
papilla.
During the cap stage, the
cells lining the concave face ef the enamel organ in contract with the dental
papilla begin to differentiate into tall columnar cells, the future
ameloblasts, which will be responsible for the production of enamel.
This, in turn, induces the
differentiation of a layer of columnar odontoblasts, the future
dentine-producing cells, in the apical region of the dental papilla. The
interface between the differentiating ameloblast and odontoblast layers marks
the position and shape of the future junction between enamel and dentine.
As the enamel organ
develops further,it assumes a characteristic bell shape, the free edge of the
"bell" proliferating so as to determine the eventual shape of the
tooth crown.
Meanwhile, the cells
forming the main bulk of the enamel organ become large and star-shaped forming
the stellate reticulum, the extracellular matrix of which is rich in
glycosaminoglycans.
Between the stellate
reticulum and ameloblast layer two or three layers of flattened cells form the
stratum intermedium whilst the outer surface of the enamel organ consists of a
simple cuboidal epithelium called the external enamel epithelium.
By the cap stage of
development, the dental lamina connecting the enamel organ with the oral mucosa
has become fragmented and, around the whole developing bud, a condensation of
mesenchyme forms the dental follicle which will eventually become the
periodontal ligament.
As ameloblasts and
odontoblasts differentiate at the tip of the crown, a layer of dentine matrix
is progressively laid down between the ameloblast and odontoblast layers.
As the odontoblasts
retreat, each leaves a long cytoplasmic extension, the odontoblastic process,
embedded within the dentine matrix thereby forming the dentine tubules. Dentine
matrix has a similar biochemical composition to that of bone and undergoes
calcification in a similar fashion. Deposition of dentine induces the
production of enamel by the adjacent aneloblasts. Each retreating ameloblast
lays down a column of enamel matrix which then undergoes mineralisation
resulting n the formation of a dense prismatic structure as described below.
With the deposition of
dentine and enamel, the overlying stellate reticulum atrophies and the enamel
organ is much reduced in thickness. A thin layer of dentine has been laid down
by the underlying odontoblastic layer of the highly cellular dental papilla.
The ameloblastic layer is about to lay down enamel in the area of the
artefactual space: note that in this area the stellate reticulum has disappered.
Note also the surrounding dental follicle and early formation of cancellous
bone, by the time that dentine and enamel formation is well under way at the
inclsual edge or tips of the cusps (as the case may be), the enamel organ will
have fully outlined the shape of the whole tooth crown. A thin, densely-stained
layer of poorly mineralised enamel can be seen covered at its external surface
by the much thinner enamel organ now. The unstained space between this and the
underlying dentine represents fully mineralised enamel laid down earlier but
dissolved away during tissue preparation.
Although enamel production
is confined to the srown, the rim of the "bell" of the enamel organ
nevertheless continues to proliferate, inducing dentine formation and thereby
determining the shape of the tooth root. This part of the enamel organ, known
as the epithelial sheath of Hertwig, disintegrates once the outline of the root
is completed. The cementum which later is formed on the root surface is derived
from the dental follicle which, as previosly stated, is of mesenchymal origin.
As the dentine of the crown and root are progressively laid down, the dental
papilla shrinks and eventually becomes the dental pulp contained within the
pulp chamber and root canals. Growth of
the tooth root is one of the principal mechanisms of tooth eruption and root
formation is not completed until some time after the crown has fully erupted
into the oral cavity.
Active ameloblasts are tall
columnar epithelial cells which form a single layer apposed to the forming
surface of the enamel. Each ameloblast elaborates a column of organic enamel
matrix which undergoes progressive mineralisation by the deposition of calcium
phosphate mainly in the form of hydroxliapatite crystals. Fully formed enamel
contains less than 3% organic material and Ii the hardest and most dense tissue in the body.
The structure of mature
enamel is not fully understood, but it appears that the process of
mineralisation of the enamel matrix is not uniform and, as a result, mature
enamel consists of highly calcified prisms separated by interprismatic material
which may differ only in the orientation of its crystals. Each prism extends
from the dentino-enamel junction to the enamel surface and may represent the
enamel laid down by a single ameloblast. Underlying the ameloblast layer are
several layers of cells, also of epithelial origin, which constitute the
remainder of the enamel organ. As enamel formation progresses, the enamel organ
becomes much reduced in thickness compared with earlier stages of its
development. At tooth eruption, the enamel organ, including the ameloblasts,
degenerates leaving the enamel exposed to the hostile oral environment,
completely incapable of regeneration.
