Chapter 5 The Skeletal System Axial Skeleton
• Be Able To:
• On a skull or diagram, identify the names of the bones of the skull.
Axial Skeleton
The Skull
• The skull is composed of 22 bones which are divided into two sets of bones: cranial bones and facial bones.
• The cranium encloses and protects the brain and the organs of hearing and
equilibrium.
• The facial bones (1) form the framework of the face (2) contain cavities for sense organs (3) provide openings or
passageways (4) secure the teeth and (5) anchor facial muscles.
• Most skull bones are flat bones.
• All bones of the skull are firmly united by interlocking joints called sutures (except for the mandible which is connected by a movable joint).
Axial Skeleton
The Cranium
• The cranium can be
divided into a base (floor) and a vault (skull cap).
• The cranium is composed of 8 large single flat bones
(except for the paired parietal and temporal bones).
• The skull has many small openings (e.g. foramina)
• The skull has many small cavities (e.g. inner ear
cavities).
• Air-filled sinuses lighten the skull.
Superior view of the cranium
Axial Skeleton
The Cranium
The Frontal Bone
• Forms anterior portion of the cranium, roofs of the orbits, and most of
anterior cranial fossa.
• It articulates posteriorly with the
paired parietal bones by the coronal suture.
Parietal Bones
• The two large parietal bones are curved, rectangular bones that form most of the superior and lateral
aspects of the skull.
• The 4 largest sutures occur where these bones articulate with other cranial bones:
1) Coronal suture – frontal and parietal.
2) Sagittal suture – parietals meet superiorly.
3) Lambdoid suture – parietal and occipital.
4) Squamous suture – parietal and temporal.
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Axial Skeleton
The Cranium
The Occipital Bone
• Forms most of the posterior wall and base of the skull externally.
• Articulates anteriorly with parietal bones and temporal bones.
• Also joins the sphenoid bone.
• At the base is a large hole
called the foramen magnum which the spinal cord passes through to join with the inferior brain.
• Occipital condyles articulate with the first vertebra of the spinal column
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Axial Skeleton
The Cranium
Temporal Bones
• Lie inferior to the parietal bones on lateral surface of the skull.
• Each temporal bone has a complicated shape and several bone markings:
1) External auditory meatus is a canal that leads to the eardrum and middle ear.
2) Styloid process serves as an attachment point for several muscles of the tongue and neck and for a ligament that secures the hyoid bone to the neck.
3) Zygomatic process is a thin bridge that meets the zygomatic bone (cheekbone).
4) Mastoid process is an anchoring site for some neck muscles.
5) Jugular foramen allows passage of the internal jugular vein and 3 cranial nerves.
6) Carotid canal transmits the internal carotid artery into the cranial cavity.
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Axial Skeleton
The Cranium
Sphenoid Bone
• Butterfly shaped bone spans the width of the skull and forms part of the floor of the cranial cavity.
• Consists of a central body with 3 processes: greater, lesser, and pterygoid wings.
• Superior surface of the body bears the Sella Turcica or “Turk’s
Saddle” which holds the pituitary gland in place.
• The foramen ovale allows fibers of cranial nerve V to pass to chewing muscles of the mandible.
• The central body contains the sphenoid sinuses.
Foramen ovale
Axial Skeleton
The Cranium
The Ethmoid Bone
• lies between the sphenoid bone and the nasal bones of the face.
• Forms most of the bony area between the nasal cavity and the orbits.
• The superior surface is called the crista galli and is punctured by tiny holes called cribiform plates that allow the olfactory nerves to pass from the smell receptors to the brain.
• The outermost covering of the brain attaches to the crista galli which secures the brain in the cranial cavity.
ethmoid bone
Axial Skeleton
Facial Bones
• The facial skeleton is made up of 14 bones, only the vomer and mandible are not paired.
• The facial skeleton of men is more elongated then women.
Maxillary bones
• Also known as maxillae they are fused medially.
• Form upper jaw and articulate with all other facial bones except for the
mandible.
• The upper teeth are carried in the alveolar margins (cavity).
• The palatine process is an extension of the maxillae.
• The sinuses of the maxillae drain into the nasal passageways.
• Sinusitis is an infection in the nasal passageways.
maxillae
Palatine process
Axial Skeleton
Facial Bones
Palatine Bones
• The paired palatine bones lie posterior to the palatine process and form the posterior portion of the hard palate.
