Anatomy & Physiology: The Skeleton (Axial Skeleton and Skull)

The Skeleton

Overview of the Skeletal System

The skeletal system is a complex framework composed of bones, cartilages, joints, and ligaments. It provides structural support, protection for internal organs, and facilitates movement through its interaction with muscles.

• Major Divisions:

  • Axial skeleton: Forms the central axis of the body.

  • Appendicular skeleton: Includes the limbs and girdles.

• Functions:

  • Support and protection of vital organs

  • Facilitation of movement

  • Mineral storage (e.g., calcium, phosphorus)

  • Blood cell production (hematopoiesis)

Axial Skeleton

The axial skeleton consists of 80 bones and is divided into three major regions:

• The Skull

• The Vertebral Column

• The Thoracic Cage

These regions collectively protect the brain, spinal cord, and thoracic organs.

The Skull

General Structure and Function

The skull is formed by two sets of bones: cranial bones and facial bones. It protects the brain and forms the structure of the face.

• Cranial bones (cranium):

  • Enclose the brain within the cranial cavity

  • Provide attachment sites for head and neck muscles

• Facial bones:

  • Form the framework of the face

  • Contain cavities for special sense organs (sight, taste, smell)

  • Provide openings for air and food passage

  • Secure teeth and anchor facial muscles for expression

• Sutures:

  • Immovable joints that firmly lock skull bones together (except for the mandible)

Divisions and Regions of the Skull

• Cranial Vault (Calvaria): Forms the superior, lateral, and posterior portions of the skull, including the forehead.

• Cranial Base: Forms the inferior aspect of the skull, divided into three fossae (anterior, middle, posterior) that cradle the brain.

• Cranial Cavity: The space enclosed by the cranial vault and base, housing the brain.

Cavities and Openings of the Skull

• Middle and Internal Ear Cavities

• Nasal Cavity

• Orbits (eye sockets)

• Sinuses

• Foramina, Canals, Fissures: Passageways for the spinal cord, blood vessels, and cranial nerves

Cranial Bones

List of Cranial Bones (8)

• Frontal bone

• Parietal bones (2)

• Occipital bone

• Temporal bones (2)

• Sphenoid bone

• Ethmoid bone

Frontal Bone

The frontal bone forms the anterior portion of the cranium and the forehead. It also forms the superior wall of the orbits and most of the anterior cranial fossa.

• Supraorbital margin: Area under the eyebrows

• Supraorbital foramen (notch): Passage for the supraorbital artery and nerve

• Glabella: Area between the orbits

• Frontal sinuses: Located just lateral to the glabella

Parietal Bones

The parietal bones form most of the superior and lateral aspects of the cranial vault. They articulate with other cranial bones at four major sutures:

• Coronal suture: Parietal and frontal bones

• Sagittal suture: Between right and left parietal bones

• Lambdoid suture: Parietal and occipital bones

• Squamous suture: Parietal and temporal bones

Occipital Bone

The occipital bone forms the posterior wall and base of the skull.

• Foramen magnum: Large opening for the spinal cord

• Occipital condyles: Articulate with the first vertebra (atlas)

• Hypoglossal canal: Passage for cranial nerve XII

• External occipital protuberance and crest: Attachment sites for ligaments and muscles

• Superior and inferior nuchal lines: Muscle attachment sites

Temporal Bones

The temporal bones are paired bones forming the inferolateral aspects of the skull and parts of the cranial base.

• Squamous part: Zygomatic process (forms zygomatic arch), mandibular fossa (temporomandibular joint)

• Tympanic part: Surrounds external acoustic meatus (ear canal)

• Petrous part: Houses middle and internal ear cavities, forms part of the middle cranial fossa

• Foramina: Jugular foramen, carotid canal, foramen lacerum, mastoid foramen

• Mastoid and styloid processes: Muscle attachment sites

Clinical Note: Mastoiditis

• Mastoid air cells in the mastoid process can become infected, potentially spreading to the brain due to thin bony separation.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone that articulates with all other cranial bones. It contains the sphenoidal sinuses and several important features:

• Sella turcica: Houses the pituitary gland

• Greater and lesser wings, pterygoid processes

• Optic canals, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum: Passageways for nerves and vessels

Ethmoid Bone

The ethmoid bone is the deepest skull bone, forming the roof of the nasal cavity and part of the anterior cranial fossa.

• Cribriform plates: Roof of nasal cavity, floor of anterior cranial fossa

• Crista galli: Attachment for dura mater

• Perpendicular plate: Superior part of nasal septum

• Ethmoidal labyrinths: Contain ethmoidal air cells (sinuses), form superior and middle nasal conchae

• Orbital plates: Contribute to medial wall of orbits

Sutural Bones

Sutural bones are tiny, irregular bones found within cranial sutures. Their significance is unknown and not everyone has them.

