Axial Skeleton Overview

Definition and Divisions

The axial skeleton forms the central axis of the human body, providing support and protection for the brain, spinal cord, and thoracic organs. It consists of 80 bones and is one of two major divisions of the skeletal system, the other being the appendicular skeleton.

• Axial skeleton: 80 bones (skull, vertebral column, thoracic cage)

• Appendicular skeleton: 126 bones (limbs and girdles)

• Includes bones, cartilage, joints, and ligaments

The Skull

Structure and Function

The skull is composed of cranial and facial bones. It protects the brain, provides attachment sites for muscles, and forms the framework of the face.

• Cranial bones: Protect the brain and provide muscle attachment for the head and neck

• Facial bones: Form the structure of the face, house sense organs, and provide passageways for food and air

• Sutures: Immovable joints between skull bones

Cranial and Facial Bones

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

• Facial bones: Maxilla (paired), zygomatic (paired), nasal (paired), mandible, lacrimal (paired), palatine (paired), inferior nasal concha (paired), vomer

Functions of Cranial Sutures

• Sutures: Fibrous joints connecting cranial bones; allow for growth during childhood

• Major sutures: Coronal, sagittal, lambdoid, squamous

• Help absorb mechanical stress and prevent fractures

Skull Cavities and Openings

• Cranial cavity: Houses the brain

• Other cavities: Orbits (eyes), nasal cavity, oral cavity, paranasal sinuses

• Foramina and canals: Allow passage of nerves and blood vessels (e.g., foramen magnum for spinal cord)

Bones Forming the Orbit

The orbit is a bony cavity that houses the eye and associated structures. Seven bones contribute to its formation:

• Frontal

• Sphenoid

• Zygomatic

• Maxilla

• Palatine

• Lacrimal

• Ethmoid

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within certain skull bones, lined by mucous membranes.

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

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

Cleft Palate

A cleft palate occurs when the right and left palatine processes of the maxilla fail to fuse during development.

• Results in an opening between the oral and nasal cavities

• Can lead to feeding difficulties and increased risk of respiratory infections

• Treated surgically; prenatal vitamins (folic acid) may reduce risk

Vertebral Column

Structure and Regions

The vertebral column supports the trunk, protects the spinal cord, and provides attachment for muscles.

• Composed of 26 bones: 24 vertebrae, sacrum, coccyx

• Regions:

  • Cervical (7 vertebrae)

  • Thoracic (12 vertebrae)

  • Lumbar (5 vertebrae)

  • Sacrum (5 fused vertebrae)

  • Coccyx (3-5 fused vertebrae)

General Structure of Vertebrae

• Body: Weight-bearing region

• Vertebral arch: Formed by pedicles and laminae

• Processes: Spinous, transverse, articular

• Vertebral foramen: Passage for spinal cord

Regional Differences in Vertebrae

• Cervical: Small, light, transverse foramina, bifid spinous process (C3-C6)

• Thoracic: Larger, heart-shaped body, long spinous process, costal facets for rib articulation

• Lumbar: Largest, thick body, short spinous process, triangular vertebral foramen

Special Cervical Vertebrae

• C1 (Atlas): No body or spinous process; articulates with occipital condyles for nodding motion (atlanto-occipital joint)

• C2 (Axis): Has odontoid process (dens) for rotation (atlanto-axial joint)

Intervertebral Discs

• Annulus fibrosus: Outer ring of fibrocartilage

• Nucleus pulposus: Inner gel-like core

• Functions: Absorb shock, allow movement, distribute weight

Spinal Curvatures

• Primary curvatures: Thoracic and sacral (present at birth)

• Secondary curvatures: Cervical and lumbar (develop after birth)

• Functions: Balance, flexibility, shock absorption

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature

• Kyphosis: Exaggerated thoracic curvature

• Lordosis: Exaggerated lumbar curvature

Thoracic Cage

Structure and Function

The thoracic cage protects thoracic organs and supports the shoulder girdle and upper limbs.

• Composed of sternum, ribs, and thoracic vertebrae

• Attachment sites for muscles of the back, chest, and shoulders

Sternum

• Manubrium: Superior portion; articulates with clavicles and first ribs

• Body: Middle portion; articulates with ribs 2-7

• Xiphoid process: Inferior tip

Ribs

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

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

• Floating ribs (11-12): No anterior attachment

Fontanelles

Definition and Function

Fontanelles are soft spots on a baby's skull where bones have not yet fused. They allow for growth and facilitate passage through the birth canal.

• Anterior fontanelle closes by 1.5-2 years

• Posterior fontanelle closes by 1 year

• Permit brain growth and molding during birth

Summary Table: Major Bones of the Axial Skeleton

Key Equations

• Number of bones in adult skeleton: $206$

• Axial skeleton: $80$ bones

• Appendicular skeleton: $126$ bones

Additional info: Some details, such as the specific names of all cranial and facial bones, the functions of fontanelles, and the clinical significance of cleft palate, were expanded for academic completeness.