Chapter 7: The Axial Skeleton – Study Notes

The Axial Skeleton

Overview and Divisions

The axial skeleton forms the longitudinal axis of the human body and is essential for supporting and protecting the brain, spinal cord, and organs in the thoracic and abdominal cavities. It also provides attachment points for muscles that move the head, neck, and trunk, and those involved in respiration.

• Axial skeleton: 80 bones, including the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Bones of the limbs and girdles (not covered in this chapter).

Bones of the Axial Skeleton

Major Components

• Skull: 8 cranial bones, 14 facial bones

• Associated bones: 6 auditory ossicles, 1 hyoid bone

• Thoracic cage: Sternum and 24 ribs

• Vertebral column: 24 vertebrae, sacrum, coccyx

The Skull

Cranial and Facial Bones

The skull consists of 22 bones: 8 cranial bones that form the cranium and 14 facial bones that protect and support the entrances to the digestive and respiratory tracts.

• Cranial bones: Surround and protect the brain.

• Facial bones: Form the structure of the face and support sensory organs.

Associated Bones

• Auditory ossicles: Three small bones in each ear (malleus, incus, stapes) involved in hearing.

• Hyoid bone: Supports the larynx (voice box) and is not directly attached to other bones.

Sutures of the Skull

Sutures are immovable joints that connect the bones of the skull. The four main sutures are:

• Lambdoid suture: Connects occipital to parietal bones

• Coronal suture: Connects frontal to parietal bones

• Sagittal suture: Between the parietal bones

• Squamous sutures: Attach temporal bones to parietal bones

Key Cranial Bones and Markings

• Occipital bone: Forms the posterior and inferior surfaces of the cranium; contains the foramen magnum for the spinal cord.

• Parietal bones: Form the superior and lateral surfaces of the cranium.

• Frontal bone: Forms the forehead and the superior part of the eye sockets.

• Temporal bones: House the structures of the ears and form part of the side and base of the skull.

• Sphenoid: Forms part of the floor of the cranium; contains the sella turcica for the pituitary gland.

• Ethmoid: Forms part of the nasal cavity and the nasal septum.

Facial Bones and Markings

• Maxillae: Support upper teeth and form part of the orbit and nasal cavity.

• Palatine bones: Form the posterior part of the hard palate.

• Nasal bones: Form the bridge of the nose.

• Vomer: Forms the inferior part of the nasal septum.

• Inferior nasal conchae: Create air turbulence in the nasal cavity.

• Zygomatic bones: Form the cheekbones.

• Lacrimal bones: Form part of the medial wall of the orbit.

• Mandible: Lower jawbone, supports lower teeth.

Orbital and Nasal Complexes

Orbital Complex

The orbits are bony sockets that protect the eyes. Each orbit is formed by seven bones: frontal, maxilla, palatine, lacrimal, ethmoid, sphenoid, and zygomatic.

Paranasal Sinuses

• Paranasal sinuses: Air-filled chambers that lighten the skull and provide resonance to the voice. Include frontal, ethmoidal, sphenoidal, and maxillary sinuses.

• Sinusitis: Inflammation of the sinuses, often due to infection.

Fontanelles

Infant Skull Features

Fontanelles are large areas of fibrous connective tissue in the infant skull that allow for rapid brain growth and flexibility during childbirth. These areas eventually ossify as the child grows.

The Vertebral Column

Structure and Functions

The vertebral column, or spine, consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx. It protects the spinal cord, supports the head and trunk, and transfers weight to the lower limbs.

Spinal Curves

• Primary curves (accommodation curves): Thoracic and sacral; present at birth and accommodate internal organs.

• Secondary curves (compensation curves): Cervical and lumbar; develop after birth to help maintain upright posture.

Abnormal Spinal Curvatures

• Scoliosis: Abnormal lateral curvature.

• Kyphosis: Excessive thoracic curvature (hyperkyphosis) or flattened thoracic curve (hypokyphosis).

• Lordosis: Excessive (hyperlordosis) or flattened (hypolordosis) cervical or lumbar curvature.

Vertebral Regions

• Cervical: 7 vertebrae (smallest, support head)

• Thoracic: 12 vertebrae (articulate with ribs)

• Lumbar: 5 vertebrae (largest, support most weight)

• Sacral: Sacrum (5 fused vertebrae)

• Coccygeal: Coccyx (3–5 fused vertebrae)

General Vertebral Anatomy

• Vertebral body: Transfers weight along the spine.

• Vertebral arch: Forms the posterior part of the vertebral foramen.

• Spinous and transverse processes: Sites for muscle and ligament attachment.

• Articular processes: Form joints with adjacent vertebrae.

• Intervertebral discs: Pads of fibrocartilage between vertebral bodies.

• Vertebral canal: Encloses the spinal cord.

The Thoracic Cage

Structure and Function

The thoracic cage consists of the thoracic vertebrae, ribs, costal cartilages, and sternum. It protects vital organs and provides attachment points for muscles involved in breathing and upper limb movement.

• Ribs: 12 pairs, divided into true ribs (1–7), false ribs (8–12), and floating ribs (11–12).

• Sternum: Flat bone at the front of the thoracic cage.

Summary Table: Major Bones of the Axial Skeleton

Region	Bones	Function
Skull	8 cranial, 14 facial, 6 auditory ossicles, 1 hyoid	Protects brain, supports sensory organs
Vertebral Column	24 vertebrae, sacrum, coccyx	Protects spinal cord, supports body
Thoracic Cage	24 ribs, sternum	Protects thoracic organs, aids respiration