Axial
skeleton:
-
The
skull is the superior section of the axial skeleton. It alone contains
twenty-eight bones, divided into three categories:
-
Cranial
bones:
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The
frontal bone forms the anterior third of the skull dome. The supraorbital
foramen is the hole on the superior edge of the orbit, and the frontal
sinus (sinus meaning cavity) is the airspace on the midline, above the
eyes
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The
middle section of the cranium is made up of the two parietal bones, mirror
images of one another, joined to one another along the sagittal suture.
They are joined to the frontal at the coronal suture, from the word corona,
or 'crown'. The suture with the occipital bone is the lamboidal suture
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The
sphenoid bone resembles nothing so much as a butterfly, forming the anterior
floor of the cranium, and stretching out to also form a section of the
outer cranium. The body of the sphenoid is a hollow, cube-shaped central
portion. The greater wings are the out-thrust extensions that joint and
form the lateral orbital wall. The lesser wings are more narrow, and form
the posterior section of the orbit's roof. The optic foramen is just what
it sounds like-- a round hole for the optic nerve in the posterior wall
of the orbit. The superior orbital fissure is a crack-like opening in the
inferior wall, lateral to the optic foramen. Below it is the inferior orbital
fissure. The round opening seen from the inside of the skull is foramen
rotundum, anterior to the foramen ovale, an oval opening also see from
the inside of the skull. The 'Turkish saddle' is the sella turcica, a double
projection on the superior aspect of the sphenoid. Inferior to this saddle
is an airspace-- the sphenoid sinus
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The
left and right temporal bones extend inward to from the cranial floor with
the sphenoid. The narrow, bridge-like projection of the temporal bone,
where it joins the facial zygomatic bone, left or right, is the zygomatic
process. The mastoid process is a large bump posterior and inferior to
the external auditory meatus. A thin projection anterior to the mastoid
process is the styloid process. The aforementioned external auditory meatus
is the tube-like opening of the ear canal. Only seen from the inside of
the skull, a 'rocky cliff' anterior to the occipital bone is the petrous
portion, with an opening called the internal auditory meatus. The squamous
suture is a joint along the top, curved edge of the temporal bone. The
mandibular fossa is a depression that receives the condyle of the mandible.
The joint formed is the temporomandiblular joint. The jugular foramen is
a round hole at the junction of the petrous portion of the temporal bones
and the occipital bone. The carotid foramen is the canal running through
the petrous portion, slightly medial to the jugular foramen
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The
posterior cranial bone is the occipital bone, curving inferiorly to the
base of the skull. The spinal cord enters the skull through the foramen
magnum, a very large hole in the occipital. The occipital condyle is a
flattened bump lateral to the foramen magnum
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The
ethmoid bone makes up the middle section of the anterior cranial floor,
extending below the orbits to form the roof of the nasal cavity. The ethmoid
sinuses are located in the main section of the bone. The perpendicular
plate projects in a thin line along the midsagittal line to form the upper
nasal septum. The crista galli is a crest on the superior midline of the
ethmoid. The superior nasal concha, or turbinate, is the middle one of
three curved projection from the nasal cavity's lateral wall. The cribiform
plate is a thin, perforated plate forming the ethmoid's superior surface.
The olefactory foramina are tiny holes in the cribiform plate for the olefactory
nerves from the upper nasal cavity
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Facial
bones:
-
Two
maxillae, maxillary bones, are also referred to as the upper jaw bones.
They support the face from the eyes down to the teeth. The palatine process
is the plate-like posterior projection of the upper jaw. The incisive foramen
is an anterior hole in the palatine process, near the midline. The ridge
in which the teeth are anchored is the alveolar process. The infraorbital
foramen is a hole in the anterior, below the orbit. The maxillary sinus
is in the 'cheek' portion
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There
are two lacrimal bones on the medial margin of the orbits, between the
ethmoid and the upper projection of the maxillary, each has a sulcus where
the tear ducts are located
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On
the midline, the right and left nasal bones are joined to form the superior
margin of the nasal passage
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Two
palatine bones make up the posterior third of the hard palate, joined together
at the midline. The joint between the palatine bones and the maxillary
palatine process is called the transverse palatine suture. The larger of
two holes in each palatine bone is the greater palatine foramen
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The
inferior nasal conchae, or turbinate bones, are a pair of thin, curved
bones that project medially from the lateral walls of the nasal cavity,
curving down towards the floor
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The
lower jaw bone is called the mandible. The ramus is the posterior arm of
the mandible, on which two major processes can be seen. The mandibular
condyle is the rounded, posterior of the two, which forms a joint with
the temporal bone. Anterior to the condyle is the coronoid process. Between
the two is the mandibular arch, or notch, a large, U-shaped curve un the
superior edge of the mandible. Mirroring those of the maxillary bones,
the mandible has teeth set into an alveolar process. The corner formed
where the mandible begins it's upward journey is called the angle. The
entire anterior portion of the mandible is called the body. On both the
inside and outside of the mandible are located paired holes, the mandibular
foramen and the mental foramen, respectively
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The
upper corner of the cheek is made up of the paired zygomatic bones. They
join to the temporal bone by way of the temporal process, forming the zygomatic
arch
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The
single vomer bone forms the lower part of the nasal septum
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Auditory
ossicles: The middle ear is located within the petrous portion of the temporal
bone, is made up of three primary parts: The malleus, or the hammer The
incus, or the anvil The stapes, or the stirrup
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The
fetal skull is only partially ossified. The bones have not yet formed sutures,
and there are six fibrous areas called fontanels:
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Two
anterolateral, or sphenoid, fontanels are located at the future junction
of the sphenoid, temporal, parietal, and frontal bone
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Two
posteriolateral, or mastoid, fontanels are located at the future junction
of the temporal, parietal, and occipital bone
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The
anterior, or coronal, fontanel is found where the left and right parietal
bones will meet the frontal
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The
posterior fontanel is located where the parietal bones will meet the occipital
bone
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The
vertebral column is made up of twenty-six bones, divided into three areas
and two additional bones:
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Seven
cervical vertebrae make up the superior section of the spine, designated
C1 through C7, though C1 and C2 have other commonly used names. C1, the
most superior vertebral bone is also referred to as the atlas. It forms
a joint with the skull's occipital condyles. The atlas has an anterior
arch and a posterior arch, fused to left and right lateral masses to form
a circle. Flat articulating facets are located on both the superior and
inferior aspects. The C2 bone is the axis, with it's distinctive dens,
or odontoid process. The dens projects upward, acting as a pivot for the
C1 and skull. The cervical curve of the spine is produced by these seven
bones
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T1
though T12 are the twelve thoracic vertebrae. As a group, they curve in
the opposite direction of the cervical column, forming the thoracic curve
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Five
large lumbar vertebrae make up L1 through L5, forming the lumbar curve
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C3
to L5 have certain common features. All have a vertebral foramen in the
center, to house the delicate spinal cord. The spinous process projects
posteriorly. The thick dick of bone forming the anterior arch is the body.
The lamina is the thin, plate-like section forming each leg of the posterior
arch. The lateral section that joins the lamina to the body is referred
to as the pedicle. A hole is formed between the laminae of successive vertebrae
for spinal nerves is the intervertebral foramen. A spine projects laterally
from the vertebrae, called the transverse process. Through it, there is
a hole named the transverse foramen. At the superior and inferior aspects
of the bone, there are located superior and inferior articular facets.
They are separated by pads of fibrocartilage called intervertebral discs
with about a 90% water content
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Five
vertebrae fuse and form the sacrum inferior to the L5. There is a slight
sacral curve, seen from the lateral perspective. The dorsal foramina are
the double row of holes that run down the sacrum. Similar, but running
on the ventral aspect, are the pelvic foramina. Formed by the fused vertebral
foramina is the sacral canal. An open space is located at the caudal (tail)
end of the of the sacral canal, named the sacral hiatus. Fusion of the
sacral spinous processes forms the bumpy medial sacral crest. The sacral
promontory is the jutting ventral lip of the superior sacral segment
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The
most inferior bone of the spine is the coccyx, or tailbone, a fusion of
several vertebrae
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Drying
of the intervertebral discs can lead to herniated, or slipped, discs in
the elderly. The S-shaped curve of the alternating concave-convex-concave
curves of the spine can be changed to create one of three major distortions.
Scolliosis refers to a lateral distortion of the curvature. Kyphosis is
a superiorly located malformation, and lordosis is inferior
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The
hyoid bone is a U-shaped bone in the throat. It is an attachment for the
tongue muscles and the connective tissue associated with the larynx. It
is not attached to the cervical column. The central body has two pairs
of projections. The greater cornu is the most lateral of the two, and larger.
The smaller, more spiky processes are the lesser cornu
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The
thoracic, or rib, cage is made up of twenty-five bones. Each of the twelve
pairs of ribs articulates with the spine. The superior seven pairs attach
directly to the sternum via costal cartilage. These are called true ribs.
False ribs, the inferior five pairs, do not connect directly to the sternum,
but rather by the cartilage of the true ribs. The floating ribs, the most
inferior three pairs, are not attached. The sternum, or breastbone, forms
an anterior protective plate over the heart. It is in actuality three fused
bones: the superior manubrium, the body (middle) gladiolus, and the inferior
xiphoid process
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