Facial Bones

List of Facial Bones (14)

• Mandible

• Maxillary bones (2)

• Zygomatic bones (2)

• Nasal bones (2)

• Lacrimal bones (2)

• Palatine bones (2)

• Vomer

• Inferior nasal conchae (2)

Mandible

The mandible is the largest and strongest facial bone, forming the lower jaw.

• Body: Contains alveolar process (tooth sockets) and mandibular symphysis (midline ridge)

• Rami: Upright portions

• Mandibular angle: Where rami and body meet

• Coronoid process: Muscle attachment

• Condylar process: Forms part of temporomandibular joint

• Mandibular and mental foramina: Passageways for nerves and blood vessels

Maxillary Bones

The maxillae are paired bones fused to form the upper jaw and central facial skeleton.

• Alveolar processes: Hold upper teeth

• Anterior nasal spine: Just below the nose

• Palatine process: Forms two-thirds of the hard palate

• Frontal process: Forms lateral bridge of nose

• Zygomatic processes: Articulate with zygomatic bones

• Maxillary sinuses: Flank the nasal cavity

• Openings: Inferior orbital fissure, infraorbital foramen, incisive fossa and canal

Zygomatic Bones

The zygomatic bones form the cheekbones and inferolateral margins of the orbits. They articulate with the zygomatic processes of the temporal, frontal, and maxillary bones.

Nasal Bones

The nasal bones form the bridge of the nose and articulate with the frontal, maxillary, and ethmoid bones. They attach to cartilage that forms the tip of the nose.

Lacrimal Bones

The lacrimal bones form the medial walls of the orbits and articulate with the frontal, maxillary, and ethmoid bones. The lacrimal fossa houses the lacrimal sac, allowing passage for tears to drain.

Palatine Bones

The palatine bones are L-shaped bones forming the posterior part of the hard palate and part of the nasal cavity and orbit.

• Horizontal plate: Posterior one-third of hard palate

• Perpendicular plate: Part of posterolateral walls of nasal cavity and small part of orbit

Vomer

The vomer is a plow-shaped bone forming part of the nasal septum.

Inferior Nasal Conchae

The inferior nasal conchae are paired bones forming part of the lateral walls of the nasal cavity. They are the largest of the three pairs of conchae; the other two pairs are formed by the ethmoid bone.

Special Bones and Cavities

Hyoid Bone

The hyoid bone is located in the anterior neck, inferior to the mandible. It is unique as it does not articulate directly with any other bone.

• Anchored by ligaments

• Serves as a movable base for the tongue

• Attachment site for muscles involved in swallowing and speech

Orbits

The orbits are cavities that encase the eyes and lacrimal glands, and serve as attachment sites for eye muscles. They are formed by parts of seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid.

Nasal Cavity

The nasal cavity is formed by several bones:

• Roof: Cribriform plates of ethmoid

• Lateral walls: Superior and middle conchae of ethmoid, perpendicular plates of palatine, inferior nasal conchae

• Floor: Processes of palatine and maxillary bones

• Nasal septum: Posterior part formed by vomer and perpendicular plate of ethmoid; anterior part by septal cartilage

Paranasal Sinuses

Paranasal sinuses are mucosa-lined, air-filled spaces in five skull bones: frontal, sphenoid, ethmoid, and paired maxillary bones.

• Warm and humidify air

• Lighten the skull

• Enhance resonance of the voice

Summary Table: Major Bones of the Skull

Bone	Type	Main Features
Frontal	Cranial	Forehead, superior orbit, frontal sinuses
Parietal (2)	Cranial	Superior/lateral vault, sutures
Occipital	Cranial	Posterior base, foramen magnum, condyles
Temporal (2)	Cranial	Inferolateral base, ear structures, mastoid process
Sphenoid	Cranial	Keystone, sella turcica, sinuses
Ethmoid	Cranial	Deepest, cribriform plate, conchae
Mandible	Facial	Lower jaw, alveolar process, rami
Maxilla (2)	Facial	Upper jaw, hard palate, sinuses
Zygomatic (2)	Facial	Cheekbones, orbit margin
Nasal (2)	Facial	Bridge of nose
Lacrimal (2)	Facial	Medial orbit, lacrimal fossa
Palatine (2)	Facial	Hard palate, nasal cavity
Vomer	Facial	Nasal septum
Inferior Nasal Conchae (2)	Facial	Lateral nasal cavity

Key Terms and Definitions

• Suture: Immovable joint between skull bones

• Foramen: Opening for nerves and blood vessels

• Process: Projection for muscle or ligament attachment

• Sinus: Air-filled cavity within a bone

Clinical Application

• Understanding the anatomy of the skeleton is essential for anticipating clinical problems, such as pelvic dimensions affecting labor and delivery.

Additional info: The notes above expand on the brief points in the slides, providing full academic context and definitions for key terms, as well as a summary table for exam review.