Chapter 7 - Axial Skeleton

The Skeleton

• Consists of:

  • Bones, cartilage, joints, & ligaments

• Composed of 206 named bones grouped into 2 divisions

  • Axial skeleton (80 bones)

  • Appendicular skeleton (126 bones)

The Axial Skeleton

• Consists of skull, vertebral column, & thoracic cage (ribs & sternum)

• Forms the long axis of the body

• Helps support the head, neck, and trunk

The Skull

• Formed by cranial and facial bones

• The Cranium

  • Considered the body’s most complex bony structure

  • Serves to surround and protect the brain

  • Provides attachment for head & neck muscles

• Facial bones serve to

  • Form framework of the face

  • Form cavities for the sense organs of sight, taste, and smell

  • Provide openings for the passage of air & food 

  • Hold the teeth in place

  • Anchor muscles of the face

Cranial Bones

• Formed from eight large bones

  • Paired bones include

    • Temporal bones

    • Parietal bones

  • Unpaired bones include

    • Frontal bone

    • Occipital bone

    • Sphenoid bone

    • Ethmoid bone

Parietal Bones & Sutures

• Parietal bones form superior & lateral parts of the skull

• Sutures - interlocking, immovable joint that will unite the skull bones firmly together; rigid zig-zaggy appearance

• Major sutures of the cranium

  • Coronal suture - Articulates the parietal and frontal bones together

    • Runs in the coronal plane

  • Squamous suture - Articulates the parietal and temporal bones together

  • Lambdoid suture - Articulates the parietal and occipital bones together

  • Sagittal suture - Articulates the parietal bones together

    • Lies right on the midline

  • Occipitomastoid suture - Between the occipital bone and mastoid process

    • ONLY suture that doesn’t concern the parietal bone

Frontal Bone (unpaired)

• Forms the forehead and roofs of orbits

• Supraorbital margin - superior margin of orbits

• Glabella - smooth part of frontal bone between superciliary arches (structures deep to the eyebrows)

• Borders the frontal sinuses

  • Located near the nasal cavities

Occipital Bone (unpaired)

• Forms the posterior portion of the cranium & cranial base

• Part of the skull that forms walls holding the cerebellum

• Foramen magnum located at its base 

  • “Magnum” = meaning large

  • Marks the connection of the brain to the spinal cord

  • Flanked by a pair of structures called “occipital condyles”

Temporal Bones (paired)

• Lie inferior to parietal bones

• Specific regions of temporal bone

  • Squamous - plate shaped portion of the bone

  • Tymphanic - surrounding the external acoustic meatus 

    • Contains styloid process which attaches to stylohyoid ligament

  • Mastoid - contains the mastoid process, a lump right behind the ear

    • Site for neck attachment

    • Contains air sinuses

  • Petrous - harbor cavities that belong to the middle & inner ear

    • Can only be seen from the inferior aspect

Sphenoid bone (unpaired)

• “Keystone” cranial bone because it forms a central wedge that articulates w/ every other cranial bone

• Spans the width of the cranial floor

• Resembles a butterfly or bat w/ spread wings

• Consists of a body and three pairs of processes

  • Greater wings

  • Lesser wings

  • Pterygoid process

• Sella turcica - “turkish saddle”

  • Contains a depression on it called the “hypophyseal fossa” 

Ethmoid bone (unpaired)

• Bone that is most deeply situated into the skull

• Delicate, thin bone

• Lies between nasal and sphenoid bones

• Forms most of the medial bony region between the nasal cavity and orbits

• Cribriform plate - superior surface of the ethmoid bone 

  • Forms the roof of the nasal cavity

  • Contain olfactory foramina (tiny holes that olfactory nerves pass through)

• Crista galli - divides the cribriform plate into 2 sections

  • Attachment site for a fibrous membrane called the falx cerebri

• Perpendicular plate - forms superior part of nasal septum

  • Flanked by lateral masses (L and R lateral mass)

    • Filled w/ ethmoidal air cells (a.k.a ethmoid sinuses)

Facial Bones

• Unpaired

  • Mandible

  • Vomer

• Paired

  • Maxillae

  • Zygomatic bones

  • Nasal bones

  • Lacrimal bones

    • Tear drop shape

  • Palatine bones

  • Inferior nasal conchae

    • Shell shaped

Mandible (unpaired)

• ONLY bone of the skull that moves

• Lower jaw bone is largest and strongest facial bone

• Composed of 2 major parts

  • Horizontal body

  • Alveolar margin

    • Superior border of the horizontal body

    • Contain “alveoli” (tooth sockets) that secure the teeth in place

• Ramus of mandible

  • Meets the body at the mandibular angle

• Temporomandibular joint (TMJ) -  formed by the the articulation of the mandibular condyle to the temporal bone

Maxillary Bones (paired)

• Form the upper jaw

• Central portion of the facial skeleton

• Also contains an alveolar margin

  • Inferior border of the maxilla

    • Contain alveoli

• Contains maxillary sinuses

  • Make up ¼ of the paranasal sinuses

• Zygomatic process that meets w/ the zygomatic bone

• Palatine process

  • Forms the anterior part of the hard palate

  • Only visible from the anterior aspect

• Articulates w/ every other facial bone except for the mandible

Zygomatic bones (paired)

• Also known as the cheekbones

• Joins w/ 3 different zygomatic processes

1. Zygomatic process belonging to the temporal bone

2. Zygomatic process belonging to the frontal bone

3. Zygomatic process belonging to the maxilla

• Zygomatic arch is held together by articulating bones

Other Bones of the Face

• Nasal Bones (paired) 

  • Form the bridge of the nose

  • Very delicate

• Palatine bone (paired)

