Axial Skeleton Overview

Definition and Components

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 the skull, vertebral column (including sacrum and coccyx), and the thoracic cage (ribs and sternum).

• Axial skeleton: Skull, vertebral column, thoracic cage

• Appendicular skeleton: Upper and lower limb bones

Classification of Bones by Shape

Types of Bones

Bones are classified based on their shapes, which relate to their functions and locations in the body.

• Long bones: Longer than wide, with a shaft and two ends (e.g., humerus)

• Short bones: Cube-shaped, found in wrist and ankle

• Sesamoid bones: Develop within tendons (e.g., patella)

• Flat bones: Thin, flattened, and curved (e.g., sternum, skull bones)

• Irregular bones: Complex shapes (e.g., vertebrae, hip bones)

• Sutural bones: Small bones within skull sutures (Wormian bones)

Skull Structure

Major Regions and Bones

The skull is composed of cranial and facial bones. The cranial bones protect the brain, while the facial bones form the structure of the face.

• Cranial bones (8): Occipital, frontal, sphenoid, ethmoid, 2 parietal, 2 temporal

• Facial bones (14): Maxilla (2), lacrimal (2), nasal (2), zygomatic (2), mandible, palatine (2), inferior nasal conchae (2), vomer

Sutures are immovable joints connecting cranial bones:

• Sagittal suture

• Lambdoid suture

• Squamous suture

• Coronal suture

Anterior View of the Skull

The anterior view displays the bones forming the forehead, orbits, nasal cavity, nasal septum, and jaws.

Key Skull Features

• Hard palate: Formed by maxillary and palatine bones

• TMJ (temporomandibular joint): Only movable joint of the face (mandible and temporal bone)

• Teeth: Anchored in mandible and maxillary bones

Specialized Bones and Structures

• Sphenoid bone: Forms part of cranial floor, contains sella turcica for pituitary gland, has multiple foramina for nerves and vessels

• Ethmoid bone: Forms part of nasal septum, nasal cavity, and medial orbit wall; contains crista galli, perpendicular plate, cribriform plates, and ethmoid air cells

• Nasal conchae: Superior and middle (ethmoid), inferior (independent); increase surface area and direct airflow in nasal cavity

• Hyoid bone: Does not articulate with other bones; supports tongue and larynx muscles

Sinuses

Paranasal Sinuses

Sinuses are air-filled cavities within certain skull bones. They lighten the skull, add resonance to the voice, and drain into the nasal cavity.

• Frontal sinus: Pain in forehead if infected

• Ethmoid sinus: Pain between eyes if infected

• Maxillary sinus: Pain in upper jaw if infected

• Sphenoid sinus: Pain in back of eye if infected

Vertebral Column

Structure and Regions

The vertebral column supports the body, protects the spinal cord, and allows flexible movement. It consists of 24 vertebrae, the sacrum, and the coccyx.

• Cervical vertebrae (C1–C7): Neck region

• Thoracic vertebrae (T1–T12): Upper back, articulate with ribs

• Lumbar vertebrae (L1–L5): Lower back, largest vertebrae

• Sacrum: Fusion of 5 sacral vertebrae

• Coccyx: Fusion of 4 small coccygeal vertebrae

  

  Curvatures of the Spine

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

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

Abnormal curvatures:

• Scoliosis: Lateral bending

• Kyphosis: Excessive thoracic curvature

• Lordosis: Excessive lumbar curvature

Structure of a Typical Vertebra

• Body: Weight-bearing portion

• Vertebral arch: Formed by pedicles and laminae

• Processes: Transverse, spinous, superior and inferior articular

• Vertebral foramen: Passage for spinal cord

• Intervertebral foramen: Passage for spinal nerves

• Intervertebral discs: Fibrocartilage pads (anulus fibrosus and nucleus pulposus) between vertebral bodies

Special Vertebrae

• Atlas (C1): Supports skull, allows "yes" motion

• Axis (C2): Has dens, allows "no" motion

• Atlas (C1) and axis (C2): "No" movement

• L5 and sacrum, sacrum and coccyx, sacrum and ilium: Support and transfer of body weight

Thoracic Cage

The thoracic cage protects thoracic organs and supports respiration. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Manubrium, body, xiphoid process

• Ribs (12 pairs):

  • True ribs (1–7): Directly attached to sternum

  • False ribs (8–12): Indirect or no attachment to sternum

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

Newborn Skull

At birth, the skull bones are not fully ossified and are separated by fontanelles (fibrous connective tissue areas) that allow for brain growth and skull expansion.

• Fontanelles: Soft spots on infant skull

• Mastoid process: Not yet formed at birth