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The Axial Skeleton
The Axial Skeleton

Forms the longitudinal part of the body

Divided into three parts

Skull

Vertebral column

Bony thorax
The Skull

Two sets of bones
o
Cranium – encloses & protects brain
o Facial bones – holds eyes in anterior position & allows the facial muscles to show
emotions.

All but 1 of the bones are joined by sutures (interlocking, immovable joints)

Only the mandible is attached by a freely movable joint
The Cranium

Frontal – forms forehead, brow bone, superior eye orbit

Parietal (2) – form most of the superior & lateral walls of the cranium

o
Meet in midline = sagittal suture
o
Meet frontal = coronal suture
Temporal (2) – inferior to parietals & join to them at the squamous sutures
o
Important bone markings found here

External acoustic (auditory) meatus – canal leading to eardrum

Styloid process – sharp needle-like projection inferior to external auditory
meatus (attachment point for many neck muscles

Zygomatic process – thin bridge of bone that joins w/ the zygomatic
(cheek) bone

Mastoid process – rough projection posterior & inferior to the external
auditory meatus



Full of air cavities (sinuses)

Attachment for some neck muscles

Close to middle ear & leads to ear infections
Jugular foramen – junction of occipital & temporal

Allows for passage of jugular vein

Largest vein of the head – drains the brain
Internal auditory meatus – anterior to jugular foramen


Carotid canal - anterior to jugular foramen


Transmits cranial nerves 7 & 8 (facial & vestibulocochlear)
Carotid artery runs through it to brain
Occipital – most posterior bone of cranium forming back wall & floor of the skull
o
Joins parietals anteriorly at lambdoid suture
o
Foramen magnum = large opening in base of the occipitals (spinal cord connects
with the brain)

Lateral to the foramen magnum are rockerlike occipital condyles which
rest on the 1st vertebra

Sphenoid – butterfly-shaped – spans the width of the skull and forms part of cranial
cavity floor
o
Sella turcica “Turk’s saddle” = small depression on the midline of the sphenoid,
holds the pituitary gland
o
Foramen ovale = large oval opening in line w/ the posterior end of the sella
turcica (allows cranial nerve 5 (trigeminal) to pass to chewing muscles of
mandible
o
Parts of the sphenoid form part of the eye orbits

2 important openings:
1. Optic canal (optic nerve)
2. Superior orbital fissure (cranial nerves 3, 4 & 6 – eye movements)
o Central part of the sphenoid riddled w/ air cavities = sphenoid sinuses

Ethmoid – irregularly shaped, anterior to sphenoid – forms roof of nasal cavity and
medial walls of the orbits.
o
Crista galli “cock’s comb” = superior ethmoid surface projection – outermost brain
covering attaches
o
Cribriform plates – holey areas on sides of crista galli= nerve fibers for smell pass
through from nose
o
Superior & middle nasal conchae – extensions of the ethmoid – form part of
lateral walls of nasal cavity & increase turbulence of air flowing
Facial Bones
*14 bones
*12 paired, only the mandible and vomer are single

Maxillae (2) / maxillary bones – fused to form upper jaw
o
Upper teeth carried in the alveolar margin
o
Palatine processes- extensions that form the anterior part of the hard palate
o
Paranasal Sinuses – drain the nasal passages, lighten the skull bones, amplify
sounds as we speak


Hollow portions of bones surrounding the nasal cavity

Sinusitis (infection of sinuses) – can result in headache or upper jaw pain
Palatine (2) – posterior to palatine processes of maxillae – form posterior part of hard
palate
o
Cleft palate= failure of these to fuse

Zygomatic (2) – cheek bones – form portion of lateral walls of orbits

Lacrimal (2) – fingernail sized bones forming part of medial walls of orbits
o

Groove serves as passageway for tears
Nasal (2)– small rectangular bones – form bridge of nose – lower part of nose made of
cartilage

Vomer “plow” (1) – median line of nasal cavity – forms most of the nasal septum

Inferior nasal conchae (2) – thin, curve bones projecting from lateral walls of the nasal
cavity

Mandible (lower jaw) – largest, strongest bone of the face – joins temporal bones on
each side of face, forming the only freely movable joints in the skull (find them!)
o
Horizontal part (body) forms the chin
o
2 upright bars of bone (rami) extend from the body to connect the mandible with
the temporal bone.
o
Lower teeth lie in alveolar margin
The Hyoid Bone

Not really part of the skull

Horseshoe shaped w/ a body and 2 pair of
horns (cornua)

Closely related to mandible and temporal
bones

Unique b/c it’s the only bone that does not
articulate w/ any other bone

Suspended in mid–neck region 2 cm above
the larynx, anchored by ligaments to the
styloid processes of the temporal bones

Serves as a movable base for the tongue &
attachment point for neck muscles (lower and
raise larynx when we swallow & speak)
The Fetal Skull

Face small compared to size of cranium (skull is large
compared to body length)

Adult skull is 1/8 total body length; newborn is 1/4

Fontanels – fibrous membranes connecting the cranial
bones
o
Baby’s pulse can be felt in these soft spots
(explains their name “little fountain”)
o
Allow fetal skull to be compressed in birth
process
o
Allow infants brain to grow
o
Largest fontanels are diamond shaped anterior
shaped fontanel and smaller triangular shaped
posterior
o
Convert to bone within 24 months after birth
The Vertebral Column

Serves as axial support of the body

Extends from the skull, which it supports, to the
pelvis, where it transmits the weight of the body to
the legs.