• A cleft palate is the result of a
failure of the palatine bones or the palatine process to fuse medially.
Zygomatic Bones
• Commonly called the cheek bones.
• Articulate with the zygomatic
processes of the temporal bones and with the zygomatic
processes of the maxillae.
• Also form part of the lateral walls of the orbits.
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Axial Skeleton
Facial Bones
Lacrimal bones
• Fingernail-sized bones that contribute to the medial walls of each orbit.
• Articulate with the frontal and ethmoid bones and with the maxillae.
• Each has a groove called the
lacrimal fossa that houses part of the passageway that permits tears to drain from the surface of the eye.
Nasal Bones
• Rectangular shaped bones are fused medially to form the bridge of the
nose.
• Articulate with the frontal and ethmoid bone and with the maxillary bones.
• They attach to the cartilages that form most of the skeleton of the external nose.
Nasal bone
Axial Skeleton
Facial Bones
Vomer
• A slender bone located within the nasal cavity
where it forms part of the nasal septum.
Inferior Conchae
• Thin, curved, paired bones in the nasal cavity that
project medially from the lateral walls of the nasal cavity.
Axial Skeleton
Facial Bones
Mandible
• Lower jawbone is the largest and strongest bone of the face.
• It has a body which forms the chin and two upright rami or branches.
• The mandibular condyle
articulates with the mandibular fossa of the temporal bone which forms the temporomandibular joint.
• The mandibular body anchors the lower teeth at the alveolar margin.
• Dentists inject Novocain into small openings called the mandibular formamina which house nerves to prevent pain.
Ramus body
Mandibular foramina
Mandibular condyle
Axial Skeleton
the Hyoid Bone
• Although not part of the skull, the
hyoid bone is associated with the mandible and temporal
bones.
• It lies inferior to the mandible in the anterior neck.
• It is anchored to the styloid processes by the stylohyoid ligaments.
• It is horseshoe shaped with a
body and two pairs of horns called cornua.
• It act as a movable base for the tongue.
• The body and tongue act as
attachment point for muscles that raise and lower the larynx.
It is the only bone that does not articulate with another bone.
Chapter 5 The Skeletal System Axial Skeleton
• Be Able To:
• Describe how the skull of a newborn infant or fetus differs from an adult.
• Explain the function of the fontanels.
Axial Skeleton
The Fetal Skull
• The skull of an infant is large
compared to the infant’s total body length.
• A newborn’s skull is not completely ossified at birth.
• Fibrous membranes that connect cranial bones are called fontanels.
• The fontanels allow the skull to be compressed during birth and allow the growth of the brain into early infancy.
• By 2 years of age the fontanels have been converted to bone.
Chapter 5 The Skeletal System Axial Skeleton
• Be Able To:
• Identify the parts of a typical vertebra and explain how the cervical, thoracic, and
lumbar vertebrae differ from each other.
• Discuss the importance of the
intervertebral disks and the spinal curvatures.
• Explain how abnormal spinal curvatures differ from one another.
Axial Skeleton
The Vertebral Column: general structure
• The vertebral column or spine is comprised of 26 irregular bones connected into a flexible, curved structure.
• The spine extends from the skull to the pelvis.
• It transmits the weight of the trunk to the lower limbs.
• Running through its central cavity is the spinal cord.
• The vertebral column also serves as points of attachment for the ribs and muscles of the back.
• In a fetus or infant, the spine
consists of 33 separate bones of which 9 fuse to form the sacrum and coccyx or tailbone.
Axial Skeleton
The Vertebral Column: divisions and curvatures
• The vertebral column has 5 major divisions:
1) 7 Cervical vertebrae of the neck 2) 12 thoracic vertebrae
3) 5 lumbar vertebrae of the lower back.
4) Sacrum articulates with the pelvis.
5) Coccyx is the terminal end.
• The vertebral column has 4 curvatures which give it an S shape:
1) The primary curvatures are the thoracic and sacral which are convex posteriorly.
2) The secondary curvartures that develop later in life are the cervical and lumbar which are concave posteriorly.
• The curvatures increase the flexibility and resiliency of the spine.
Axial Skeleton
The Vertebral Column: general vertebral structure
• Each vertebra consists of an anterior body or centrum and a posterior vertebral arch.
• The body and vertebral arch enclose the vertebral foramen.
