Anatomia - generalities of the encephalon

Structure and contents of the neurocranium

The neurocranium contains the encephalon, the meninges, the intracranial portion of the cranial nerves (there are twelve pairs of nerves that emerge mainly from the brain stem, with the exception of the first two, and converge information principally to the head and neck. They possess a real and apparent origin), vessels, and the cerebrospinal fluid associated with the ventricles.

Ventricles

The ventricles are in direct communication with the central canal of the spinal cord. They include the 4^th ventricle (located between the brainstem and cerebellum) which is continuous with the 3^rd ventricle via the aqueduct of Silvio (found in the diencephalon). It is then continuous with the two lateral ventricles found in the two cerebral hemispheres through the two foramens of Monro.

Neurocranial fossa

The neurocranial fossa presents a base and a vault. The vault is formed by the frontal, parietal, and occipital bones. The base is subdivided into three main cavities:

• Anterior cranial fossa: Composed of the frontal bone, ethmoid bone, and lesser wing of the sphenoid bone, it presents the lamina cribrosa and the crista galli.
• Middle cranial fossa: Extends from the lesser wings of the sphenoid bone to the petrous part of the temporal bone. It contains the sella turcica that hosts the hypophysis and many fissures and foramina for the passage of neurovascular structures.
• Posterior cranial fossa: Mainly formed by the occipital bone, characterized by the presence of the foramen magnum.

The role of meninges

The encephalon is invested by the meninges. The most external layer is the dura mater, which adheres to the periosteum of the different bones that compose the neurocranium and with the sutures and sagittal sulcus. It also forms the septa that partially subdivide the cavities into lodges. The septa are:

• The cerebral falx: A large semilunar sagittal septum that extends from the crista galli to the tentorium cerebelli, running inside the interhemispheric fissure, dividing the two cerebral hemispheres.
• The cerebellar falx: A sagittal septum placed in the posterior cranial fossa under the tentorium cerebelli, attached to the internal occipital crest, running between the two cerebellar hemispheres.
• The tentorium cerebelli: Starts from the internal occipital protuberance and continues laterally in the transverse occipital sulcus, terminating at the level of the posterior clinoid process. The anterior free margin delimits together with the sella turcica the oval foramen of Pacchioni. The tentorium cerebelli subdivides the cranial cavity into a supratentorial lodge (where the brain or forebrain is found) and a subtentorial lodge (where the brain stem and the cerebellum are localized).
• The diaphragm of the sella turcica: A small septum that extends over the sella turcica.

Supratentorial lodge

The brain or forebrain is composed of the telencephalon and diencephalon.