Chapter 7: The Axial Skeleton

axial skeleton

~ 206 bones

• forms longitudinal axis of the body

• skull, hyoid bone, vertebral column, and throacic cage

• framework that supports:

  • the brain, spinal cord, and organs of the ventral body cavity

appendicular skeleton

bones of the limbs and girdles that connect them to the axial skeleton

what is the function of bone markings?

attachment of muscles, grooves for nerves, blood vessels, and bone articulations

how can bone markings be useful to forensic scientists, anthropologists and pathologists? 

• helps to determine size, age and sex

• helps to determine general appearance of an individual 

bone markings: head

description: expanded articular end of epiphysis, often separated from the shaft by a narrower neck

purpose: serves as an articulating surface that forms a joint

bone markings: tuberosity

description: rough projection

purpose: serves as an attachment point for muscles, tendons and ligaments

bone markings: tubercle

description: small, rounded projection 

purpose: provides an attachment point for smaller muscles, tendons, and ligaments

bone markings: condyle

description: smooth, grooved articular process shaped like a pulley

purpose: forms a smooth, articular surface for joint movement and bears the force of the joint

bone markings: fossa

description: shallow depression

purpose: provides a surface that receives and protects organs or structures 

bone markings: foramen 

description: rounded passageway for blood vessels and/or nerves

purpose: a passageway for nerves, blood vessels, and sometimes ligaments and tendons to move through the body

number of bones in the skull

22 bones

helmet-like covering that encloses cranial cavity: houses and protects brain, 8 bones

cranium (or brain case)

helmet-like covering that encloses cranial cavity: houses and protects brain, 8 bones

frontal bone

number: 1

function: forms forehead and upper parts of orbits

special features:

• contains frontal sinus mucus cavity

• formed by union of 2 bones at metopic suture: usually disappears by age 8 when bones fuse

parietal bone

number: 2 

function: forms sides and roof of cranium 

special features: none

temporal bone

number: 2 

function: forms lateral wall of cranium and part of cranial floor

special features:

• zygomatic process: forms posterior part of the zygomatic arch

• mastoid process: rounded projection behind ear

• styloid process: slender spike extending downward

• external auditory canal: canal leading to middle ear

• mandibular fossa: depression on inferior surface which forms socket for mandible

occipital bone

number: 1

function: forms much of the posterior and inferior surfaces of the cranium

special features:

• occipital condyle: oval process that articulate with the first cervical vertebra 

• foramen magnum: large hole; connects the cranial cavity; surrounds connection between brain and spinal cord

ethmoid bone

number: 1

function: irregularly shaped; forms inner surface of cranial floor, medial walls of orbit, roof of nasal cavity, and superior part of nasal septum

special features:

• cribiform plate: forms floor of cranium and roof of nasal cavity; contains perforations = olfactory bulb

• crista galli: upward projecting triangular process

• superior and middle nasal conchae: forms lateral masses that form walls of nasal cavity

• perpendicular plate: slender process in midline that forms upper part of nasal septum

• ethmoid sinus: mucus lined cavities

sphenoid bone

number: 1

function: keystone of cranial floor; acts as a cross brace to strengthen skull by extending from one side of the cranium to other; resembles bat with wings extended; forms side of skull, most of cranial floor and part of the orbit

special features:

• body: central portion

• sella turcica: saddle shaped depression on superior surface 

• greater wing: lateral projections that form part of orbit and lateral walls of skull anterior to temporal bone

• sphenoid sinus: mucus lined cavities

number of facial bones

14 bones

nasal bone

number: 2

function: forms bridge of nose

special features:

• none

maxillary bone

number: 2

function: forms upper jaw and parts of orbit, anterior part of roof of mouth (palate) = hard palate

special features:

• maxillary sinus: mucus lined cavity

• palatine process: horizontal projection that forms bony roof of mouth = anterior 2/3 of hard palate

zygomatic bone

number: 2

function: forms part of zygomatic arch and part of outer wall and floor of orbit

special features:

• none

mandible bone

number: 1

function: lower jaw

special features:

• main horizontal part: forms chin = body

• alveolar processes: sockets for teeth

• mandibular condyle: (condylar process) articulates with mandibular fossa of temporal bone

lacrimal bone

smallest of the facial bones

number: 2

function: tiny bones that form medial walls of orbit

special features:

• none

palatine bone

number: 2

function: forms posterior part of hard palate and small part of orbit floor

special features:

• none

inferior nasal conchae bone

number: 2

function: forms lower part of lateral walls of nasal cavity

• helps to create turbulence in air passing through nasal cavity = promotes warming and humidification of inhaled air

special features:

• none

vomer bone

number: 1

function: forms lower part of nasal septum

special features:

• none

special features of the skull

sutures and wormian (sutural bones)

sutures

immoveable joints between the skull bones

squamosal suture

boundary between temporal and parietal bones

sagittal suture

boundary between parietal bones

lambdoidal suture

boundary between parietal and occipital bones

coronal suture

boundary between frontal and parietal bones

wormian (sutural bones)

usually present along lambdoidal suture

orbit

bony recess made up of seven bones → contain the eyes

• frontal = roof

• ethmoid = medial wall

• lacrimal = medial wall

• palatine = posterior process

• maxillary = floor and medial orbital rim

• zygomatic = lateral wall and rim

• sphenoid = posterior portion

the nasal septum

superior portion = perpendicular plate of the ethmoid

inferior portion = vomer

the bridge 

formed by paired nasal bones

the nasal cavity

• frontal 

• sphenoid

• superior, middle, inferior nasal conchae

• maxilla 

• palatine

• nasal

• ethmoid 

paranasal sinuses

• lightens the skull

• extensive area of mucus epithelium 

fontanelles 

large fibrous areas between cranial bones 

functions:

