THE AXIAL SKELETON
BONES OF THE SKULL, VERTEBRAL COLUMN AND THORACIC CAGE
Preparatory Review
THE SKELETAL SYSTEM
There are 206 bones in the adult human skeleton. The bones of the human body are
characterized by shape or physical characteristics. The major bone classifications are long,
short, irregular, flat, and sesamoid. Long bones have a long slender shape and are found in the
arms, legs, thighs, fingers, and toes. Short bones are approximately equal in length and width.
They can be found in the wrist and ankle. Flat bones have flat thin surfaces, which are found in
the skull, sternum, and ribs. Irregular bones have complex shapes with many bumps and ridges.
The vertebrae of the spinal column and sphenoid bone of the skull are considered to be irregular
bones. Sesamoid bones are located where tendons cross over joints. They typically function to
increase the mechanical advantage of a muscle. In other words, this type of bone aligns the
muscle’s tendon to be in a position where it can generate more force. The patella is considered a
sesamoid bone. The forces placed on the skeleton ultimately will dictate the shape of the bones.
These forces include both gravitational and muscular stressors. Notice the prominent ridges on
many of the bones of the body. These ridges are usually the result of powerful muscles, which
pull on these bones to create movement.
The skeleton can be divided into an appendicular and axial skeleton. The appendicular skeleton
is composed of the pectoral (shoulder), pelvic girdles (hip), and bones of the extremities. The
appendicular skeleton will be covered in more detail in the next laboratory. The axial skeleton is
composed of the bones of the skull, hyoid, vertebrae, ribs, sternum, sacrum, and coccyx. The
skull alone is composed of 22 bones. It can be divided into the cranium, which encases the brain,
and facial bones. With the exception of the temporomandibular joint, the bones of the skull are
connected by relatively immovable joints called sutures. A more detailed description of the
different joints will be discussed later in the lab.
CRANIAL BONES
The cranium is composed of eight bones. The frontal bone forms the forehead and superior
portion of the eye socket. It also makes up the anterior portion of the cranial fossa. Look at the
top of the skull and notice the coronal suture, which runs along the frontal (coronal) plane. This
suture separates the frontal bone anteriorly and parietal bones posteriorly. The sagittal suture
connects the 2 parietal bones in the midline. Identify the temporal bones, which are located just
inferiorly to the parietal bones. This irregularly shaped bone makes up the lateral wall of the
cranium and has several important landmarks including the external auditory meatus (canal)
and mastoid process. Look at the back of the skull to view the occipital bone. It creates the
posterior and inferior portion of the cranium. The posterior aspect of this bone has several
prominent ridges, which are the attachment points for muscles of the posterior cervical spine.
Look at the underside of the skull to identify the large opening at the base of the skull. This
portion of the occipital bone is called the foramen magnum, which allows passage of the spinal
cord into the cranial cavity. Lateral and slightly anterior to the foramen magnum are the occipital
condyles, which articulate with the first cervical vertebra. Remove the top part of the skull.
Identify the sphenoid bone, which has an irregular shape that resembles a butterfly. It is part of
the cranial floor located in between the frontal bone and temporal bones. It can be palpated
posterior lateral to the eyes but anterior to the temporal bones (temples). At the center of the
sphenoid bone there is a bony depression called the sella turcica (Turkish saddle). The pituitary
gland is located here. Just anterior to the sella turcica are 2 small holes for the optic nerves,
which are responsible for the conduction of nerve impulses from the eye to the area of the brain
that processes vision. The ethmoid bone is located anterior to the sella turcica within the center
of the frontal bone. The ethmoid is also visible in the orbits of the eyes and nasal cavity. It is
located just medially to the sphenoid bone within the eye socket. It also projects down into the
nasal cavity forming the superior portion of the nasal septum (perpendicular plate). The nasal
septum divides the nasal cavity into a right and left side.
Clinical application: A tumor affecting the pituitary gland has several characteristic symptoms. It
may present as a persistent deep headache because as the gland grows it becomes compressed
within the sella turcica. If the tumor becomes large enough it can compress on nearby structures
such as the optic chiasm, which is where both optic nerves cross. This results in tunnel vision.
Finally, the gland itself can become dysfunctional, often increasing its normal secretions. For
example, it may produce higher than normal amounts of growth hormone. This results in a
medical condition called gigantism.
FACIAL BONES
There are 14 bones that make up the face and are collectively called the facial bones. The inferior
and lateral portion of the eye socket is created by the zygomatic bone (cheek bone). The
zygomatic arch is composed of two processes, one from the temporal and one from the
zygomatic bone. The maxillary bone forms the upper jaw, containing the upper teeth, and
contributes toward the anterior portion of the hard palate (palatine process of maxillary bone).
