Download Anatomy Skull and Spinal Cord 1. Skull

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Anatomy
Skull and Spinal Cord
1. Skull
The skull is anterior to the spinal column and is the bony structure that encases
the brain. Its purpose is to protect the brain and allow attachments for the facial
muscles. The two regions of the skull are the cranial and facial region. The
cranial portion is the part of the skull that directly houses the brain and the
facial portion includes the rest of the bones of the skull.
The skull is supported on the summit of the vertebral column, and is of an oval
shape, wider behind than in front. It is composed of a series of flattened or
irregular bones which, with one exception (the mandible), are immovably
jointed together. It is divisible into two parts: (1) the cranium, which lodges and
protects the brain, consists of eight bones, and (2) the skeleton of the face, of
fourteen, as follows:
Skull, 22 bones
Cranium, 8 bones
- Occipital.
- Two Parietals.
- Frontal.
- Two Temporals
- Sphenoidal
- Ethmoida
Face, 14 bones
- Two Nasals
- Two Maxillæ
- Two Lacrimals
- Two Zygomatics
- Two Palatines.
- Two Inferior Nasal Conchæ
- Vomer
- Mandible
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SKULL
1. Frontal Bone 2. Supra-Orbital Foramen 3. Orbit (Orbital Cavity)
4. Superior Orbital Fissure 5. Inferior Orbital Fissure 6. Zygomatic Bone
7. Infra-Orbital Foramen 8. Maxilla 9. Mandible 10. Mental Foramen
11. Incisive Fossa 12. Symphysis 13. Vomer 14. Inferior Nasal Concha
15. Middle Nasal Concha 16. Perpendicular Plate of Ethmoid
17. Nasal Bone 18. Lacrimal Bone
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1. Parietal Bone
2. Temporal Bone
3. Occipital Bone
4. Mandible Maxilla
5. Frontal Bone Sphenoid
6. Bone zygomatic bone
A. Frontal bone
During development there are two frontal bones which fuse after birth. This
bone forms the ridges above the orbits and the roof of the orbit. Branches of
the ophthalmic division of the trigeminal nerve emerge from the orbit to
innervate the skin of the forehead (supra-orbital and supra-trochlear nerves).
The supra-orbital nerve lies in the supra-orbital notch of the frontal bone.
B. Parietal bone
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The paired parietal bones form the top and sides of the skull. The suture
between them is the sagittal suture. The suture between the pair of parietal
bones and the frontal bone is the coronal suture.
On the outside the bones carry large muscles which elevate the mandible
during chewing (temporalis muscles).
On the inside, running along the sagittal suture, lies the superior sagittal
sinus, a major route of venous drainage from the brain.
At birth there is a soft spot where the sagittal suture meets the coronal suture,
the anterior fontanelle. In the adult this fills in and the point is then termed
the bregma.
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C. Temporal bone
The temporal bone has a vertical squamous part forming part of the side of the
skull, and a horizontal base, the petrous temporal bone. On the exterior the
temporal bone has an opening which leads into the middle ear, the external
auditory meatus. The ear is attached at this point. A process runs forward
forming part of the cheek bone, the zygomatic process of the temporal bone.
Projecting inferiorly from behind the external auditory meatus is the mastoid
process. The mastoid process is a spongy bone containing air cells. The
drawing out of the mastoid process is related to the insertion of the
sternocleidomastoid muscle, responsible for rotation of the head to the
opposite side. Within the substance of the petrous temporal bone lie the
components of the inner ear, the cochlear and vestibular systems. On the
inside of the squamous part run branches of the middle meningeal artery
which may be ruptured in bone fractures.
The sigmoid sinus, a continuation of the transverse sinus lies in a groove on
the inside of the bone. The head of the mandible articulates with the temporal
bone forming the temporomandibular joint (TMJ). Anterior to the joint is the
articular tubercle. At the internal auditory meatus the vestibulo-cochlear
(VIII) and facial nerves (VII) enter the bone.
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On the inferior surface of the bone between the mastoid and styloid processes
is the stylomastoid foramen through which the facial nerve leaves the
temporal bone.
The temporal bone is commonly fractured, leading to complications. The
facial nerve enters the temporal bone at the internal auditory meatus. A
common site of injury is close to the geniculate ganglion.
The integrity of the greater superficial petrosal nerve and chorda tympani
should be tested. The integrity of the motor component of the facial nerve can
be tested by testing the muscles of facial expression.
