Notes on the Web
Skeletal System - Axial Skeleton

Bruce G. Stewart

Related Textbook Readings

• Marieb (2004) Chapter ___

Lecture Outline

I.  Skull

A. General

1. Rests on the superior end of the vertebral column

2. Contains 22 bones

a. cranial bones – 8 bones enclose and protect the brain

* frontal bone, parietal bones (2), temporal bones (2), Occipital bone, sphenoid bone & ethmoid bone

b. facial bones – 14 bones

* nasal bones (2), maxillae (2), zygomatic bones (2), mandible, lacrimal bone (2), palatine bones (2), inferior nasal conchae (2), and vomer

B. Sutures – means seam or stitch; immovable joint found only between skull bones; little connective tissue

1. Four prominent ones

a. coronal suture – between the frontal bone & two parietals

b. sagittal suture – between the parietal bones

c. lambdoidal suture – between the parietals and the occipital bones

2. Other sutures – names descriptive (e.g. frontonasal suture)

3. Note: review figures

C. Fontanels – membrane-filled spaces between cranial bones at birth (‘soft spots’);

1. General

a. areas of dense connective tissue where intramembranous ossification will occur

b. functions

* enable fetal skull to compress as it passes through the birth canal

* permit rapid brain growth during infancy

* facilitate determination of degree of brain development by their state of closure

* serve as landmarks (anterior fontanel) for withdrawal of blood from the superior sagittal sinus

* aid in determining the position of the fetal head prior to birth

2. Six major fontanels

a. anterior (frontal) fontanel – between angles of the two parietal bones and frontal bone

* diamond – shaped & largest

* closes in 18 to 24 months

b. posterior (occipital) fontanel – between parietal bones occipital

* diamond – shaped & smaller

* closes in about two months

c. anterolateral ( sphenoidal) fontanels – at junction frontal, parietal, temporal, and sphenoid bones

* paired, small, and irregular in shape

* close in about three months

d. posterolateral (mastoid) fontanels – at junction of parietal, occipital, and temporal bones

* paired, small, and irregular

* close in 12 months

D. Cranial bones

1. Frontal bone – forms forehead, roofs of the orbits, and most of the anterior part of the cranial floor

a. supraorbital margin – thickening of the frontal bone the forms the roof of the orbit

* supraorbital foramen (notch) – medial to the orbits midpoint; carries the supraorbital nerve & artery

b. frontal sinuses – mucus- lined cavities that serve as sound chambers for voice – resonance

2. Parietal bones – form much of the sides and roof of cranial cavity

a. internal surface contains many eminences and depressions for blood vessels to meninx (dura mater)

3. Temporal bones  - inferior to parietals & meet them at squamous sutures s

a. form inferolateral aspects of skull & parts of cranial floor

b. word comes from Latin word temporum in reference to time because gray hair usually appears here first

c. complicated shape with four major regions

* squamous region – upper side

# includes zygomatic process of arch (cheekbone)

# mandibular fossa articulates with condyle of mandible

--temporomandibular joint

* tympanic region – lower side

# surrounds the external auditory meatus

# styloid process – provides neck muscle attachment site

* mastoid region – back, lower side

# mastoid process – neck muscle attachment

# stylomastoid foramen – facial nerve exit

# mastoid sinuses – near inner ear; subject to infection; separated from brain by only thin bone plate

* petrous region – inferior most area; a bony wedge between sphenoid & occipital

# together with sphenoid, form the middle cranial fossa for temporal lobes of brain

# also includes the middle & inner ear cavities

# many foramina including:

-- jugular foramen which allows passage of internal jugular vein & three cranial nerves

-- carotid canal – just anterior to jugular foramen; transmits internal carotid artery; provides 80% of blood to cerebral hemispheres & passes close to inner ear cavities

-- foramen lacerum – not important but may see between temporal & sphenoid

-- internal auditory meatus – superolateral to jugular foramen; transmits the internal auditory artery and cranial nerves VII & VIII

4. Sphenoid bone – spans width of cranial floor & a ‘keystone’ bone (articulates with all other bones)

a. a body & three processes: greater wings, lesser wings & pterygoid processes

* body

# sphenoid sinuses

# sella turcica – supports pituitary

* greater wings

# form part of middle cranial fossa

# form laterodorsal walls of orbits

# form part of external wall of cranium

* lesser wings

# form part of the anterior fossa of the cranial floor

# form part of the medial wall of orbits

b. have several important foramina

* optic foramen – anterior to sella turcica

# transmits optic nerve & ophthalmic nerve

* superior orbital fissure

# transmits cranial nerves that control eye movements

* foramen rotundum (base of greater wings) & foramen ovale (posterior to foramen rotundum) transmit branches of cranial nerve V

