The Axial Skeleton: Structure and Components

The Axial Skeleton

Overview of the Axial Skeleton

• The axial skeleton forms the central axis of the human body and consists of 80 bones.

• It is divided into three major regions:

  • Skull

  • Vertebral column

  • Thoracic cage

• Functions include protection of vital organs (brain, spinal cord, thoracic organs), support, and attachment for muscles.

The Skull

Number of Bones in the Adult Human Body

• The adult human skeleton contains 206 bones.

• The skull is composed of 22 bones: 8 cranial and 14 facial bones.

Components of the Skull

• Cranial bones (8): Enclose and protect the brain.

• Facial bones (14): Form the framework of the face, contain cavities for special sense organs, and provide openings for air and food passage.

Cranial Fossae

• The cranial floor is divided into three fossae:

  • Anterior cranial fossa

  • Middle cranial fossa

  • Posterior cranial fossa

• These fossae support the brain's lobes.

Major Cavities of the Skull

• Cranial cavity: Houses the brain.

• Orbits: Contain the eyes.

• Nasal cavity: Passage for air and olfaction.

• Oral cavity: Mouth region.

• Paranasal sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

Paranasal Sinuses

• Located in the frontal, sphenoid, ethmoid, and maxillary bones.

• Functions: Lighten the skull, warm and moisten air, and enhance resonance of the voice.

Foramina vs. Fissures

• Foramina: Round or oval openings for nerves and blood vessels.

• Fissures: Narrow, slit-like openings for passage of nerves and vessels.

Cranial Sutures

• Immovable joints connecting skull bones.

• Major sutures: Coronal, sagittal, lambdoid, and squamous.

Fontanelles

• Soft spots in the fetal and infant skull where bones have not yet fused.

• Allow for brain growth and skull compression during birth.

The Hyoid Bone

• Located in the anterior neck, not directly attached to other bones.

• Serves as an attachment for tongue and neck muscles.

Locate and Name All Bones of the Skull

• Cranial bones: Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid.

• Facial bones: Nasal (2), maxillae (2), zygomatic (2), mandible, lacrimal (2), palatine (2), inferior nasal conchae (2), vomer.

Foramina of the Skull

• Supraorbital foramen: Passage for supraorbital nerve and vessels above the orbit.

• Optic canals (sphenoid): Passage for optic nerves.

• Foramen magnum (occipital): Large opening for spinal cord.

• Infraorbital foramen (maxilla): Passage for infraorbital nerve and vessels.

• Incisive fossa/canal (maxilla): Passage for nasopalatine nerves and blood vessels.

Zygomatic Arch and Bones Involved

• Formed by the zygomatic bone and the zygomatic process of the temporal bone.

• Provides the prominence of the cheek.

Mandible

• Coronoid process: Insertion for temporalis muscle.

• Condylar process: Articulates with temporal bone to form temporomandibular joint.

• Body: Horizontal portion holding lower teeth.

• Foramina: Mental foramen (anterior), mandibular foramen (medial surface).

Maxillary Bones (Maxillae)

• Alveolar processes: Hold upper teeth.

• Anterior nasal spine: Bony projection at the base of the nasal cavity.

• Palatine process: Forms anterior hard palate.

• Frontal process: Articulates with frontal bone.

• Zygomatic processes: Articulate with zygomatic bones.

• Maxillary sinuses: Largest paranasal sinuses.

Zygomatic Bones

• Articulate with:

  1. Frontal bone

  2. Maxilla

  3. Temporal bone

Nasal Bones

• Form the bridge of the nose; small, rectangular bones.

Lacrimal Bones

• Located in the medial walls of orbits.

• Lacrimal fossa: Houses the lacrimal sac for tear drainage.

Palatine Bones

• Horizontal plate: Forms posterior part of hard palate.

• Perpendicular plate: Forms part of nasal cavity and orbit.

Inferior Nasal Conchae

• Located in the lateral walls of the nasal cavity.

• Increase surface area for warming and humidifying air.

Regions of the Nasal Cavity

• Roof: Formed by ethmoid bone.

• Floor: Formed by palatine processes of maxillae and palatine bones.

• Lateral walls: Formed by maxillae, inferior nasal conchae, ethmoid, and palatine bones.

• Nasal septum: Formed by perpendicular plate of ethmoid, vomer, and septal cartilage.

Major Bones Forming the Nasal Septum

• Perpendicular plate of ethmoid bone

• Vomer

• Septal cartilage (not a bone, but important structurally)

Orbits Are Formed By

• Frontal bone

• Sphenoid bone

• Zygomatic bone

• Maxilla

• Palatine bone

• Lacrimal bone

• Ethmoid bone

Ethmoid Bone

• Cribriform plates: Form roof of nasal cavity; contain olfactory foramina.

• Crista galli: Attachment for brain's dura mater.

• Perpendicular plate: Forms superior part of nasal septum.

• Orbital plates: Form part of medial orbit walls.

Sphenoid Bone

• Greater wings: Form part of cranial floor and orbits.

• Lesser wings: Form part of anterior cranial fossa.

• Pterygoid processes: Attachment for chewing muscles.

• Sella turcica: Houses the pituitary gland.

• Optic canals: Passage for optic nerves.

Temporal Bones

• Three major regions:

  1. Squamous part

  2. Tympanic part

  3. Mastoid part

Occipital Bone

• Also known as "C0" because it articulates with the first cervical vertebra (atlas).

• Foramen magnum: Passage for spinal cord.

• External occipital crest: Midline ridge for ligament attachment.

• Superior and inferior nuchal lines: Sites for muscle attachment.

The Vertebral Column

General Characteristics

• Composed of 26 irregular bones (vertebrae) in adults.

• Functions: Protects spinal cord, supports head and trunk, provides attachment for ribs and muscles.

Regions and Curvatures

• Five regions:

  1. Cervical (7 vertebrae)

  2. Thoracic (12 vertebrae)

  3. Lumbar (5 vertebrae)

  4. Sacrum (5 fused vertebrae)

  5. Coccyx (4 fused vertebrae)

• Curvatures: Cervical and lumbar (concave posteriorly), thoracic and sacral (convex posteriorly).

Ligaments of the Vertebral Column

• Anterior and posterior longitudinal ligaments: Run along the length of the vertebral column; prevent hyperextension and hyperflexion.

• Short ligaments: Connect adjacent vertebrae.

Intervertebral Discs

• Composed of fibrocartilage.

• Act as shock absorbers and allow flexibility.

General Structure of Vertebrae

• Body: Weight-bearing region.

• Vertebral arch: Encloses the vertebral foramen.

• Vertebral foramen: Canal for spinal cord.

• Vertebral canal: Successive vertebral foramina form the canal.

• Intervertebral foramina: Openings for spinal nerves.

Vertebral Processes

• Functions: Muscle attachment, articulation with ribs and other vertebrae.

• Names and locations:

  • Spinous process: Posterior projection.

  • Transverse processes (2): Lateral projections.

  • Superior and inferior articular processes (2 each): Form facet joints.

Facet Joints

• Formed by articulation of superior and inferior articular processes.

• Orientation varies by region, affecting movement.

Cervical Vertebrae

• Smallest and lightest vertebrae.

• C1 (atlas): No body or spinous process; supports the skull.

• C2 (axis): Has odontoid process (dens) for rotation of the head.

• Vertebra prominens (C7): Has a prominent spinous process.

Thoracic Vertebrae

• Articulate with ribs.

• Have demifacets for rib articulation.

Lumbar Vertebrae

• Largest vertebrae; bear most weight.

Sacrum

• Formed by fusion of five vertebrae.

• Superior articular process: Articulates with L5.

• Auricular surfaces: Articulate with hip bones.

• Sacral foramina: Passage for nerves and vessels.

• Alae: Wing-like projections.

• Sacral canal: Continuation of vertebral canal.

• Sacral hiatus: Inferior opening of sacral canal.

Coccyx

• Formed by fusion of 3-5 small vertebrae.

• Remnant of a tail; provides attachment for ligaments and muscles.

Thoracic Cage

Functions

• Protects heart, lungs, and major blood vessels.

• Supports shoulder girdles and upper limbs.

• Attachment for muscles of the neck, back, chest, and shoulders.

Sternum

• Flat bone in the anterior midline of the thorax.

• Parts:

  • Manubrium

  • Body

  • Xiphoid process

Ribs

• 12 pairs of ribs:

  • True ribs (1-7): Attach directly to sternum via costal cartilages.

  • False ribs (8-12): Attach indirectly or not at all to sternum.

Main Parts of a Rib

• Shaft: Main body of the rib.

• Head: Articulates with vertebral bodies.

• Tubercle: Articulates with transverse process of vertebra.

Summary Table: Major Bones and Features of the Axial Skeleton

Region	Major Bones	Key Features
Skull	Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid, Maxilla, Mandible, Zygomatic, Nasal, Lacrimal, Palatine, Inferior Nasal Conchae, Vomer	Cranial fossae, sutures, foramina, sinuses, orbits, nasal cavity
Vertebral Column	Cervical, Thoracic, Lumbar vertebrae, Sacrum, Coccyx	Curvatures, intervertebral discs, processes, facet joints
Thoracic Cage	Sternum, Ribs	Manubrium, body, xiphoid process, true/false ribs

Additional info: Academic context and definitions have been added for clarity and completeness. This guide covers all major features and bones of the axial skeleton as outlined in the provided materials.