• distortion of skull for delivery 

• allows for brain growth to continue after birth

anterior fontanelle

occipital fontanelle

two sphenoid fontanelles

two mastoid fontanelles

hyoid bone

number: 1

function: horseshoe shaped bone in neck between mandible and larynx 

special features: 

• supports tongue and serves as attachment for muscles 

vertebral column

number: 26

functions: a flexible segmented rod is shaped like an enlarged “S”

special features:

• forms axis of body and encloses spinal cord

consists of…

• cervical = 7

• thoracic = 12

• lumbar = 5

• sacral = 5 fused

• coccyx = 3-5 fused-

  • sacral and coccyx not completely fused until 25-30 yrs

why is the spinal column curved?

allows the body to remain in balance when standing upright

concave

curves posteriorly - toward viewer

convex

curves anteriorly - away from viewer

development of the curves

~ 3 months - convex cervical develops when infant can hold its head up

thoracic (concave)

~ 1 yr - convex lumbar develops when child can stand and walk

sacral (concave)

primary curves

ones you are born with

• thoracic and sacral 

concave 

secondary curves

ones you develop after birth

• cervical and lumbar 

• = compensation curves 

convex 

kyphosis

• hunchback

exaggeration of thoracic curve 

lordosis 

• swayback

exaggeration of lumbar curve 

scoliosis

abnormal lateral curvature of spine

features of typical vertebrae

• body

• vertebral arch

• seven processes of vertebral arch

• vertebral foramen

• vertebral articulation 

body

• thick, disc-shaped anterior part of vertebrae 

• bears weight 

vertebral arch

• pedicles: two short thick projections which extend posteriorly from body

• lamina: flat part that joins to form the dorsal wall of the vertebral arch

seven processes of vertebral arch

• transverse process (2): lateral projections from laminae 

• spinous process (1): sharp process projecting posterior and inferior from midline of lamina

• superior articular process (2): projects upward from laminae

• inferior articular process (2): project downward from laminae ( articulates with superior articular processes of vertebrae below )

vertebral foramen

hole in the center of vertebra formed by the union of the body, pedicles and lamina

vertebral articulation

• separated by intervertebral discs (fibrocartilage pads)

• inferior articular processes of one vertebra join with superior articular process of the next

• alignment of vertebral foramina forms vertebral canal

cervical vertebrae

• small

• spinous process is split (bifid) (C2 to C6)

• transverse foramen is transverse processes = blood vessels (C1 to C7)

C1: atlas

• no body

• no spinous process

• large round vertebral foramen 

• contains superior articular surfaces for articulation with occipital condyles 

how does the atlas get its name?

it holds the head up like the greek god atlas holding the world on his shoulders

what action is provided by the joint between the atlas and the occipital condyles?

nod head

C2: axis

• has a body

• contains a peg-like projection called the odontoid process or dens that extends into the atlas

what action is provided by the joint between the axis and the atlas?

rotation of head

in babies the dens is not fully fused to the axis. why is shaking dangerous to a baby?

you could break the dens and cause serious brain damage

C7: the vertera prominens

long non-bifid spinous process (does have transverse foramina)

thoracic vertebrae 

• larger than cervical vertebrae

• spinous process is long, pointed and directed inferiorly

• long and heavy transverse processes that have facets for articulations with ribs

• bodies have facets or partial facets (demi-facets) for articulations with ribs

lumbar vertebrae

• largest and strongest vertebrae because they support weight of the body

• thick, oval, flat bodies

• spinous process is square and projects posteriorly 

sacrum

5 fused sacral vertebrae

sacrum anterior

(concave side facing pelvic cavity)

• transverse lines: points where vertebrae have fused

• anterior sacral foramina: four pairs of foramina that communicate with posterior foramina 

sacrum posterior 

• median sacral crest: ridge formed by spinous processes of fused vertebrae

• posterior sacral foramina

• sacral canal: inferior extension of vertebral canal

sacrum superior

sacral promontory: superior border on anterior surface

sacrum lateral

auricular surface

coccyx

3-5 fused coccygeal vertebrae

• vestigial tail

thorax

bony cage formed by sacrum, costal cartilages, ribs and bodies of thoracic vertebrae

sternum

manubrium: triangular superior portion

body: middle, tongue shaped, largest portion

xiphoid process: inferior, smallest portion

ribs

24 = 12 pairs

general features:

• head: projection at posterior end of rib

• tubercle: knoblike elevation 

• body/shaft: flat, main part of rib

articulation:

• head of ribs fits into facets of bodies of one to two adjacent thoracic vertebrae

• tubercle of rib fits into facets of transverse process of vertebrae 

vertebrosternal ribs 1-7 (true ribs)

anterior articulation: costal cartilages of sternum

posterior articulation: thoracic vertebrae 

vertebrochondral ribs 8-10 (false ribs)

anterior articulation: costal cartilage of rib 7

posterior articulation: thoracic vertebrae

vertebral/floating 11-12 (false ribs)

anterior articulation: NOT attached directly

posterior articulation: thoracic vertebrae