Hearing loss is common in fractures of the temporal bone since the
auditory mechanism lies entirely within the temporal bone. Vertigo may
follow damage to the vestibular apparatus in the temporal .
D. Occipital bone
This bone forms part of the base and the back of the skull. A fall on the
back of the head might cause a fracture of this bone. The basal part of the
bone forms a ring (the foramen magnum) through which passes the spinal
cord. On the inside surface of the occipital bone, venous drainage from the
brain runs in the transverse sinuses. The hypoglossal nerve leaves the skull
through the hypoglossal canal.
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E. The sphenoid
The sphenoid lies in the base of the skull forming part of the floor of the
middle cranial fossa. On its upper surface the bone carries a chamber for the
pituitary gland. Anterior to the pituitary fossa the bone is hollowed out as
the sphenoidal air sinus. Foramina in the bone transmit nerves and vessels.
The superior orbital fissure transmits the ophthalmic division of the
trigeminal nerve, the abducent, trochlear and oculomotor nerves. The
foramen rotundum transmits the maxillary division of the trigeminal nerve.
The pterygoid canal transmits the nerve of the pterygoid canal (sympathetic
fibres plus the greater superficial petrosal branch of the facial nerve). The
foramen ovale transmits the mandibular division of the trigeminal nerve.
The optic foramen transmits the optic nerve and ophthalmic artery. The
ophthalmic veins drain back to the cavernous sinus which lies on either side
of the sella turcica.
The cavernous sinus drains through the superior petrosal sinus to the
transverse sinus and through the inferior petrosal sinus to the jugular vein.
The internal carotid artery lies for part of its course in the cavernous sinus.
Running along the dura of the cavernous sinus are the ophthalmic,
oculomotor and trochlear nerves.
The abducent nerve runs through the sinus. Although the sphenoid lays
centrally it is still prone to fracture. The many neurovascular structures that
traverse the sphenoid bone are vulnerable to injury.
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Sphenoidal fractures are frequently associated with vascular injuries
such as traumatic pseudo-aneurysm of the internal carotid artery,
carotid-cavernous sinus fistula, bleeding from the middle meningeal
artery, and nerve palsies due to damage to the optic, trigeminal,
oculomotor, abducent and trochlear nerves.
F. The maxilla
The maxilla forms the upper jaw and part of the cheek bone. It carries the
upper set of teeth. The bone forms the medial floor of the orbit and part of the
lateral wall and floor of the nasal cavity. The maxillary sinus is a chamber
within the bone lined by a mucus producing epithelium.
The drain of the maxillary sinus in the adult is above the level of the floor.
Cilia in the epithelium move the mucus up and out of the ostium into the
hiatus semilunaris below the middle concha. The opening can become
blocked resulting in painful sinusitis. The opening can be cleared or another
opening created closer to the floor of the maxillary sinus through the lateral
wall of the nose below the inferior concha. Several nerves pass through the
maxilla, including the infraorbital and superior alveolar nerves.
Facial trauma may result in maxillary fractures. The maxillae are
hollowed out as the maxillary sinuses. The bone also transmits the
maxillary artery and branches of the maxillary nerve.
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G. The mandible
The mandible articulates through its head with the articular process of the
temporal bone. The bone is flattened as the ramus and changes direction at the
angle. On its inside surface the mandible has a foramen for the inferior
alveolar nerve and vessels.
The mandibular foramen is partly covered over by a spike of bone, the
lingula. As the inferior alveolar nerve enters the mandibular foramen it gives
off a motor nerve to mylohyoid and the anterior belly of digastric. This nerve
lies in a shallow groove in the bone. Anteriorly on the external surface lies the
mental foramen. The terminal branches of the inferior alveolar nerve emerge
to supply the skin as the mental nerve.
The mandible can be dislocated or fractured. The mandible is most commonly
dislocated in an anterior direction, with the head of the mandible passing over
the articular tubercle. The mandible may be fractured anterior to the masseter
muscle. Fracture will result in injury to the inferior alveolar nerve and vessels.
Normal action of the mandible in chewing is produced by the muscles of
mastication.
The temporalis muscle fills the temporal fossa on the side of the skull and
inserts onto the coronoid process and anterior edge of the ramus of the
mandible. It elevates and retracts the mandible. The masseter muscle arises
from the inside of the zygomatic arch and inserts into the lower edge of the
body of the mandible at the angle. It elevates the jaw.
