Skull Anatomy: Structure, Features, and Cranial Nerves

Skull Anatomy Overview

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton includes the skull, vertebral column, and rib cage, providing protection for vital organs and serving as attachment points for muscles. The appendicular skeleton consists of the limbs and girdles, facilitating movement.

• Axial Skeleton: Skull, vertebral column, rib cage.

• Appendicular Skeleton: Limbs (arms & legs), pectoral and pelvic girdles.

• Function: Protection, support, and muscle attachment.

Skull Structure and Function

Cranium vs. Facial Bones

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

• Cranium: Encloses and protects the brain.

• Facial Bones: Form the face and provide attachment points for muscles.

Associated Bones of the Skull

• Frontal Bone: Forehead and upper part of the eye sockets.

• Parietal Bones (2): Sides and roof of the skull; joined at the sagittal suture.

• Occipital Bone: Back and base of the skull; contains the foramen magnum.

• Temporal Bones (2): Sides of the skull near the ears; house ear structures.

• Sphenoid Bone: Butterfly-shaped bone at the base of the skull; articulates with almost every other cranial bone. Contains the sella turcica (pituitary gland sits here).

• Ethmoid Bone: Located inside the skull, between the nasal cavity and orbits; contains the cribriform plate and perpendicular plate.

• Hyoid Bone: Unique bone in the neck; does not articulate with any other bone. Important for speech and swallowing.

Additional info: The hyoid bone's forensic significance: fracture may indicate strangulation.

Paired vs. Unpaired Bones

• Paired Bones: Parietal, temporal, maxilla, nasal, zygomatic.

• Unpaired Bones: Frontal (in adults), occipital, sphenoid, ethmoid, mandible, vomer.

Sutures of the Skull

Definition and Key Sutures

Sutures are immovable joints where two or more skull bones meet. They provide stability and allow for growth during development.

• Sagittal Suture: Between the two parietal bones; runs along the midline.

• Coronal Suture: Between the frontal and parietal bones; runs in the coronal plane.

• Lambdoid Suture: Between the occipital and parietal bones; shaped like the Greek letter lambda.

• Squamous Suture: Between the temporal and parietal bones; weakest part of the temporal bone.

Landmarks and Processes

• External Occipital Protuberance (EOP): Bump on the back of the occipital bone; muscle attachment site.

• Mastoid Process (Temporal Bone): Large projection behind the ear; attachment for neck muscles.

• Zygomatic Process (Temporal Bone): Projects anteriorly to form the zygomatic arch (cheekbone).

• Styloid Process (Temporal Bone): Slender, pointed projection; attachment for muscles/ligaments of tongue and hyoid bone.

Key Bone Features & Structures

Processes and Projections

• Processes: Bony projections for muscle/ligament attachment.

• Protuberances: Bony landmarks (e.g., EOP).

Ethmoid Bone Features

• Cribriform Plate: Horizontal plate with olfactory foramina for passage of olfactory nerves (smell).

• Perpendicular Plate: Forms superior part of the nasal septum.

• Crista Galli: Vertical projection; attachment for dura mater.

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

• Inferior Nasal Conchae: Located inside the nasal cavity; increases surface area to warm, humidify, and filter air.

Foramina (Holes) and Cranial Nerves

Definition and Function

Foramina are openings in the skull that allow passage of nerves, blood vessels, and other structures.

Key Foramina and Associated Cranial Nerves

Foramen	Associated Nerve(s)	Function
Cribriform Plate	Olfactory nerves (CN I)	Smell
Optic Canal	Optic nerve (CN II)	Vision
Superior Orbital Fissure	Oculomotor (CN III), Trochlear (CN IV), Abducens (CN VI), Ophthalmic branch of Trigeminal (CN V1)	Eye movement, sensation
Foramen Rotundum	Maxillary branch of Trigeminal (CN V2)	Facial sensation
Foramen Ovale	Mandibular branch of Trigeminal (CN V3)	Facial sensation, chewing
Foramen Magnum	Spinal cord	Brain-spinal cord connection
Hypoglossal Canal	Hypoglossal nerve (CN XII)	Tongue movement
Stylomastoid Foramen	Facial nerve (CN VII)	Facial expression
Jugular Foramen	Glossopharyngeal (CN IX), Vagus (CN X), Accessory (CN XI)	Swallowing, head/shoulder movement

Skull Base & Internal Features

• Foramen Magnum: Large opening in the occipital bone; passage for spinal cord.

• Occipital Condyles: Rounded projections on the occipital bone; articulate with the atlas (C1 vertebra).

• Hypophyseal Fossa: Depression within the sella turcica; houses the pituitary gland.

• Petrous Part (Temporal Bone): Thick, dense portion housing inner ear structures (hearing and balance).

Functions of the Skull

• Protection: Rigid barrier protecting the brain.

• Sensory Input: Pathways for cranial nerves controlling senses and muscles in the head and neck.

• Motor Control: Attachment points for muscles controlling movement, facial expressions, and chewing.

• Structural Support: Maintains shape of the head and face.

Facial Bones (Brief Overview)

• Maxilla: Upper jaw.

• Mandible: Lower jaw.

• Nasal Bones: Bridge of the nose.

Cranial Nerves: Overview

Twelve pairs of cranial nerves emerge directly from the brain, controlling sensory and motor functions.

• Sensory: Smell, vision, taste, hearing, touch, pain.

• Motor: Eye movement, facial expression, swallowing, tongue movement, head/shoulder movement.

Additional Notes

• Pneumatic Bones: Bones containing air spaces lined by mucosa (frontal, ethmoid, maxillary, sphenoid).

• Things to Skip (Not on Test): Palatine process of maxilla, pterion brand, convoy fragment.

Summary Table: Key Skull Bones and Features

Bone	Location	Key Features
Frontal	Forehead	Forms upper eye sockets
Parietal (2)	Sides/roof	Joined at sagittal suture
Occipital	Back/base	Foramen magnum, EOP
Temporal (2)	Sides (near ears)	Mastoid, styloid, zygomatic processes
Sphenoid	Base	Sella turcica, articulates with all cranial bones
Ethmoid	Between nasal cavity/orbits	Cribriform plate, perpendicular plate, crista galli
Hyoid	Neck	No direct articulation; speech/swallowing

Key Equations (LaTeX Format)

• $\text{Number of Cranial Nerves} = 12$

• $\text{Number of Skull Bones} = 22$

Additional info: The sphenoid bone is called the "keystone" of the cranial floor because it articulates with all other cranial bones and forms a central base for the skull's structure.