5. Ethmoid bone – complex shape; between sphenoid & nasals

a. forms most of bony area between nasal cavity & orbit

* horizontal plate – superior surface

# crista galli – projection that anchors brain

# cribriform plates – has many tiny olfactory foramina which allow nerves to nasal cavity

* perpendicular plate – inferior part; forms superior part of nasal cavity

* perpendicular plate – inferior part; forms superior part of nasal  septum

* lateral masses – riddled with ethmoid sinuses

# laterally form part or orbit

# medially have coiled superior & middle nasal conchae (turbinates)

E. Facial Bones- 14 with only the mandible & vomer unpaired; more elongate in men

1. Mandible – lower jaw

a. body & two upright rami

* mandibular angle – where body & ramus meet

* coronoid process – anterior part of ramus where temporalis inserts

* mandibular condyle – articulates with mandibular fossa of the temporal bone (temporomandibular joint)

* alveolar margin of body – superior & has alveoli ( sockets)

* mandibular symphysis – fusion point

* mandibular foramina – medial surface of each ramus; transmits nerves for tooth sensation

# where dentists inject novacaine for lower jaw work

* mental foramina – lateral aspects of body; transmit nerves to chin

2. Maxillary bones – upper jaw; keystone bones since all facial bones except mandible articulate with them

a. alveolar margins for upper teeth

b. palatine processes – form part of hard palate

c. frontal processes – form lateral part of nose

d. maxillary sinuses – largest of paranasal sinuses; from orbits to upper teeth

e. articulate laterally with zygomatic bones via zygomatic processes

f. inferior orbital fissure – transmits cranial nerve V, zygomatic nerve & blood vessels to maxillary and zygomatic regions of face

g. infraorbital foramen – infraorbital nerve & artery

3. Zygomatic bones (malar) – cheek bones and inferolateral margins of orbits

a. between temporal & maxillae

4. Nasal bones – bridge of nose

a. between frontals, maxillae, & perpendicular plate of ethmoid

5. Lacrimal bones – minor bones that form part of medial walls of orbits

a. lacrimal sulcus – transmits tears from eye into nasal cavity

6. Palatine bones – hard part of palate posteriorly; small part of orbit and a part of posterolateral wall of nasal cavity

7. Vomer – plow-shaped bone in nasal cavity; forms part of nasal septum (discussed more later)

8. Inferior nasal conchae – thin, curved bones from lateral wall of nasal cavity below the middle conchae of the ethmoid bone; largest of the 3 pairs of conchae

II. Orbits – eye sockets

A. sockets for eyeball & associated structures

B. formed by seven bones of skull: frontal & sphenoid (roof), zygomatic & sphenoid ( lateral wall), maxilla, zygomatic & palatine (floor), and maxilla, lacrimal, ethmoid, & sphenoid (medial wall)

B. principle openings: optic foramen (junction of roof & medial wall), superior orbital fissure, inferior orbital fissure, supraorbital foramen, canal for naso lacrimal duct (in nasal bone)

III. Hyoid Bone – single u shaped bone that is suspended from styloid process of temporal bone

A. only axial bone the is free ( doesn’t articulate directly with other bones)

B. supports the tongue & some of its muscles

C. provides attachment points for some muscles of neck & pharynx

D. composed of body & two pairs of paired projections, the lesser cornu & greater cornu

E. often breaks during strangulation murders

IV. Vertebral Column – spine

A. General

1. along with sternum and ribs forms the trunk of  body 

2. makes up 2/5 of body height

3. composed of series of bones (vertebrae) & intervertebral discs

4. 71 cm mean in males; 61 cm mean in females

5. functions

a. enclose and protect spinal cord

b. support head

c. points of attachments for ribs & muscles of back

d. outlets for nerves via intervertebral foramina

B. vertebral types – 33 prior to fusion of two groups

1. cervical vertebrae – (7) in neck region

2. thoracic vertebrae – 12 posterior to thoracic cavity

3. lumbar vertebrae – 5 in lower back

4. sacral vertebrae – 5 fused to form sacrum in adults

5. coccygeal vertebrae – 4 fused to form coccyx

C. intervertebral discs – between vertebrae

1. annulus fibrosus – outer fibrous ring of fibrocartilage

2. nucleus pulposus – inner soft pulp, highly elastic

3. functions

a. form strong joints

b. permit movements

c. absorb vertical shock

D. Normal curves – two convex & two concave (see figure in your textbook)

1. increase strength

2. fetus has only single anteriorly concave curve posteriorly convex

3. development of others

a. cervical curve – 3^rd postnatal month when infant begins to hold head erect (concave posteriorly)

b. lumbar curve – when child sits up, stands & walks (concave)

c. thoracic & sacral curves – primary curves retaining the fetal anterior concavity. (convex)

