Comprehensive Notes on Axial Skeleton

Axial Skeleton

• Composed of 80 bones including the bones of the head and trunk.
• Forms the longitudinal axis of the body, also known as the midline.
• Main parts:
  • Skull and associated bones (auditory ossicles and hyoid bone).
  • Thoracic (rib) cage.
  • Vertebral column (spine).
• Main roles:
  • Protects vital organs.
  • Maintains posture.
• Foramen: natural hole in the body.

Skull

• Total of 22 bones.
  • 8 Cranial bones.
    • Frontal bone: 1
    • Parietal bone: 2
    • Occipital bone: 1
    • Sphenoid bone: 1
    • Ethmoid bone: 1
    • Temporal bone: 2
  • 14 Facial bones.
    • Maxillae: 2
    • Palatine bones: 2
    • Nasal bones: 2
    • Inferior nasal conchae: 2
    • Zygomatic bones (cheekbones): 2
    • Lacrimal bones: 2
    • Vomer: 1
    • Mandible: 1
• Associated with the skull:
  • 6 auditory ossicles (incus, malleus, stapes - 3 in each side) and hyoid bone.

Thoracic Cage

• Sternum and 24 ribs.
  • Breastbone: manubrium, body, xiphoid process.
  • Ribs: 1-7 true, 8-12 false.

Vertebral Column

• 24 vertebrae, sacrum, and coccyx.

Functions of the Axial Skeleton

• Protects organs (visceral) and cavities (parietal).
• Maintains posture (3 axes → joints (articulations)).

Cranial Bones

• 8 cranial bones form the cranium:
  • Frontal bone
  • Parietal bones (2)
  • Temporal bones (2)
  • Occipital bone
  • Sphenoid bone
  • Ethmoid bone

Facial Bones

• 14 facial bones:
• Nine superficial bones (for muscle attachment facilitating facial expressions, chewing (mastication), and jaw movement):
  • Maxillae (2)
  • Lacrimal (2)
  • Nasal (2)
  • Zygomatic (2)
  • Mandible (1)
• Five deeper facial bones (help separate oral and nasal cavities and form the nasal septum for easier breathing):
  • Palatine (2)
  • Inferior nasal conchae (2)
  • Vomer (1)
• Septoplasty: surgery to correct a deviated septum.

Sphenoid Bone

• Shape: Butterfly or bat-shaped.
• Role: Connects several bones and protects the pituitary gland and optic nerve.

Sutures

• Immovable joints of the skull (synarthrosis).
• Four major sutures (with wormian bones):
  • Lambdoid suture: Connects occipital and parietal bones at the back of the skull.
  • Coronal suture: Connects the frontal and parietal bones on top of the skull.
  • Sagittal suture: Connects the two parietal bones along the midline.
  • Squamous sutures: Connects the temporal and parietal bones on the sides of the skull.

Fontanelles

• Also known as "soft spots".

Sinuses

• Also known as paranasal sinuses.
• Four major types:
  • Frontal
  • Ethmoidal
  • Sphenoidal
  • Maxillary
• Air-filled chambers in the skull that warm air, trap allergens/pathogens, and serve as resonating chambers for speech.

Cranial Bones Functions

• Occipital bone: Protects the back/base of the brain.
• Parietal bone: Forms the sides and roof of the skull.
• Frontal bone: Shapes the forehead and protects the eyes and other facial structures.
• Temporal bone: Houses hearing and balance structures (incus, malleus, stapes).
• Ethmoid bone: Cuboidal shaped, associated with the sense of smell (olfaction), and separates the nasal cavity from the brain.

Foramen Magnum

• Located at the base of the skull, connects the spinal cord to the brain.
• A foramen is a natural hole.

Auditory Ossicles

• Three tiny bones in the tympanic cavity (middle ear) – incus (anvil), malleus (hammer), and stapes (stirrup).
• Six total.
• Main role: Transfer sound waves from the eardrum to the inner ear (amplification).
• Three parts of the ear: external, middle, and inner ear.