It is best visualized by looking at the underside of the skull. The palatine bone can be seen
posterior to the maxillary bone forming the posterior portion of the hard palate. The vomer is a
blade-shaped bone located just inferior to the perpendicular plate, forming the lower portion of the
nasal septum. Lateral to the nasal septum are prominent bony projections called nasal
conchae. The ethmoid bone forms the superior and middle nasal conchae. A separate bone
forms the inferior nasal concha. They create a turbulence of inspired air, which aids in both the
cleaning and warming of the air that enters the nose. The nasal bones form the bridge of the
nose. Return to the orbit of the eyes and identify the lacrimal bone, which is located just anterior
to the ethmoid bone forming part of the anterior medial portion of the eye socket. The mandible
forms the lower jawbone. Open and close the mouth of the model and observe how the
mandibular condyle (condyloid process) articulates with the temporal bones, forming the
temporomandibular joints. The coronoid process is located just anterior, serving as a site of
muscular attachment for the temporalis muscle which is a muscle of mastication (chewing).
A XI AL S KELETON (E X AM INATION OF THE S PINE )
The spine typically has an S-shaped configuration. In both the cervical and lumbar spine, the
curve projects anteriorly (lordosis). The thoracic spine’s curve projects posteriorly (kyphosis)
These curves naturally develop as a result of the stresses of weight bearing. Variations in spinal
curves may be seen in people with scoliosis (excessive curving of the spine laterally) or
osteoporosis (collapsing of the vertebra, typically seen in the thoracic vertebra, creating an
excessive thoracic spine kyphosis)
THE VERTEBRAL COLUMN
The vertebral column consists of 24 individual articulating vertebrae, the sacrum, and the coccyx.
These bones function to protect the spinal cord and are important sites of muscular attachment,
making movement of the spine and maintaining an upright posture possible. The 24 individual
vertebrae are divided into 3 major areas. There are 7 vertebrae located in the neck called
cervical vertebrae. Just inferior to the cervical vertebrae are the 12 thoracic vertebrae. Finally,
the lumbar vertebrae make up the last 5 non-fused vertebral bones located in the lower back
area. The most caudal portion of the spinal column is composed of 2 fused bones called the
sacrum superiorly and coccyx inferiorly. The vertebrae of these two bones are considered fused
because they do not contain an intervertebral disc between them. There are 33 bones total that
make up the vertebral column, if you count the 5 sacral and 4 coccygeal bones.
Most vertebrae have several characteristic features with slight variations in anatomy due to
location and regional functions (Table 7.1 and Figure 7.4). The body of the vertebra is located
anteriorly and is designed for weight bearing. The size of the body is proportional to the amount
of weight it needs to support. The vertebral foramen is located posterior to the body. The spinal
cord and its nerve roots occupy this space. The spinous process is the prominent projection that
can be palpated and seen posterior in the midline. These are very visible when asking a thin
person to bend forward. They serve as attachment sites for many muscles and ligaments that
support the spine. The transverse processes are the lateral projections off the vertebra. They
also serve as attachment sites for many muscles and ligaments that support the spine. The
lamina is the portion of bone that connects the spinous and transverse processes. Articular
facets are projections off the body that enable the vertebra to move in relation to each other. The
superior articular facet of the bottom vertebra will articulate with the inferior articular facet of the
top vertebra. The facet joints are important for movement such as bending and rotating.
The sacrum is a bone composed of five-fused vertebra that articulate with L5 (lowest lumbar
vertebra) superiorly and the coccyx inferiorly. There are four pairs of sacral foramen that allow
both blood vessels and nerves to pass into the lower extremities. The coccyx is also known as
the tailbone.
The atlas and axis are considered atypical vertebrae. The atlas is the first cervical vertebra (C 1).
It lacks a vertebral body. The only structure it needs to support is the skull. The superior articular
facet of the atlas articulates with the occipital condyles at the base of the skull. (Remember, the
ancient Greek god Atlas holds up the world—so the atlas holds the skull.) The axis is the second
cervical vertebra (C2). The odontoid process (dens) is unique to this vertebra. It projects
superiorly to articulate with the atlas. Fifty percent of the rotation in the cervical spine occurs
between the atlas and axis. The articulation between the atlas and axis is called the atlantoaxial
joint.
Intervertebral discs are located between the vertebral bodies. These provide cushioning and
space which allows for both mobility of the spine and a space (intervertebral foramen) for the
peripheral nerves to exit the spinal column. The intervertebral disc is composed of two parts. The
annulus fibrosus is the tough, outer layer that anchors the vertebra together while allowing for a
small degree of movement. The nucleus pulposus is located in the center of the disc. It contains
a viscous gelatinous substance that contributes to both the height and shock-absorbing qualities
of the disc.