The medial pterygoid muscle arises from the medial side of the lateral
pterygoid plate to insert on the medial side of the angle of the mandible,
forming a sling together with masseter.
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The lateral pterygoid muscle arises from the lateral side of the pterygoid plate
and neighbouring maxilla to insert into the capsule and disc of the TMJ and
the neck of the mandible. It protracts the jaw, and unilaterally moves the jaw
from side to side. The mandibular division of the trigeminal nerve supplies all
these muscles.
H. The zygomatic bone
The zygomatic bone forms the lateral part of the orbit and part of the
zygomatic arch.
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2. The Vertebrae
The vertebral column or spine is made up of a series of bones called
vertebrae. The spine is a strong and flexible column of bones made up of the
cervical, thoracic, lumbar, sacral, and coccygeal regions. The cervical or
neck region is normally made up of 7 bones, the thoracic or chest region has
12 bones, and the lumbar or low back region is made up of 5 bones. The
sacral region consists of 5 fused bones, and the coccygeal region 3 to 5 tiny
bones.
A. Purpose of the Vertebrae
Although vertebrae range in size; cervical the smallest, lumbar the largest,
vertebral bodies are the weight bearing structures of the spinal column.
Upper body weight is distributed through the spine to the sacrum and pelvis.
The natural curves in the spine, kyphotic and lordotic, provide resistance and
elasticity in distributing body weight and axial loads sustained during
movement.
The vertebrae are composed of many elements that are critical to the
overall function of the spine, which include the intervertebral discs and
facet joints.
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B. The Cervical Column
The cervical spine is further divided into two parts; the upper cervical region
(C1 and C2), and the lower cervical region (C3 through C7). C1 is termed the
Atlas and C2 the Axis. The Occiput (CO), also known as the Occipital Bone,
is a flat bone that forms the back of the head.
1. Atlas (C1)
The Atlas is the first cervical vertebra and therefore abbreviated C1. This
vertebra supports the skull. Its appearance is different from the other spinal
vertebrae. The atlas is a ring of bone made up of two lateral masses joined at
the front and back by the anterior arch and the posterior arch.
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2. Axis (C2)
The Axis is the second cervical vertebra or C2. It is a blunt tooth–like
process that projects upward. It is also referred to as the ‘dens’ (Latin for
‘tooth’) or odontoid process. The dens provides a type of pivot and collar
allowing the head and atlas to rotate around the dens.
There are seven vertebrae forming the cervical vertebral column. The
first of these, the atlas, forms a ring since the body is absent. It articulates
above by a pair of articular facets, with the occipital bone. Nodding
movements are possible at these joints. The transverse processes transmit
the vertebral arteries which run around the posterior arch to enter the
skull through the foramen magnum. The atlas articulates inferiorly with
the axis. The dense of the axis projects upwards to articulate with the
anterior arch of the atlas. Movements of rotation are possible at the
atlanto-axial joint.
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The other cervical vertebrae are more alike in that they all show a small
body and perforated transverse processes. They articulate with each other
through the intervertebral discs and through facets on the articular processes.
The facets lie in the coronal plane, permitting lateral bending and
flexion/extension. The spinous processes of the cervical vertebrae are bifid,
C7 being easily palpable.
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C. The Spinal column or the Vertebral column
The spinal column (or vertebral column) extends from the skull to the
pelvis and is made up of 33 individual bones termed vertebrae. The
vertebrae are stacked on top of each other group into four regions:
1. Thoracic Vertebrae (T1 – T12)
The thoracic vertebrae increase in size from T1 through T12. They are
characterized by small pedicles, long spinous processes, and relatively
large intervertebral foramen (neural passageways), which result in less
incidence of nerve compression. The rib cage is joined to the thoracic
vertebrae. At T11 and T12, the ribs do not attach and are so are called
"floating ribs." The thoracic spine's range of motion is limited due to the
many rib/vertebrae connections and the long spinous processes.
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1 Vertebral Body 2 Spinous Process 3 Transverse Process
4 Pedicle 5 Foramin 6 Articular Process
2. Lumbar Vertebrae (L1 – L5)
The lumbar vertebrae graduate in size from L1 through L5. These
vertebrae bear much of the body's weight and related biomechanical
stress. The pedicles are longer and wider than those in the thoracic spine.
The spinous processes are horizontal and more squared in shape. The
intervertebral foramen (neural passageways) are relatively large but nerve
root compression is more common than in the thoracic spine.
