(21) Skull Anatomy

ANTERIOR ASPECT OF THE SKULL

Orbit

The orbit is the bony cavity that contains the eyeball and its associated structures.

Notable Landmarks and Foramina

• Supraorbital margin: This is the upper edge of the orbit, formed specifically by the frontal bone.

• Supraorbital notch/foramen: Located along the supraorbital margin, this opening transmits the supraorbital nerve (which is a branch of the ophthalmic division of the trigeminal nerve, $\text{CN V}_1$) and its accompanying vessels.

• Infraorbital foramen: This opening is found within the maxilla, inferior to the orbit. It transmits the infraorbital nerve (a branch of the maxillary division of the trigeminal nerve, $\text{CN V}_2$) and its associated vessels.

Boundaries of the Orbit

• Lateral boundary: Formed by the zygomatic and frontal bones.

• Inferior boundary: Formed by the maxilla and zygomatic bones.

• Medial boundary: Formed by the maxilla, lacrimal, and frontal bones.

Bones within the Orbit

There are $6$ bones that contribute to the structure of the orbit:

• Frontal bone

• Ethmoid bone

• Sphenoid bone

• Lacrimal bone

• Maxilla

• Zygomatic bone

Openings of the Orbit

The following openings communicate with the orbit:

• Superior orbital fissure

• Inferior orbital fissure

• Optic canal

Zygomatic Bone (Cheek)

1. The zygomatic bone articulates with three other bones: the maxilla, frontal, and temporal bones.

2. Zygomatic Arch: The lateral articulation between the zygomatic bone and the temporal bone forms the zygomatic arch.

Nose

The structure of the nose involves several components:

1. Nasal bones

2. Maxillae

3. Piriform aperture: The large anterior opening of the nasal cavity.

4. Nasal septum: This structure divides the nasal cavity into halves.     - Anteriorly: Composed of cartilage.     - Posteriorly: Composed of the ethmoid and vomer bones.

Maxilla

The maxilla forms the upper jaw and contributes significantly to the facial structure. It consists of:

1. Body: Contains the maxillary sinus.

2. Zygomatic process: Articulates with the zygomatic bone.

3. Frontal process: Articulates with the frontal bone.

4. Palatine process: A horizontal projection that forms part of the hard palate.

5. Alveolar process: The portion that holds the upper teeth.

Mandible

The mandible forms the lower jaw and includes:

1. Body: Features the mental foramen, which transmits the mental nerve (a branch of the mandibular division of the trigeminal nerve, $\text{CN V}_3$).

2. Angle: The corner where the body and ramus meet.

3. Ramus: The vertical portion of the mandible.

4. Coronoid process: The anterior projection of the superior ramus.

5. Condylar process: The posterior projection of the superior ramus that articulates with the temporal bone.

LATERAL SKULL

The lateral aspect of the skull is composed of several major bones:

• Frontal bone

• Parietal bone

• Temporal bone:     - Zygomatic process: Joins with the temporal process of the zygomatic bone to form the zygomatic arch.     - External acoustic (auditory) meatus: The opening for the ear canal.     - Styloid process: A thin, pointed projection of bone.     - Mastoid process: A rounded projection located posterior to the external acoustic meatus.

• Sphenoid bone

• Occipital bone

SUTURES AND CRANIAL LANDMARKS

Sutures are the fibrous joints between the bones of the cranium. These joints allow for very little movement.

The Pterion

The pterion is a crucial clinical landmark located where the sphenoid, frontal, parietal, and temporal bones all converge.

• Clinical Significance: It serves as a landmark for the underlying middle meningeal artery, which is located in the epidural space. Trauma to this region can lead to arterial rupture and subsequent medical emergencies.

ANTERIOR CRANIAL FOSSA

Landmarks

• Contents: It contains the frontal lobes of the cerebral hemispheres (specifically the frontal poles).

• Boundaries: Formed by the frontal bone anteriorly and the lesser wings of the sphenoid bone posteriorly.

• Medial Division: It is divided medially by the crista galli and the cribriform plates of the ethmoid bone.

• Floor: The floor of the anterior cranial fossa also forms the roofs of the orbits.

Cranial Foramina and Neurovascular Structures

• Cribriform plate: Transmits the olfactory nerves ($\text{CN I}$).

MIDDLE CRANIAL FOSSA

Landmarks

• Contents: Contains the anterior parts of the temporal lobes of the cerebral hemispheres.

• Boundaries: Formed by the greater wings of the sphenoid bone anteriorly and the temporal bone posteriorly.

• Medial Division: Divided medially by the body of the sphenoid bone.

• Sella Turcica: The upper part of the sphenoid body is the sella turcica (translated as "Turkish saddle"), which contains the pituitary gland.

• Floor: The floor is formed by the petrous part of the temporal bone, which serves as the roof for the chambers of the middle and inner ear.

• Clinical Note: The middle cranial fossa is a common site for basilar skull fractures.

Cranial Foramina and Neurovascular Structures

• Optic canal:     - Transmits the optic nerve ($\text{CN II}$).     - Transmits the ophthalmic artery.

• Superior orbital fissure:     - Transmits the oculomotor nerve ($\text{CN III}$).     - Transmits the trochlear nerve ($\text{CN IV}$).     - Transmits the ophthalmic division of the trigeminal nerve ($\text{CN V}_1$).     - Transmits the abducens nerve ($\text{CN VI}$).

• Carotid groove: Contains the internal carotid artery.

• Foramen rotundum: Transmits the maxillary division of the trigeminal nerve ($\text{CN V}_2$).

• Foramen ovale: Transmits the mandibular division of the trigeminal nerve ($\text{CN V}_3$).

• Foramen spinosum:     - Transmits the middle meningeal artery. This artery grooves the inner surface of the squamous portion of the temporal bone.     - Clinical Note: If the temporal bone is fractured, the middle meningeal artery can be torn, producing an epidural hematoma.

• Foramen lacerum: This opening is filled by cartilage in life, which is absent in a dried skull.

POSTERIOR CRANIAL FOSSA

Landmarks

• Contents: Contains the cerebellum, pons, and medulla.

• Boundaries: Formed by the temporal and sphenoid bones anteriorly, and the occipital bone posteriorly.

• Medial Division: Divided medially by the foramen magnum.

• Floor and Posterior Walls: These areas are grooved for the dural venous sinuses.

Cranial Foramina and Neurovascular Structures

• Internal acoustic (auditory) meatus:     - Transmits the facial nerve ($\text{CN VII}$).     - Transmits the vestibulocochlear nerve ($\text{CN VIII}$).

• Foramen magnum:     - Marks the juncture of the medulla with the spinal cord.     - Transmits the spinal roots of the accessory nerve ($\text{CN XI}$).     - Transmits the spinal arteries.     - Transmits the vertebral arteries.

• Jugular foramen:     - Transmits the sigmoid sinus / internal jugular vein.     - Transmits the glossopharyngeal nerve ($\text{CN IX}$).     - Transmits the vagus nerve ($\text{CN X}$).     - Transmits the accessory nerve ($\text{CN XI}$).

• Hypoglossal canal:     - Transmits the hypoglossal nerve ($\text{CN XII}$).