Clinical application: A herniated disc can occur if there was trauma to the outer annulus. This
allows for the nucleus pulposus to get pushed out posteriorly, putting pressure on the nerve
exiting the spine (Figure 7.5). The disc height will also become diminished, resulting in a smaller
intervertebral foramen, which can also compress the nerves exiting the spine at that level. A
laminectomy is a surgical procedure in which the lamina is removed to help decompress the
spinal nerves at the level of compression.
A XI AL S KELETON (E X AM INATION OF THE T HORACIC C AGE )
The thoracic cage consists of the sternum, ribs, and thoracic vertebrae (Figure 7.7). They
collectively function to protect the organs in the thoracic cavity. The sternum (breast bone) is a
flat bone created by the fusion of the manubrium (superior), body (middle), and xiphoid
process (inferior). There are 12 pairs of ribs that help protect the organs in the thoracic cavity.
The first 7 ribs, known as the true ribs, directly attach to the sternum via the costal cartilage.
Ribs 8–12 are considered false ribs because they attach indirectly to the sternum or have no
sternal attachments at all. Ribs 11 and 12 are considered floating ribs, because they have no
attachments to either the sternum or adjacent ribs.
Clinical application: Repetitive motions of a specific joint can often cause it to become inflamed.
The bursae associated with the joint may also swell making all movement of the joint painful. This
is a condition called bursitis. Osteoarthritis is a degenerative joint disease characterized by
clicking, stiffness, and joint pain.
Key muscles:
Suboccipital muscles ( rectus capitis major, minor, Obliquus capitis superior and
inferior)
Lonus capitis and cerivcis
Scalene muscles ( anterior middle and posterior)
Semispinalis and splenius muscles
trapezius
accesory (11th cranial) (C3, 4)
sternocleidomastoid
spinal part of accessory (C2, 3)
Acland Videos:
The Spine
• 3.1.1 The vertebral column, features of a typical vertebra (4:45)
• 3.1.2 Cervical and thoracic vertebrae (5:01)
• 3.1.3 Lumbar vertebrae, sacrum (3:35)
1. How many vertebrae make up the vertebral column? How many cervical ,
thoracic and lumbar vertebrae are there?
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2. Describe some characteristics of the cervical vertebrae.
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3. Describe some characteristics of the thoracic vertebrae.
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4. Name the motions of the thoracic spine.
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5. Describe some differences of the lumbar vertebrae.
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6. Name the motions of the lumbar spine
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7. The 5th lumbar vertebra articulates with which bones?
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8. The intervertebral disc has 2 parts: Name them
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9.
10.
3.1.4 Ligaments of the vertebral column, intervertebral disks (4:41)
3.1.5 Intervertebral joints (0:47)
3.1.6 Review of bones, joints, and ligaments of the vertebral column (1:45)
1. Describe the 2 parts of the intervertebral disc.
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2. Name 5 ligaments that support the spine. Describe their general locations.
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3.2.1 The bones of the thorax (4:53)
3.2.2 Costovertebral joints (1:08)
3.2.3 First rib and clavicle (2:53)
3.2.4 Review of bones of the thorax (1:15)
1. Name the bones that compose the thorax.
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2. Name the 3 types of ribs.
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3. Name the 2 articulations that form between the ribs and vertebra.
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• 4.1.5 Cervical vertebrae (4:57)
• 4.1.6 Bones of the upper thorax (1:24)
• 4.1.7 Cervical spine: intervertebral joints (2:23)
4.1.10 Review of the bones, joints and ligaments involved in support and
movement of the head (3:00)
4.1.11 Sub occipital muscles (1.18)
4.1.12 Anterior Neck muscles (1.34)
4.1.13 Posterior neck muscles (4.05)
1. What is the name of the first and second cervical vertebrae?
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2. Name the motions of the cervical spine.
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3. Name the anterior neck muscles Describe the direction of their muscle
fibers. Which anatomical plane do they run? What are their actions?
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4. Name the posterior neck muscles. Describe the direction of their muscle
fibers. Which anatomical plane do they run? What are their actions?
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5. Name the sub occipital muscles. Describe the direction of their muscle
fibers. Which anatomical plane do they run? What are their actions?
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Identify the following anatomical structures on the skeleton using The
Visible Body Skeleton Program.
Cranial Bones
 Neural arch
Identify the following anatomical
 Transverse process:
landmarks.
 Transverse foramen:
Occipital (1)
 Spinous process:
 Foramen magnum:
 Pedicle:
 Occipital condyles:
 Superior articular process:
 External occipital
 Inferior articular process:
protuberance and nuchal
Sacrum (1)
lines:
 Base:
 Temporal (2)
 ILA (inferior lateral angle):
 Styloid process:
 Superior articular process:
 Mastoid process:
 Auricular (or articulating)
Vertebrae
(sacroiliac joint)
Cervical (7), Thoracic (12), Lumbar
 Sacral canal:
(5)
 Sacral foramina:
 Vertebral foramen:
Coccyx (3-5 fused)
 Body:
 Lamina: