Base of Skull

The base of the skull, also known as the cranial base or basicranium, is the inferior portion of the skull that supports the brain and contains numerous openings (foramina) for the passage of nerves and blood vessels. It plays a critical structural and protective role and is divided into external and internal surfaces, each with unique anatomical landmarks and clinical relevance.

Structure

The skull base is formed by parts of several bones, both paired and unpaired, and is best studied from two perspectives:

• Internal base of the skull: Observed from inside the cranial cavity; divided into three cranial fossae — anterior, middle, and posterior.
• External base of the skull: Viewed from the underside; shows foramina, processes, and muscular attachments.

The primary bones involved in forming the base include the frontal, ethmoid, sphenoid, temporal, occipital, and parts of the parietal bones.

Location

The base of the skull is located at the inferior aspect of the cranium. It lies below the brain and above structures of the neck and upper face. It forms the floor of the cranial cavity and articulates inferiorly with the cervical vertebrae at the atlanto-occipital joint.

Function

• Support: Provides a rigid platform for the brain and anchors the cranial fossae that hold different brain regions.
• Passage: Allows exit and entry of cranial nerves, blood vessels, and the spinal cord through various foramina and canals.
• Protection: Shields neurovascular structures and supports the base of the brain.
• Attachment: Serves as an attachment site for muscles of the head and neck, including those for chewing and swallowing.

Anatomy of the Internal Base of the Skull

1. Anterior Cranial Fossa

This is the most shallow and anterior of the three fossae. It supports the frontal lobes of the brain and is formed by:

• Frontal bone (orbital plates)
• Ethmoid bone (cribriform plate)
• Lesser wings of the sphenoid

Key features:

• Cribriform plate: Contains foramina for the olfactory nerves (CN I).
• Crista galli: A vertical projection for attachment of the falx cerebri.

2. Middle Cranial Fossa

This deeper fossa supports the temporal lobes and the pituitary gland. It is formed by:

• Greater wings of the sphenoid
• Body of the sphenoid
• Temporal bones (petrous part)

Key features and foramina:

• Optic canal: Transmits optic nerve (CN II) and ophthalmic artery.
• Superior orbital fissure: CN III, IV, V1, VI pass through here.
• Foramen rotundum: Maxillary nerve (CN V2).
• Foramen ovale: Mandibular nerve (CN V3).
• Foramen spinosum: Middle meningeal artery.
• Sella turcica: Houses the pituitary gland.

3. Posterior Cranial Fossa

This is the deepest and largest fossa, supporting the cerebellum, pons, and medulla. It is formed by:

• Occipital bone
• Petrous and mastoid portions of the temporal bones

Key features and foramina:

• Foramen magnum: Allows passage of the medulla oblongata, spinal accessory nerves (CN XI), and vertebral arteries.
• Jugular foramen: CN IX, X, XI, and internal jugular vein.
• Internal acoustic meatus: CN VII (facial) and VIII (vestibulocochlear).
• Hypoglossal canal: Hypoglossal nerve (CN XII).

Anatomy of the External Base of the Skull

The external base can be viewed from an inferior perspective and shows important foramina and muscular attachments.

Key Bony Landmarks

• Occipital condyles: Articulate with the atlas (C1) to form the atlanto-occipital joint.
• External occipital protuberance: A prominent bump used as a surface landmark.
• Styloid process: Serves as an attachment for ligaments and muscles of the tongue and neck.
• Mastoid process: Contains mastoid air cells and provides attachment for neck muscles.
• Pterygoid processes (sphenoid): Serve as origin points for pterygoid muscles involved in mastication.

Important Openings

• Carotid canal: Internal carotid artery enters here.
• Jugular foramen: Exit for several cranial nerves and veins.
• Foramen lacerum: Filled with cartilage in life; internal carotid passes across it.
• Stylomastoid foramen: Facial nerve (CN VII) exits the skull here.

Development

The base of the skull develops through a mix of endochondral and intramembranous ossification. Most of the base, including the occipital, sphenoid, and petrous temporal parts, undergo endochondral ossification, meaning they form from a cartilaginous model. These regions form early and fuse during development, contributing to the stability of the cranial cavity.

Sutures at the base, such as the spheno-occipital synchondrosis, fuse during adolescence or early adulthood. Abnormal timing of fusion can affect cranial shape and brain development.

Clinical Relevance

• Basilar skull fractures: Can result from significant trauma and may involve the temporal bone, occipital bone, or sphenoid. Symptoms include CSF leakage from nose or ears (rhinorrhea, otorrhea), raccoon eyes, or Battle's sign.
• Increased intracranial pressure: May compress nerves at the foramina of the base, leading to cranial nerve palsies.
• Chiari malformations: Involve descent of cerebellar tissue through the foramen magnum, compressing the brainstem.
• Tumors at the skull base: Can affect the pituitary gland (e.g., pituitary adenoma), cranial nerves, or brainstem, depending on location.
• Surgical approaches: Skull base surgery involves complex approaches (e.g., transnasal, retrosigmoid) to access deep tumors or vascular lesions.

Surface Anatomy and Landmarks

• Basion: Midpoint of the anterior margin of the foramen magnum.
• Opisthion: Midpoint of the posterior margin of the foramen magnum.
• Pterion: Junction of frontal, parietal, temporal, and sphenoid bones—site of middle meningeal artery.
• Inion: External occipital protuberance, palpable on the back of the head.