26 irregular bones connected & reinforced by
ligaments creating a flexible, curved structure.

Spinal cord runs through central cavity, protected
by vertebrae

Before birth = 33 separate vertebrae but 9 later
fuse to form 2 composite bones – the sacrum (5
fused) & the coccyx (4 fused).

Each vertebrae is given a name according to its
location


24 single vertebrae
o
7 cervical vertebrae
o
12 thoracic vertebrae
o
5 lumbar vertebrae
Vertebrae separated by pads of flexible
fibrocartilage – intervertebral discs –cushion &
absorb shocks while allowing flexibility.

o
Young person – discs = 90% water content – spongy & compressible.
o
As you age – water content decreases – harder & less compressible.
Can lead to herniated (“slipped”) discs.

Can also occur from exceptional twisting forces.

If disc presses on spinal cord or nerves = numbness & excruciating pain.
Disks & S-shaped curvature of spine prevent shock to head when we walk or run.
o
o


Primary curvatures

Thoracic & sacral regions

Present at birth
Secondary curvatures

Cervical curvature appears when baby begins to raise its head.

Lumbar curvature when baby begins to walk.
Abnormal spinal curvatures
o
Scoliosis - abnormal lateral curvature of the spine.
o
Kyphosis - Abnormal rearward curvature of the spine, resulting in hunchback.
o
Lordosis - Abnormal forward curvature of the spine in the lumbar region.
Vertebrae

All vertebrae have a similar structural
pattern.
o
Body: disclike, weight bearing
part facing anteriorly.
o
Vertebral arch: formed from the
joining of all posterior
extensions, the laminae &
pedicles.
o
Vertebral foramen: canal through
which the spinal cord passes.
o
Transverse processes: 2 lateral projections
from the vertebral arch.
o
Spinous process: single projection arising from
the posterior aspect of the vertebral arch (fused
laminae).
o Superior & inferior articular processes: paired
projections lateral to the vertebral foramen
allowing a vertebra to form joints w/ adjacent
vertebrae.
Cervical Vertebrae

7 (C1 to C7) – form the neck region.

First 2 – atlas & axis – are different because they
perform functions not shared by any other cervical
vertebrae.

Atlas (C1) has no body; the superior surfaces of its
transverse processes contain large depressions
that receive the occipital condyles of the skull;
allows you to nod “yes.”

Axis (C2) has a large upright process (dens or
odontoid process), which acts as a pivot point;
allows you to indicate “no.”

C3 through C7 are the smallest, lightest vertebrae

All transverse processes of cervical vertebrae only contain foramina through which
vertebral arteries pass to the brain.
Thoracic Vertebrae

12 with body somewhat heart shaped w/ 2 costal
facets on each side, which receive the heads of the
ribs.

Spinous process is long & hooks sharply downward
(from the side looks like a giraffe’s head).
Lumbar Vertebrae

5 w/ massive blocklike bodies & short hatchetshaped spinous processes (looks like moose head
from side).

Sturdiest vertebrae – most stress here.
Sacrum

Formed by the fusion of 5 vertebrae.

Winglike alae articulate laterally w/ the hipbones
forming the sacroiliac joints.

Forms the posterior wall of the pelvis.

Median sacral crest roughens the posterior midline & are flanked by sacral foramina.

Vertebral canal continues inside the sacrum as the sacral canal – terminates in large
inferior opening called the sacral hiatus.
Coccyx

Formed by fusion of 3 to 5 tiny, irregularly shaped vertebrae

This is the human “tailbone” – a remnant of the tail other vertebrate animals have.
Bony Thorax


Made-up of three parts
o
Sternum
o
Ribs
o
Thoracic vertebrae
Often called the “thoracic cage” b/c it forms a cone-shaped cage of slender bones to
protect the major organs of the thoracic cavity.

Sternum – “breastbone”
o
Flat bone that is a result of the fusion of 3 bones – the manubrium, body and
xiphoid process
o
Attached to the first 7 pairs of ribs
o
Three important bony landmarks:
1. .jugular notch (concave upper border of the manubrium) – can be felt easily –
generally at level of T3
2. .sternal angle - where manubrium & body meet - formed at level of 2nd ribs
(reference to locate 2nd intercostal space for listening to heart valves)
3. .xiphisternal joint – body and xiphoid process fuse (level of T9)
o
Sternal puncture used to get bone marrow tissue to diagnose certain blood
diseases

Ribs – 12 pair – form walls of bony thorax
o
Articulate w/ vertebral column posteriorly & curve downward toward anterior body
surface.
o
True ribs = first 7 pair – attach directly to sternum by costal cartilage
o False ribs = next 5 pair – attach indirectly to sternum or not at all (last 2 pair are
called “floating ribs” b/c they are
the ones not attached at all)
o Contrary to popular myth – men &
women have the same number of
ribs!! 