Learning Objective: To describe the anatomy, contents, foramina, and clinical significance of the posterior cranial fossa — the deepest part of the cranial cavity that houses the brainstem and cerebellum.

The floor of the cranial cavity is divided into three fossae:

• Anterior cranial fossa – accommodates the frontal lobes
• Middle cranial fossa – accommodates the temporal lobes and pituitary gland
• Posterior cranial fossa – accommodates the brainstem and cerebellum

The posterior cranial fossa is the most posterior and deepest of the three fossae.

Borders

The posterior cranial fossa is formed by the occipital bone and two temporal bones.

Contents

• Brainstem (midbrain, pons, medulla oblongata)
• Cerebellum
• Associated cranial nerves (VII–XII) and arteries (vertebral, basilar)

The cerebellum lies in the cerebellar fossae, separated medially by the internal occipital crest. The brainstem continues through the foramen magnum to become the spinal cord.

Foramina and Transmitted Structures

Temporal Bone

• Internal acoustic meatus → CN VII, CN VIII, labyrinthine artery

Occipital Bone

• Foramen magnum → medulla, meninges, vertebral arteries, ascending spinal accessory nerve, spinal arteries, dural veins
• Jugular foramen → CN IX, CN X, CN XI (descending), internal jugular vein, sigmoid sinus, inferior petrosal sinus
• Hypoglossal canal → CN XII (hypoglossal nerve)
• Cerebellar fossae → houses the cerebellar hemispheres

Clinical Relevance

Cerebellar Tonsillar Herniation (Coning)

• Definition: Downward displacement of cerebellar tonsils through the foramen magnum.
• Causes: Raised intracranial pressure due to hydrocephalus, space-occupying lesion, or congenital malformation.
• Effects: Compression of the pons and medulla, leading to cardiorespiratory arrest if untreated.
• Imaging: Seen on MRI as tonsillar descent below the foramen magnum.

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