ODONTOGENESIS
Ectoderm and mesoderm partake in
odontogenesis. The development of teeth starts in the 5th week of
pregnancy. The fetal crown-rump length at this stage is about 7–10mm. At this
time, the multilayered nonkeratinizing squamous epithelium of oral cavity
proliferates and forms a narrow layer, which corresponds to the line of the
adult jaw. Concomitantly, the epithelium fuses with the underlying jaw
mesenchyme. At early stage of the tooth germ dental lamina is formed of the
deciduous and permanent teeth ridge, which are both oriented toward the inside.
The adjacent mesenchyme around the tooth germ has condensed to
the dental papilla . The multilayered internal enamel epithelium 3
is located at the inner surface of the cap enameloid. The layer of cells, which
borders on the surrounding mesenchyme , forms the external enamel epithelium
at the outer surface of the cap
enameloid.
1
External enamel epithelium
4 Dental sac
2
Internal enamel epithelium 5
Dental papilla
3
Stellate reticulum 6
Permanent teeth ridge
7
Multilayered nonkeratinizing squamous epithelium
Frontal section through the head of a hamster fetus on the 15th day of
gestation. The bell-shaped enamel organs
1
Oral cavity 4 Nasal
cavity
2
Tongue 5 Bell-shaped
enamel organ
3
Palate 6 Lower
jaw
Odontogenesis—Enamel Organ
Tooth germ of a human fetus in the 5th month of pregnancy. The
metachromatic intercellular substance inside the nonvascularized enamel organs
has increased and has pushed the epithelial cells away from each other. This
creates the stellate reticulum (pulpa enamelea) , in which cells of
reticular character form a web-like
structure. The stellate reticulum at the outer convex surface of the enamel
organ is separated from the adjacent connective tissue of the dental sac 3 by
the external enamel epithelium 2.
1
Stellate reticulum
2
External enamel epithelium
3
Connective tissue of the dental sac
4
Epithelial cells of the stellate reticulum
Air-Filled Ground Dental Section, Unstained
Figure (a) shows osteocytes in the fibrous bone of the tooth enamel. The
osteocytes feature long cytoplasmic processes. Figure (b) depicts the ground
section of the crown of a tooth with the enamel (top) and dentine (bottom).
There is a clear border between both the hard substances. Dentine is traversed
by air-filled, parallel dentine canals 1 . Their ends may branch and fan
out like bushels. The dentine canals cause a radial striation of the dentine.
The long odontoblast processes are located in the dentine canals. They are also
called Tomes fibers. The odontoblast processes reach to the enamel
border or the cement border, respectively. In contrast to enamel, dentine is a
living tissue. The odontoblasts are able to produce newdentine throughout life.
The tooth enamel is covered by an about 1μm-thick
fibrous enamel cuticle 2 . Noncalcified areas of dentine are called
interglobular dentine 4 or Tomes granular layer.
Vertical
section through the primordium in the feline lower jaw. The figure shows the
development of dentine and tooth enamel. There are the following layers:
1
Stellate reticulum. Interstitial fluids have accumulated and pushed
apart the cells of the originally dense epithelium of the enamel organ. The
nowstar-shaped cells form a reticular connective tissue.
2
The internal enamel epithelium initially borders on the mesenchyme of
the dental papilla. It consists of pseudostratified columnar ameloblasts
(adamantoblasts, enameloblasts).
3
Enamel. The enamel cap (stained dark red) is generated by ameloblasts.
4
Dentine (stained red). Predentine is already mineralized.
5
Predentine (stained blue) is a soft intercellular substance that is rich
in fibrils and comparable to the osteoid.
6
Odontoblast layer. The pseudostratified columnar cells are responsible
for the formation of dentine.
7
Dental papilla and stellate reticulum, respectively, consist of
very delicate fibrous connective tissue. Their star-shaped, branched cells are
probably fibroblasts.
Enamel
and dentine can be particularly well demonstrated showing an incisor in the 5th
fetal month. At the end of development, dentine is a 1–5–nm wide layer between
stellate reticulum and enamel, and between stellate reticulum and dental
cement. The following tissues are seen in this figure: 1 Loosely arranged
mesenchyme cells and the connective tissue fibers of the dental sac.