E. Typical Vertebra

1. body (centrum) – thick disc-shaped anterior portion

a. superior & inferior surfaces roughened foe disc attachment

b. anterior & lateral surfaces with nutrient foramina for blood vessels

2. vertebral (neural) arch – posterior from body

a. with body surrounds spinal cord

b. formed by two short, thick pedicles

c. pedicles meet laminae which are flat posterior

d. space between vertebral arch and body is vertebral foramen which together with other vertebral foramina form the vertebral (spinal) canal

e. superior and inferior notched help form intervertebral foramina

3. processes – arise from vertebral arch

a. transverse process – lateral on each side

b. spinous process – single, posterior to inferior projection

c. these first three form muscle attachment points

d. next four form joints

e. superior articular processes – articulate with next superior vertebra

f. inferior articular processes – articulate with next inferior vertebra

g. facets – articular surfaces

F. Vertebral characteristics (by region)

1. Cervical region

a. bodies small but vertebral arches large

b. transverse foramina in addition to vertebral foramen

* transmits vertebral artery and vein & nerves

c. spinous process often bifid

d. first cervical vertebra (C1) – atlas

* supports head, lacks body & is ring of bone

#anterior and posterior arches

#lateral masses

* superior articular facets – articulate with axis

* inferior lateral facets – articulate with axis

e. second cervical vertebra (C2) – axis

* dens – peg like process that projects up through the ring of the altus

# forms pivot for rotation of head

# whiplash injuries that are due from this process can be fatal (damage medulla)

f. C3 through C6 typical but C7 ( the vertebra prominens) has large, nonbifid spinous process (feel it at base of neck)

2. Thoracic Region – vertebrae are larger & stronger than in cervical region

a. spinous process is laterally flattened & pointed inferiorly

b. transverse processes longer & heavier

* except for T11 & T12, there are facets that articulate with tubercles of ribs

* facets or demifacets articulate with heads of ribs

# specifics vary among T vertebrae

3. Lumbar region – largest & strongest in vertebral column

a. projections are short & thick

b. inferior processes are directed laterally

c. spinous process is quadrilateral & nearly straight

4. Sacrum & Coccyx

a. sacrum (= holy bone) – formed from fusion of S1-S5

* begins 16 –18 years of age; completed by mid –20s

* posterior portion of the pelvic cavity between coxal bone

* transverse lines – anterior side is concave & has 4 lines that mark vertebral joinings

* anterior sacral (pelvic) foramina – at ends of transverse lines

* ala (=wing) – formed by transverse process of S1

* posterior surface

# median sacral crest – spinous processes

# lateral sacral crests – transverse processes

# posterior (dorsal) foramina – go to ventral

# sacral canal – continuous with vertebral canal

# sacral hiatus – open laminae of S5 and sometimes S4

# sacral promontory – superior border of sacrum

-- auricular surface – articulates with ilium

# sacral tuberosity – ligaments attachment to coxal (forms sacroiliac joint)

b. coccyx – triangular fusion of Co1-Co4

* fusion generally occurs between 20-30 years

V. Thorax – chest skeleton including sternum, costal cartilage, ribs, and the bodies of the thoracic vertebrae

A. Function – encloses and protects the organs in the thoracic cavity and upper abdominal; supports the bones of the girdle & upper appendages

B. Sternum – breastbone

1. flat, narrow bone ca. 15cm long

2. medial line of the anterior thoracic wall

3. three portions

a. manubrium – superior

b. body – middle and largest

c. xiphoid process – inferior & smallest

4. sternal angle – junction of manubrium and body

5. jugular (suprasternal) notch – depression on superior surface

6. clavicular notches – articulate with medial ends of clavicle   

C. Ribs – twelve pairs that make up sides of thoracic cavity

1. increase in length 1-7 then decrease

2. 1-7 articulate with cartilage (costal) to sternum

a. true ribs

3. 8-12 called false ribs

a. 8,9,& 10 – vertebrochondral ribs whose costal cartilage attaches to the 7^th rib

b. 11 & 12 – floating (vertebral) ribs have anterior ends that don’t even attach indirectly to sternum (only to thoracic vertebrae)

4. typical structure (3-9 at least)

a. head – projection at posterior end of rib

* has one or two facets

b. neck – a constricted portion just lateral to head

* tubercle – where neck joins head

# nonarticular part – ligament attachment

# articular part – articulates with the facet of the transverse process of the inferior of the 2 vertebrae to which it connects

c. body (shaft) – main part of the rib

* costal groove – inner surface groove that protects blood vessels

5. intercostal spaces – intercostal muscles, blood vessels & nerves

Reminder about Textbook Study

As with other topics, your textbook has excellent presentations of the materials on the axial skeleton of the skeletal system. While you should focus on the specific material in the Notes on the Web, you should always use your textbook as a resource for illustrations and for understanding content that your notes cover.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class.

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