Sphenoid Bone Details

• Bat or butterfly-shaped.
• Sella turcica: Part of the sphenoid bone where the pituitary gland is located at the base of the brain.

Ethmoid Bone Details

• Cubed-shaped.
• Contains olfactory nerves (smell) and protects the eyes.

Facial Bones – Functions

• Maxillae: Upper jaw, forms the hard palate and part of the eye orbits.
• Palatine bones: Form part of the hard palate and nasal cavity.
• Nasal bones: Bridge of the nose.
• Vomer: Nasal septum that divides the nasal cavity into equal left and right sides.
• Inferior nasal conchae: Filter and warm air.
• Zygomatic bones: Cheekbones and part of the eye sockets.
• Lacrimal bones: Hold tear glands and form the medial wall of eye sockets.
• Mandible: Lower jaw.
• Hyoid bone: Supports the tongue (swallowing (deglutition) and speaking (articulation)).

Orbits (Eye Sockets)

• Seven bones form each orbit:
  • Frontal bone
  • Maxilla
  • Zygomatic bone
  • Lacrimal bone
  • Ethmoid bone
  • Sphenoid bone
  • Palatine bone

Paranasal Sinuses

• Para = above, nasal = nose.
• Air-filled chambers connected to nasal cavities.
• Lighten skull bones.
• Contain mucous epithelium which releases mucus into nasal cavities.
• Clean air (pathogens/allergens), affect tone and voice.
  • Frontal sinus: Frontal bone (anterior).
  • Ethmoidal cells: Ethmoid bone.
  • Sphenoidal sinus: Sphenoid bone.
  • Maxillary sinus: Maxillae.

Fontanelles Details

• Large areas of fibrous connective tissue that cover unfused sutures in the infant skull.
• Allow the skull to flex during birth (soft spots).
• Facilitate childbirth and accommodate brain growth.
• Four types:
  • Anterior: Largest, closes at 18-24 months after birth.
  • Posterior: Closes at 2-3 months after birth.
  • Sphenoidal: Anterolateral fontanelle.
  • Mastoid: Posterolateral fontanelle.

Vertebral Column Details

• Protects the spinal cord (signals to/from brain and rest of the body).
• Supports head and body (posture, twisting, bending).
• 26 bones: 24 vertebrae (unfused), the sacrum and coccyx (fused).
• Vertebrae:
  • Cervical (neck): C1 (atlas), C2 (axis).
  • Thoracic (chest).
  • Lumbar (lower back).
• Sacrum: Forms the posterior part of the pelvis and connects the spine to the pelvic bones.
• Coccyx: Shock absorption and weight distribution.

Abnormal Curvatures of spine

• Scoliosis (s shape)
• Hypnosis (hunchback)
• Lordosis ( swayback )

Vertebrae Types

• Cervical vertebrae:
  • Foramen: Largest foramen (movement and where spinal cord and brain meet).
  • Body: Smallest.
• Thoracic vertebrae:
  • Foramen: Circular.
  • Body: Heart-shaped.
• Lumbar vertebrae:
  • Foramen: Triangular.
  • Body: Biggest (supports body weight).

Thoracic Cage Details

• Also known as ribs, it forms the skeleton of the chest.
• Consists of:
  • Thoracic vertebrae: 12 total.
  • Ribs: 12 pairs (true 1-7, false 8-10, floating 11-12).
  • Costal cartilages (made of hyaline cartilage).
  • Sternum: Flat bone with 3 parts – the body, the xiphoid process, and manubrium.
• Ribs:
  • 12 pairs of long curved flat bones: true ribs (1-7), false ribs (8-10), floating ribs (11-12).
  • Directly and indirectly attached to sternum = 10 pairs; 2 pairs not attached to sternum.
  • cost/o = ribs, costal = ribs, thorac = ribs

Sternum Details

• Manubrium: Uppermost part that articulates with clavicles and first ribs.
• Body: Largest part that attaches clavicles and first ribs.
• Xiphoid process: Smallest and inferior part that attaches abdominal muscles and helps with respiration.