3
Single-layered columnar ameloblasts (adamantoblasts).
4
The enamel (stained light red) was produced by ameloblasts.
5
The following layer consists of dentine (stained violet). It clearly shows fine
light lines. These are dentine canals, which contain the odontoblast
processes, also called Tomes fibers.
7
Alayer of dentine-forming odontoblasts is located underneath the
predentine. Their cytoplasmic processes appear prominently as fine dark lines.
PHARYNX. The
pharynx, a transitional space between the oral cavity and the respiratory and
digestive systems, forms an area of communication between the nasal region and
the larynx. It is divided into oropharynx (the opening of the mouth into the
pharynx) and nasopharynx (nasal opening). The Eustachian tube from the middle
ear opens into the pharynx on each side. The oropharynx and pharynx proper are
lined by largely stratified squamous epithelium of the mucous type, except in
those regions of the respiratory portions that are not subject to abrasion.
These latter areas have a ciliated pseudostratified epithelium with goblet
cells.
The mucosa of the pharynx
has many small mucous glands in its dense connective tissue layer. The
submucosa is well endowed with lymphoid tissue (tonsils). The constrictor and
longitudinal muscles of the pharynx are located outside this layer.
Tonsils are organs composed
of aggregates of incompletely encapsulated lymphoid tissues that lie beneath,
and in contact with, the epithelium of the initial portion of the digestive
tract. They surround the entrance to the proper digestive tube being organized
into pharyngeal lymphoepithelial rink of Pirogov-Waldeier Depending on their
location, tonsils in the mouth and pharynx are called palatine (2), tube (2)
and unpaired pharyngeal, laryngeal and lingual.
Palatine tonsils. The two
palatine tonsils are located in the lateral walls of the oral part of the
pharynx in the gap between the glossopalatine and pharynopalatine arches on
each side. Under the squamous stratified epithelium, the dense lymphoid tissue
in these tonsils forms a band that contains lymphoid nodules, generally with
germinal centers. Each tonsil has 10-20 epithelial invaginations that penetrate
the parenchyma deeply, forming crypts, whose lumens contain desquamated
epithelial cells, live and dead lymphocytes, and bacteria. Crypts may appear as
purulent spots in tonsils. Separating the lymphoid tissue from subjacent
structures is a band of dense connective tissue, the capsule of the tonsil.
This capsule usually acts as a barrier against spreading tonsillar infections.
Inflammatory process of
palatine tonsils is known as angina and very often such chronicle process is
accompanied with rheumatism.
Pharyngeal tonsil is a single tonsil situated in the superior-posterior portion of the
pharynx. It is covered by ciliated pseudostratified columnar epithelium typical
of the respiratory tract, and areas of stratified epithelium can also be
observed.
The pharyngeal tonsil is
composed of pleats of mucosa and contains diffuse lymphoid tissue and nodules.
It has no crypts, and its capsule is much thinner than those of the palatine
tonsils.
Hypertrophy of the
pharyngeal tonsil resulting from chronic inflammation is called adenoid.
Lingual tonsil are smaller then the previous ones. It is situated at the base of the
tongue and is covered by stratified squamous epithelium. Lingual tonsil has a
single crypt.
Epiglottis
The
epiglottis is the flap of cartilage lying behind the tongue and in front of the
entrance to the larynx (voice box). At rest, the epiglottis is upright and
allows air to pass through the larynx and into the rest of the respiratory
system. During swallowing, it folds back to cover the entrance to the larynx,
preventing food and drink from entering the windpipe. The throat contains both
an air passage (the wind pipe) and a food passage (the esophagus). If these
passages were both open when a person swallowed, air could enter the stomach
and food could enter the lungs. Part of the safety hatch that seals off the
windpipe is the "epiglottis," a little valvelike cartilage, which
works with the larynx to act as a lid every time we swallow. The larynx draws
upward and forward to close the windpipe. This keeps solid food and liquid out
of the respiratory tract. At the end of each swallow, the epiglottis moves up
again, the larynx returns to rest, and the flow of air into the windpipe
continues. The uvula (Latin for "little grape") is a fleshy piece of
muscle, tissue and mucous membrane that hangs down from the palate. It is the
part that moves upward when we say, "Ah!" It flips up and helps close
off the nasal passages when we swallow. Contrary to the depictions seen in
cartoons, the uvula does not vibrate during singing and shouting and, in fact,
has nothing to do with the voice.