Cranial Fossae

The cranial fossae are three depressions on the internal base of the skull that accommodate different parts of the brain. These fossae - anterior, middle, and posterior - are separated by bony ridges and are critical for understanding the organization of the cranial cavity. Each fossa has its own set of foramina for the passage of nerves and vessels and houses specific brain regions. A clear understanding of these structures is essential for students of anatomy, radiology, neurology, and surgery.

Overview

The cranial base can be divided into three fossae:

• Anterior cranial fossa

• Middle cranial fossa

• Posterior cranial fossa

These fossae are formed by the cranial bones and shaped to cradle the brain. The anterior fossa supports the frontal lobes, the middle fossa supports the temporal lobes and pituitary gland, and the posterior fossa houses the cerebellum, pons, and medulla.

Anterior Cranial Fossa

The anterior cranial fossa is the most shallow and anterior part of the cranial base. It is formed primarily by the frontal bone (orbital plates), ethmoid bone (cribriform plate), and lesser wings of the sphenoid.

Contents:

• Frontal lobes of the cerebral hemispheres

• Olfactory bulbs

Key features:

• Cribriform plate (ethmoid): Contains numerous tiny foramina for the passage of the olfactory nerve fibers (CN I).

• Crista galli: A vertical projection of the ethmoid bone that anchors the falx cerebri.

Middle Cranial Fossa

The middle cranial fossa is deeper and wider than the anterior fossa. It is formed by the sphenoid bone (body and greater wings) and the petrous portion of the temporal bone. It lies between the lesser wings of the sphenoid and the petrous ridges of the temporal bones.

Contents:

• Temporal lobes of the brain

• Pituitary gland (in sella turcica)

Key features and foramina:

• Optic canals: Transmit the optic nerves (CN II) and ophthalmic arteries.

• Superior orbital fissure: Transmits cranial nerves III, IV, V1, VI.

• Foramen rotundum: Maxillary nerve (CN V2).

• Foramen ovale: Mandibular nerve (CN V3).

• Foramen spinosum: Middle meningeal artery.

• Sella turcica: Bony depression that houses the pituitary gland.

Posterior Cranial Fossa

The posterior cranial fossa is the deepest and largest. It is formed by the occipital bone and parts of the temporal bones. This fossa is located posterior to the petrous ridges and supports the hindbrain structures.

Contents:

• Cerebellum

• Pons

• Medulla oblongata

Key features and foramina:

• Foramen magnum: The largest opening; transmits the medulla, spinal accessory nerves (CN XI), and vertebral arteries.

• Internal acoustic meatus: Facial (CN VII) and vestibulocochlear (CN VIII) nerves.

• Jugular foramen: Glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI) nerves, and internal jugular vein.

• Hypoglossal canal: Hypoglossal nerve (CN XII).

Clinical Relevance

• Trauma: Fractures involving the fossae can lead to cranial nerve injury or cerebrospinal fluid (CSF) leaks.

• Tumors: Tumors of the pituitary gland can expand within the sella turcica, affecting the optic chiasm.

• Posterior fossa syndrome: Tumors or hemorrhages here can compress vital brainstem structures, leading to life-threatening complications.