Copy and Return to Teacher

Copy and Return to Teacher

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

·  Transmits cranial nerves 7 & 8 (facial & vestibulocochlear)

§  Carotid canal - anterior to jugular foramen

·  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  Primary curvatures

·  Thoracic & sacral regions

·  Present at birth

o  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 hatchet-shaped 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!! J