The Axial Skeleton

Overview and Divisions

The axial skeleton forms the longitudinal axis of the human body and is essential for support, protection, and movement. It consists of 80 bones, including the skull, vertebral column, and thoracic cage. The appendicular skeleton, by contrast, supports the limbs.

• Axial skeleton: Skull, associated bones, thoracic cage, vertebral column

• Appendicular skeleton: Limbs, shoulder, and pelvic girdles

Skull Anatomy

Cranial and Facial Bones

The skull is composed of 22 bones: 8 cranial bones that protect the brain and 14 facial bones that support the entrances to the digestive and respiratory tracts. Additionally, there are 7 associated bones (6 auditory ossicles and the hyoid bone).

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

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

• Associated bones: Hyoid, auditory ossicles

Major Sutures of the Skull

Sutures are immovable joints connecting skull bones. The four main sutures are lambdoid, coronal, sagittal, and squamous.

• Lambdoid suture: Occipital to parietal bones

• Coronal suture: Frontal to parietal bones

• Sagittal suture: Between parietal bones

• Squamous suture: Temporal to parietal bones

Bone Markings and Foramina

Bone markings include sinuses, foramina, and fissures, which serve as passageways for nerves and blood vessels or as attachment sites for muscles.

• Sinuses: Air-filled chambers that lighten bone and aid in speech

• Foramina: Openings for nerves and vessels (e.g., foramen magnum, supra-orbital foramen)

• Fissures: Narrow slits for nerves and vessels

Individual Cranial Bones

Occipital Bone

The occipital bone forms the posterior and inferior surfaces of the cranium. It articulates with the parietal, temporal, sphenoid bones, and the atlas (first cervical vertebra).

• External occipital protuberance: Midline bump

• Occipital condyles: Articulate with the atlas

• Foramen magnum: Passage for brain and spinal cord connection

Parietal Bone

Parietal bones form the superior and lateral surfaces of the cranium and provide attachment for the temporalis muscle.

• Superior and inferior temporal lines: Muscle attachment

Frontal Bone

The frontal bone forms the anterior cranium and the roof of the eye sockets. It contains the frontal sinuses and articulates with several other bones.

• Forehead: Anterior superior portion

• Supra-orbital margin: Protects the eyes

• Glabella: Most forward projection

• Supra-orbital foramen/notch: Passage for vessels and nerves

Temporal Bone

Temporal bones form parts of the lateral cranium and zygomatic arches, surround the internal ear, and articulate with the mandible.

• Squamous part: Borders squamous suture

• Zygomatic process: Forms zygomatic arch

• Mastoid process: Muscle attachment, contains air cells

• Styloid process: Ligament and muscle attachment

• Foramina: Carotid canal, external acoustic meatus, stylomastoid foramen

Sphenoid Bone

The sphenoid forms part of the cranial floor, unites cranial and facial bones, and contains sphenoidal sinuses.

• Sella turcica: Holds the pituitary gland

• Lesser and greater wings: Form cranial floor and orbit walls

• Pterygoid processes: Muscle attachment

• Foramina: Optic canal, foramen rotundum, foramen ovale, foramen spinosum

Ethmoid Bone

The ethmoid forms the anteromedial floor of the cranium, roof of the nasal cavity, and part of the nasal septum and medial orbital wall.

• Cribriform plate: Roof of nasal cavity

• Crista galli: Attachment for falx cerebri

• Ethmoidal labyrinths: Air-filled cavities

• Olfactory foramina: Passage for olfactory nerves

Facial Bones

Maxillae

Maxillae support the upper teeth, form the upper jaw and hard palate, and contain the largest sinuses.

• Orbital rim: Protects the eye

• Alveolar process: Supports upper teeth

• Palatine processes: Form most of the hard palate

• Infra-orbital foramen: Passage for nerves

Palatine Bones

Palatine bones form the posterior hard palate and contribute to the floors of the orbits.

• Horizontal plate: Posterior hard palate

• Perpendicular plate: Extends to orbital process

Nasal Bones, Vomer, Inferior Nasal Conchae, Zygomatic Bones, Lacrimal Bones

These bones support the nose, form the nasal septum, create air turbulence, form cheekbones, and contribute to the orbits.

• Nasal bones: Bridge of nose

• Vomer: Inferior nasal septum

• Inferior nasal conchae: Warm and humidify air

• Zygomatic bones: Cheekbones, lateral orbit walls

• Lacrimal bones: Medial orbit walls

Mandible

The mandible forms the lower jaw and articulates with the temporal bones. It supports the lower teeth and provides attachment for facial muscles.

• Body: Horizontal portion

• Ramus: Ascending part

• Condylar process: Articulates with temporal bone

• Coronoid process: Muscle attachment

• Mental foramen: Passage for nerves

Hyoid Bone

The hyoid bone supports the larynx and attaches muscles of the larynx, pharynx, and tongue. It does not articulate directly with other bones.

• Body: Muscle attachment

• Greater horns: Support larynx

• Lesser horns: Ligament attachment

Orbital and Nasal Complexes

Orbital Complex

The orbital complex consists of seven bones forming the bony sockets that protect the eyes.

• Frontal bone: Roof

• Maxilla and palatine bones: Floor

• Maxilla, lacrimal, ethmoid: Medial wall

• Sphenoid: Posterior wall

• Zygomatic bone: Lateral wall

Nasal Complex and Paranasal Sinuses

The nasal complex is formed by several bones enclosing the nasal cavities and paranasal sinuses, which are air-filled chambers connected to the nasal cavities.

• Frontal, sphenoid, ethmoid: Superior wall

• Maxillae, lacrimal, ethmoid, inferior nasal conchae: Lateral walls

• Maxillae, nasal bones: Bridge

• Paranasal sinuses: Frontal, ethmoidal, sphenoidal, maxillary

• Sinusitis: Inflammation of sinuses

Fontanelles and Skull Development

Infant Skull and Fontanelles

The infant skull has many ossification centers and areas of fibrous connective tissue called fontanelles, which allow for rapid brain growth and flexibility during childbirth.

• Fontanelles: Sphenoidal, mastoid, anterior, posterior

• Craniostenosis: Premature closure of fontanelles

• Microcephaly: Undersized skull due to halted brain development

Vertebral Column

Curves of the Vertebral Column

The vertebral column consists of 26 bones and supports the head, neck, and trunk. It has four curves: cervical, thoracic, lumbar, and sacral, which help maintain upright posture and accommodate internal organs.

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

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

Clinical Note: Spinal Disorders

Spinal disorders include kyphosis (excessive thoracic curvature), lordosis (excessive lumbar curvature), and scoliosis (lateral curvature).

• Kyphosis: Hump-backed, often due to osteoporosis

• Lordosis: Sway-back, can result from pregnancy or obesity

• Scoliosis: Crookedness, may be developmental or idiopathic

Vertebral Regions and Anatomy

The vertebral column is divided into five regions: cervical (7), thoracic (12), lumbar (5), sacral (sacrum), and coccygeal (coccyx). Each vertebra consists of a vertebral body, arch, and processes.

• Vertebral body: Transfers weight

• Vertebral arch: Pedicles and laminae

• Spinous and transverse processes: Muscle attachment

• Articular processes: Articulation with neighboring vertebrae

Thoracic Cage

Structure and Function

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

• True ribs: First seven pairs, attach directly to sternum

• False ribs: Pairs 8–12, do not attach directly

• Floating ribs: Pairs 11–12, no sternum attachment

• Sternum: Manubrium, body, xiphoid process

Rib Structure

Each rib has a head, neck, tubercle, and body. The head articulates with the vertebrae, and the body provides muscle attachment.

• Head: Articulates with vertebrae

• Tubercle: Articulates with transverse process

• Body: Shaft, muscle attachment

Sternum Anatomy

The sternum is a flat bone in the anterior thoracic wall, consisting of the manubrium, body, and xiphoid process. It provides attachment for the diaphragm and rectus abdominis muscle.

• Manubrium: Superior portion, articulates with clavicles and first rib pair

• Body: Attaches to manubrium and costal cartilages of ribs 2–7

• Xiphoid process: Inferior end, muscle attachment

Summary Table: Axial Skeleton Components

Additional info: This guide expands brief lecture points into full academic explanations, includes relevant images for anatomical clarity, and organizes content for exam preparation.