Cranial cavity

The cranial cavity is the inside surface of the base of the skull, providing a stable platform for the brain. The cranial cavity, along with the vault of the cranium or calvaria above it, functions as a protective casing for the human brain.

The bones of the base of the skull contribute to the floor of the cranial cavity. This floor can be divided into three main sections: the anterior, middle, and posterior cranial fossae.

The anterior cranial fossa is formed by various bones, including the orbital plates of the frontal bones, the cribriform plate of the ethmoid bone, and the lesser wing of the sphenoid bone. The frontal bone's orbital plates contribute to the roof of the eye socket, where the frontal lobes of the brain rest. The cribriform plate forms the roof of the nasal cavity and allows olfactory neurons to pass through, entering the olfactory bulb located just above it.

The middle cranial fossa is mainly formed by the sphenoid bone and the temporal bones. It houses the temporal lobes of the brain within it. The sphenoid bone's body, in the central part of this region, contains two important features: the suprachiasmatic sulcus, a shallow depression hosting the optic chiasma anteriorly, and the sella turcica, a deeper depression covered by the diaphragma sellae that contains the pituitary gland. At its rear, the temporal bone contains a bony ridge called the petrous temporal bone, which contains the middle and inner ear structures.

The posterior cranial fossa is primarily formed by the occipital bone and the posterior sloping part of the petrous temporal bones. It contains vital structures like the brainstem and the cerebellum, and is covered by the tentorium cerebelli, which separates these structures from the occipital lobe of the brain.

The floor of the cranial cavity also contains openings called foramina. These foramina allow the cranial nerves to pass through and innervate various structures outside of the skull. Here are some important foramina:

• Olfactory nerves pass through the olfactory foramina in the cribriform plate of the ethmoid bone.

• Optic nerves pass through the optic canals located in the lesser wing of the sphenoid bone.

• Several nerves, including the oculomotor, trochlear, abducent, and ophthalmic nerve branches (V1), pass through the superior orbital fissure to enter the eye socket on either side.

• The maxillary (V2) and mandibular (V3) nerves pass through the foramen rotundum and foramen ovale, respectively.

• The middle meningeal artery passes through the foramen spinosum.

• The facial and vestibulocochlear nerves pass through the internal acoustic meatus on each side. The vestibulocochlear nerve innervates the inner ear within the temporal bone, while the facial nerve exits the skull base through the stylomastoid foramen.

• The jugular foramen is located at the junction of the petrous temporal and occipital bones. It allows for the passage of the glossopharyngeal, vagus, and accessory nerves, as well as the internal jugular vein.

• The hypoglossal nerve exits the cranial cavity through the hypoglossal canal on both sides.

• Lastly, the foramen magnum, located in the center of the occipital bone, allows the brainstem to continue descending into the vertebral canal as the spinal cord. Other structures passing through the foramen magnum include the meninges, the spinal accessory nerve, and the vertebral arteries.