Student copy. Lecture BB1. The Skull

Introduction

The skull is a vital structure comprising multiple bones that provide protection and support for the brain and face. This overview will detail its anatomy, the differences between fetal and adult skulls, and the associated foramina and cranial nerves.

Learning Objectives

• Naming and Locating Skull Bones: Gain an understanding of both the neurocranium and viscerocranium components.

• Comparing Skull Structures: Identify differences in skull anatomy between newborns and adults.

• Locating Foramina: Understand the location and significance of various foramina within the skull.

• Cranial Nerves: Identify cranial nerves corresponding to each foramen in the skull.

Skull Anatomy

The skull consists of 22 bones, which can be categorized into:

• Neurocranium: Forming the cranial vault and base (containing 8 bones).

• Viscerocranium: Comprising the facial skeleton.

Functions of the Skull

• Protects the delicate brain.

• Serves as an attachment site for head and neck muscles.

• Forms the face's structural framework.

• Contains cavities for special senses (sight, taste, and smell).

• Provides openings for the passage of air and food.

• Secures teeth and anchors facial muscles for expression.

Bone Composition

Bones of the Calvarium (Cranial Vault)

• Frontal

• Sphenoid

• Temporal (x2)

• Parietal (x2)

• Occipital

• Ethmoid

Bones of the Cranial Base

• Anterior Cranial Fossa

• Middle Cranial Fossa

• Posterior Cranial Fossa

Cranial Sutures

The skull bones are united by fibrous sutures:

• Coronal Suture: Between frontal and parietal bones.

• Sagittal Suture: Between the two parietal bones, extending from coronal to lambdoid sutures.

• Lambdoid Suture: Between parietal and occipital bones.

• Squamous Suture: Between parietal and temporal bones.

Special Features of Temporal Bone

• Outer, middle, and inner ear structures are housed within the temporal bone, critical for hearing and balance.

Sphenoid Bone

Recognized as the keystone of the cranial floor, the sphenoid bone connects with all other cranial bones. Key landmarks include:

• Greater Wing: Lateral projections contributing to the lateral skull border.

• Lesser Wing: Smaller projections located above the greater wings.

• Pterygoid Plates: Long downward projections for muscle attachment.

• Sella Turcica: Houses the pituitary gland.

Occipital Bone

Articulates with:

• Parietal and temporal bones

• Sphenoid bone at the front

• Atlas (the first cervical vertebra) below

Key Landmarks

• Foramen Magnum: Allows spinal cord passage.

• External Occipital Protuberance: A prominent projection on the occipital bone's back.

• Nuchal Lines: Serve as muscle attachment points.

The Meninges

Membranous coverings of the brain and spinal cord are classified as:

• Dura Mater: The outermost layer.

• Arachnoid Mater: The middle layer.

• Pia Mater: The innermost layer, termed leptomeninges when referring to the latter two.

Dural Folds

Dura mater extends into the cranial cavity and aids in stabilizing the brain, housing collecting veins (dural sinuses). The main dural folds are:

• Falx Cerebri: Between cerebral hemispheres.

• Tentorium Cerebelli: Between cerebrum and cerebellum.

• Falx Cerebelli: Between cerebellar hemispheres.

Facial Skeleton

The anterior part of the skull encompasses bones surrounding the orbit, oral cavity, and nasal cavities. Key bones include:

• Frontal

• Ethmoid

• Maxilla

• Zygomatic

• Sphenoid

Orbit Boundaries

• Roof: Orbital plate of the frontal bone, lesser wing of the sphenoid.

• Floor: Orbital surface of maxilla, zygomatic bone.

• Medial Wall: Lacrimal bone, orbital plate of ethmoid.

• Lateral Wall: Zygomatic bone, greater wing of sphenoid.

Oral Cavity Structure

Formed by the maxilla, mandible, and contributions from the palatine bone. Houses upper and lower teeth.

Mandible Composition

The strongest facial bone, housing the lower teeth, includes:

• Body: Curved horizontal portion.

• Rami: Two upward projections perpendicular to the body.

• Condylar Process: Articulates with the mandibular fossa of the temporal bone.

• Coronoid Process: Muscle attachment point.

Nasal Cavity Structure

Comprising several bones like the perpendicular plate of the ethmoid, vomer, and parts of the maxilla and palatine bone. The nasal cavity has vital structural components:

• Floor: Formed by the palatine process of maxilla and palatine bone.

• Roof: Crafted from the cribriform plate of the ethmoid bone.

• Lateral Wall: Contains the nasal conchae, enhancing airflow.

Paranasal Sinuses

Hollow spaces within the frontal, sphenoid, ethmoid, and maxillary bones serve several functions:

• Increase surface area of nasal mucosa.

• Warm air before it enters the lungs.

• Act as shock absorbers.

Fetal Skull Development

The skull features fontanelles—membranous regions allowing growth and flexibility for birth.

Key Fontanelles

1. Anterior Fontanelle: Largest, closes by 18-24 months.

2. Posterior Fontanelle: Closes by 2-3 months.

3. Anterolateral Fontanelle: Closes by 6 months.

4. Posterolateral Fontanelle: Closes by 6-18 months.

Foramina and Cranial Nerves

The following foramina facilitate the passage of specific cranial nerves:

• Optic Canal: Optic nerve (CN II).

• Superior Orbital Fissure: Oculomotor (CN III), Trochlear (CN IV), Abducens (CN VI), Ophthalmic branch (CN V).

• Jugular Foramen: Glossopharyngeal (CN IX), Vagus (CN X), Accessory (CN XI).

Cranial Nerve Functions

• Olfactory (I): Smell.

• Optic (II): Vision.

• Oculomotor (III): Eye movement and pupil constriction.

• Trigeminal (V): Sensation and mastication.

• Facial (VII): Facial expression, taste.

• Glossopharyngeal (IX): Taste, parasympathetic supply to the parotid gland.

• Vagus (X): Motor and sensory functions in thoracic and abdominal cavities.

• Hypoglossal (XII): Tongue movement.

Conclusion

Understanding the skull's anatomy, including its bones, foramina, and associated cranial nerves is essential for comprehending its role in protecting the brain and facilitating various functions.