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Functions of the Vertebral or Spinal Column Include
Protection
•
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Spinal Cord and Nerve Roots
Many internal organs
Base for
Attachment
•
•
•
Ligaments
Tendons
Muscles
Structural
Support
•
•
•
Head, shoulders, chest
Connects upper and lower body
Balance and weight distribution
•
•
•
•
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Flexion (forward bending)
Extension (backward bending)
Side bending (left and right)
Rotation (left and right)
Combination of above
•
•
Bones produce red blood cells
Mineral storage
Flexibility and
Mobility
Other
The spine has several special roles in the human body. It protects the spinal
cord (which connects nerves to the brain); provides the support needed to
walk upright; enables the torso to bend; and supports the head. Viewed
from the side, the spine has a natural "S" curve.
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3. There are three main parts to a vertebra:
•
•
•
Vertebral Body (weight bearing structure)
Vertebral Arch (protective in function)
Vertebral Processes
A. Vertebral Body
The vertebral body is a thick, disc shaped cylindrical block of bone flattened
at the back and with roughened top and bottom surfaces. It is made up of
spongy bone on the inside, which enables it to resist compression, and a thin
outer covering of compact bone. It is the weight bearing part of a vertebrae.
VERTEBRA
Side View
View From Above
B. Vertebral arch
The vertebral arch, or neural arch as it is sometimes called, extends
backwards from the vertebral body. It is made up of two short, thick
processes called pedicles that stick out backwards from the vertebral body
and join the laminae. The laminae are flat and join together to form the back
part of the vertebral arch.
Together the vertebral arch and vertebral body surround the spinal column.
The space occupied by the spinal column is called the vertebral foramen.
The vertebral foramen, when stacked on top of each other, forms the
vertebral or spinal canal. On each side of the vertebral column there is an
opening between each vertebra called the intervertebral foramen which
enables the spinal nerves to pass through.
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VERTEBRA
View From Above
Side View
C. Vertebral processes
There are seven different processes that come from the vertebral arch. A
transverse process extends sideways on each side from the junction of a
lamina and pedicle. A single spinous process extends back and downwards
from the junction of the laminae. These three processes have spinal muscles
attached to them.
The other four articular processes form joints with other vertebrae. The two
articular processes on top form a joint with the two articular processes on the
bottom of the vertebrae above, and the two bottom processes form a joint
with the two processes on top of the vertebrae below.
Vertebral discs
Adjacent vertebral bodies are attached to each other by an
intervertebral disc. The disc is made up of a central mass of pulpy
tissue called the nucleus pulposus and a tough outer covering of fibrocartilage called the annulous fibrosis. The layers of the annulus are
thinner at the back, making this area potentially more prone to damage.
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Intervertebral disc
Front View
View from Above
Side View
The discs are shock absorbers, giving resilience to the spinal column as well
as flexibility. When a body is erect, the various parts of the disc are under
uniform pressure; but when the spine is flexed, extended or bent to the side,
one part of the disc is under increased compression whereas another part is
under tension.
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Vertebral Discs under Various Loads
Spinal cord & spinal nerves
The spinal cord is cylindrical in shape and starts as an extension of the
lower part of the brain stem and finishes at around the second lumbar
vertebrae. The cord is made up of some 31 segments that each give
rise to a pair of spinal nerves. The spinal cord is protected by the spinal
column, the spinal meninges (dense fibrous covering), the
cerebrospinal fluid (special fluid surrounding the brain and spine), and
the vertebral ligaments.
The main function of the spinal cord is to convey motor impulses from
the brain to the muscles, and sensory information from the body to the
brain. There are 31 pairs of spinal nerves named and numbered
according to the level and region of the spine from which they emerge.
There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs
of lumbar and sacral nerves, and 1 pair of coccygeal nerves. With the
exception of the first pair of cervical nerves, all spinal nerves leave the
vertebral column from the intervertebral foramen.
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SPINAL CORD AND NERVES - BACK VIEW
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4. Sacrum and Coccyx
Sacral/Sacrum
There are five vertebrae that join together to form the sacrum, a wedgeshaped part of the spine that rests at the top of the pelvis. In the female the
sacrum is shorter and wider than in the male.
Coccyx
The coccyx is often referred to as the tailbone. It consists of four vertebrae.
Lateral surfaces of sacrum and coccyx.
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