• The vertebral arch is formed by two pedicles that compose the sides and two laminae that compose the median plane of the vertebral arch.
• The spinous process is a single posterior projection at the junction of two fused
laminae.
• A transverse process extends laterally from each side of the vertebral arch.
• Successive vertebrae join at their bodies via the superior and inferior articular
processes.
• Intervertebral foramina are small openings for spinal nerves to pass from the spinal cord.
• Intervertebral disks are pads of
fibrocartilage that cushion and absorb shock for the vertebrae.
As we age water content of the disks decreases and
they become harder.
Axial Skeleton
The Vertebral Column: cervical vertebrae
• Cervical vertebrae identified as C1-C7 are the smallest and lightest
vertebrae.
• C3-C7 vertebrae are oval in shape, have a short bifid spinous process, large vertebral foramen, and contain a transverse vertebral foramen.
• C7 is does not have a bifid spinous process.
• C1 or the atlas has no body and no spinous process and has articular surfaces on both its superior and inferior surfaces.
• The superior articular facets receive the occipital condyles.
• C2 or the axis has a body, spine, and a the odontoid process or dens.
• The odontoid process allows the skull to pivot for rotation on the atlas.
C2 or axis
C1 or atlas
dens
Articular facet
Axial Skeleton
The Vertebral Column: thoracic vertebrae
• T1 through T12 or the thoracic vertebrae all articulate with the ribs.
• Thoracic vertebrae increase in size from the first to the last.
• The spinous process is long and points sharply inferiorly.
• The body is heart-shaped and has 2 facets on each side that articulate with the head of the ribs.
Demi-facet body
Vertebral foramen
Axial Skeleton
The Vertebral Column: lumbar vertebrae
• L1 through L5 or the lumbar vertebrae, or the small of the back, receive the most stress.
• Their bodies are massive and kidney shaped.
• The pedicles and laminae are shorter and thicker than the others.
• The spinous processes are shorter and flatter and can be seen when someone bends forward.
• The facets of the articular
processes are locked to provide stability.
Spinous process
Axial Skeleton
The Vertebral Column: sacrum and coccyx
• The sacrum is formed by 5 fused vertebrae S1-S5.
• The sacrum shapes the posterior wall of the pelvis and strengthens and
stabilizes it.
• It articulates superiorly with L5 and inferiorly with the coccyx.
• The sacral promontory is the anterosuperior margin of the first fused vertebra.
• 4 ridges called transverse lines mark vertebral fusion.
• The vertebral canal continues inside the sacrum in the sacral canal.
• Our vestigial tailbone or coccyx is a small triangular bone that consists of 3 to 5 fused vertebrae.
Transverse line
Sacral canal
coccyx
Axial Skeleton
The Bony Thorax
• The bony thorax or thoracic cage is comprised of the
thoracic vertebrae dorsally, the ribs laterally, and the sternum and costal cartilages
anteriorly.
• Costal cartilage secures the ribs to the sternum.
• The bony thorax forms a
protective cage around the vital organs, supports the shoulder girdles and upper limbs, and provides muscle attachment sites.
• Intercostal muscles between each rib aid in breathing.
Articulated thoracic vertebra and rib
rib
Axial Skeleton
The Bony Thorax: sternum
• The sternum or
breastbone is a flat bone that lies in the anterior
midline of the thorax.
• It is comprised of 3 fused bones: superior
manubrium, body or midportion, and inferior xiphoid process.
• In some people the xiphoid process projects dorsally which can be a danger.
Manubrium
Body
Xiphoid process
Axial Skeleton
The Bony Thorax: ribs
• Twelve pairs of ribs form the sides of the thoracic cage.
• All ribs attach posterioly to the thoracic vertebra and curve anteriorly.
• The superior seven rib pairs attach directly to the sternum by costal cartilage, a.k.a. true ribs.
• The remaining 5 pairs of ribs either attach indirectly to or not all to the sternum, a.k.a.
false ribs.
• Rib pairs 8-10 attach to the sternum indirectly, a.k.a. vertebrochondral ribs.
• rib pairs 11 and 12 are vertebral or false ribs because they have no anterior attachment.
• The bulk of the rib is the shaft
• The head of the rib articulates with the body and demi-facet of the thoracic vertebrae.
• A tubercle articulates with the transverse process of the thoracic vertebrae.
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