  • Horizontal plate of this bone forms the posterior aspect of the hard palate

  • Perpendicular plate forms part of the lateral wall of the nasal cavity

• Vomer (unpaired)

  • Forms the inferior region of the nasal septum

    • Completion of the septum is aided by septal (hyaline) cartilage

• Inferior nasal conchae (unpaired)

  • Projecting medially from the lateral portion of the nasal cavity

  • Superior to the hard palate

• Lacrimal bones (paired)

Nasal Cavity

• Superior to the hard palate

• Harbor sensory receptors that help in sense of smell

• Contains the following:

  • Roof (floor of the nasal cavity)

  • Floor (roof of the oral cavity)

  • Lateral walls - formed by the superior nasal concha and middle nasal concha

    • Contains canal-like passageways called “meatuses” in between, w/ purpose of increasing air turbulence

Paranasal Sinuses

• Air-filled sinuses located within

  • Frontal

  • Ethmoid

  • Sphenoid

  • Maxillary

• Lined w/ mucus membrane that warms and moistens inhaled air

• Help lighten the skull

The Hyoid Bone

• Connects to temporal bone via stylohyoid ligament

• Special because it is the only bone w/ no direct articulation with any other bone

  • Not considered part of the skull, but still associated with it

• Structures:

  • Centrally positioned body

  • Greater horns

  • Lesser horns (location of connection to stylohyoid ligament)

• Acts as a movable base for the tongue
  
  

The Vertebral Column (Spine)

• Formed from 26 bones in the adult

• Acts as the main support of the body axis

• Transmits weight of trunk to the lower limbs’

• Vertebral cavity harbors and protects the spinal cord

• Articulates w/ the ribs

• Get larger as you move down inferiorly (adjusts to the increasing weight in each section)

• Held in place by ligaments of dense regular CT

  • Anterior and posterior longitudinal ligaments - run straight, attach to the vertebral bodies 

  • Ligamentum flavum (a.k.a. adjacent vertebrae)

• 5 major regions

  • Cervical; C1-C7

  • Thoracic; T1-T12

  • Lumbar; L1-L5

  • Sacral; 5 fused vertebrae

  • Coccyx; (usually) 4 fused vertebrae

Intervertebral discs

• Cushion-like pads between vertebrae that absorb shock

• Found between all vertebrae EXCEPT between C1 and C2

  • Composed of

    • Nucleus pulposus forming the inner sphere 

    • Annulus fibrosus forming concentric rings that limit the expansion of nucleus pulposus

• Herniated disc - Nucleus pulposus squeezes out of its usual position and compresses spinal nerve fluid, causing excruciating pain

General Structure of Vertebrae

• Common structures include

  • Body

  • Vertebral arch

  • Vertebral foramen → forms vertebral canal w/ all 26 vertebrae

  • Spinous process

  • Transverse process 

  • Superior & inferior articular process 

  • Superior & inferior articular facet → on the surface of the process

Differentiating Between Vertebrae

• Cervical vertebrae - smallest and lightest 

  • C1 (atlas)

    • Only vertebrae that lacks a body and spinous process

    • Vertebral foramen is ring-shaped

    • Supports the skull

      • Superior articular facets receive the occipital condyles

    • Allows flexion & extension of neck

  • C2 (axis) 

    • Has a normal spinous process (not bifid)

    • Dens (odontoid process) projects superiorly through C1

      • Result of the body of the atlas fusing with the axis

      • Participates in side to side head movement (indicating no)

  • C3-C6 are typical cervical 

vertebrae

• Body is wider laterally

• Spinous processes are 

short and bifid

• Vertebral foramen are 

large and triangular

• Transverse processes 

contain transverse foramina

• C7

  • “Vertebra prominens” - very large & long spinous process (not bifid)

• Thoracic vertebrae

  • All articulate w/ ribs

  • Have heart-shaped bodies from the superior 

view

• Spinous processes are long and point 

inferiorly

• Vertebral foramen is circular

• Transverse processes will form the letter “W”

• Lumbar vertebrae

  • Bodies are very thick and robust

  • Transverse processes are thin/flat and 

tapered

• Spinous processes are thick, blunt, and point 

posterior

• “Hatchet” shaped

• Vertebral foramina are triangular

• Sacrum

  • Shapes the posterior wall of pelvis

  • Superior surface articulates w/ L5

  • Inferiorly articulates w/ coccyx

  • Sacral promontory (only visible from anterior aspect)

  • Sacral foramina - allow passage of sacral nerves

• Coccyx

  • The “tailbone”

  • Offers only slight support to pelvic organs

The Thoracic Cage

• Forms the framework of the chest

• Components

  • Thoracic vertebrae - posteriorly

  • Ribs - laterally

  • Sternum and costal cartilage - anteriorly

• Protects thoracic organs

• Supports shoulder girdle & upper limbs

• Attachment sites for skeletal muscles at the back, neck, shoulder, and chest

Sternum

• Shaped like a dagger

• Manubrium - handle of the “dagger”

  • Jugular notch - indentation at the superior portion of manubrium

  • Clavicular notch - indentation for association w/ the clavicle

  • Attached to rib #1

• Major portion is made up of the body

  • Contains notches to receive ribs 2-7

• Xiphoid process - inferiorly lying

  • Made up of costal cartilage

  • Does Not completely become bone until we're 40

The Ribs

• All ribs attach to vertebral column posteriorly

• True ribs (pairs 1-7)

  • “True” because they directly connect to the sternum via costal cartilage

• False ribs (pairs 8-12)

  • “False” because they are indirectly connected to the sternum, and 

• Floating ribs (pairs 11 & 12)

  • No anterior connection to the sternum AT ALL, literally floating